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Managing Carbon Monoxide Pollution in Meteorological and Topographical Problem Areas (2003)

Chapter: 1. Ambient Carbon Monoxide Pollution in the United States

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Suggested Citation:"1. Ambient Carbon Monoxide Pollution in the United States." Transportation Research Board and National Research Council. 2003. Managing Carbon Monoxide Pollution in Meteorological and Topographical Problem Areas. Washington, DC: The National Academies Press. doi: 10.17226/10689.
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Suggested Citation:"1. Ambient Carbon Monoxide Pollution in the United States." Transportation Research Board and National Research Council. 2003. Managing Carbon Monoxide Pollution in Meteorological and Topographical Problem Areas. Washington, DC: The National Academies Press. doi: 10.17226/10689.
×
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Suggested Citation:"1. Ambient Carbon Monoxide Pollution in the United States." Transportation Research Board and National Research Council. 2003. Managing Carbon Monoxide Pollution in Meteorological and Topographical Problem Areas. Washington, DC: The National Academies Press. doi: 10.17226/10689.
×
Page 18
Suggested Citation:"1. Ambient Carbon Monoxide Pollution in the United States." Transportation Research Board and National Research Council. 2003. Managing Carbon Monoxide Pollution in Meteorological and Topographical Problem Areas. Washington, DC: The National Academies Press. doi: 10.17226/10689.
×
Page 19
Suggested Citation:"1. Ambient Carbon Monoxide Pollution in the United States." Transportation Research Board and National Research Council. 2003. Managing Carbon Monoxide Pollution in Meteorological and Topographical Problem Areas. Washington, DC: The National Academies Press. doi: 10.17226/10689.
×
Page 20
Suggested Citation:"1. Ambient Carbon Monoxide Pollution in the United States." Transportation Research Board and National Research Council. 2003. Managing Carbon Monoxide Pollution in Meteorological and Topographical Problem Areas. Washington, DC: The National Academies Press. doi: 10.17226/10689.
×
Page 21
Suggested Citation:"1. Ambient Carbon Monoxide Pollution in the United States." Transportation Research Board and National Research Council. 2003. Managing Carbon Monoxide Pollution in Meteorological and Topographical Problem Areas. Washington, DC: The National Academies Press. doi: 10.17226/10689.
×
Page 22
Suggested Citation:"1. Ambient Carbon Monoxide Pollution in the United States." Transportation Research Board and National Research Council. 2003. Managing Carbon Monoxide Pollution in Meteorological and Topographical Problem Areas. Washington, DC: The National Academies Press. doi: 10.17226/10689.
×
Page 23
Suggested Citation:"1. Ambient Carbon Monoxide Pollution in the United States." Transportation Research Board and National Research Council. 2003. Managing Carbon Monoxide Pollution in Meteorological and Topographical Problem Areas. Washington, DC: The National Academies Press. doi: 10.17226/10689.
×
Page 24
Suggested Citation:"1. Ambient Carbon Monoxide Pollution in the United States." Transportation Research Board and National Research Council. 2003. Managing Carbon Monoxide Pollution in Meteorological and Topographical Problem Areas. Washington, DC: The National Academies Press. doi: 10.17226/10689.
×
Page 25
Suggested Citation:"1. Ambient Carbon Monoxide Pollution in the United States." Transportation Research Board and National Research Council. 2003. Managing Carbon Monoxide Pollution in Meteorological and Topographical Problem Areas. Washington, DC: The National Academies Press. doi: 10.17226/10689.
×
Page 26
Suggested Citation:"1. Ambient Carbon Monoxide Pollution in the United States." Transportation Research Board and National Research Council. 2003. Managing Carbon Monoxide Pollution in Meteorological and Topographical Problem Areas. Washington, DC: The National Academies Press. doi: 10.17226/10689.
×
Page 27
Suggested Citation:"1. Ambient Carbon Monoxide Pollution in the United States." Transportation Research Board and National Research Council. 2003. Managing Carbon Monoxide Pollution in Meteorological and Topographical Problem Areas. Washington, DC: The National Academies Press. doi: 10.17226/10689.
×
Page 28
Suggested Citation:"1. Ambient Carbon Monoxide Pollution in the United States." Transportation Research Board and National Research Council. 2003. Managing Carbon Monoxide Pollution in Meteorological and Topographical Problem Areas. Washington, DC: The National Academies Press. doi: 10.17226/10689.
×
Page 29
Suggested Citation:"1. Ambient Carbon Monoxide Pollution in the United States." Transportation Research Board and National Research Council. 2003. Managing Carbon Monoxide Pollution in Meteorological and Topographical Problem Areas. Washington, DC: The National Academies Press. doi: 10.17226/10689.
×
Page 30
Suggested Citation:"1. Ambient Carbon Monoxide Pollution in the United States." Transportation Research Board and National Research Council. 2003. Managing Carbon Monoxide Pollution in Meteorological and Topographical Problem Areas. Washington, DC: The National Academies Press. doi: 10.17226/10689.
×
Page 31
Suggested Citation:"1. Ambient Carbon Monoxide Pollution in the United States." Transportation Research Board and National Research Council. 2003. Managing Carbon Monoxide Pollution in Meteorological and Topographical Problem Areas. Washington, DC: The National Academies Press. doi: 10.17226/10689.
×
Page 32
Suggested Citation:"1. Ambient Carbon Monoxide Pollution in the United States." Transportation Research Board and National Research Council. 2003. Managing Carbon Monoxide Pollution in Meteorological and Topographical Problem Areas. Washington, DC: The National Academies Press. doi: 10.17226/10689.
×
Page 33
Suggested Citation:"1. Ambient Carbon Monoxide Pollution in the United States." Transportation Research Board and National Research Council. 2003. Managing Carbon Monoxide Pollution in Meteorological and Topographical Problem Areas. Washington, DC: The National Academies Press. doi: 10.17226/10689.
×
Page 34
Suggested Citation:"1. Ambient Carbon Monoxide Pollution in the United States." Transportation Research Board and National Research Council. 2003. Managing Carbon Monoxide Pollution in Meteorological and Topographical Problem Areas. Washington, DC: The National Academies Press. doi: 10.17226/10689.
×
Page 35
Suggested Citation:"1. Ambient Carbon Monoxide Pollution in the United States." Transportation Research Board and National Research Council. 2003. Managing Carbon Monoxide Pollution in Meteorological and Topographical Problem Areas. Washington, DC: The National Academies Press. doi: 10.17226/10689.
×
Page 36
Suggested Citation:"1. Ambient Carbon Monoxide Pollution in the United States." Transportation Research Board and National Research Council. 2003. Managing Carbon Monoxide Pollution in Meteorological and Topographical Problem Areas. Washington, DC: The National Academies Press. doi: 10.17226/10689.
×
Page 37
Suggested Citation:"1. Ambient Carbon Monoxide Pollution in the United States." Transportation Research Board and National Research Council. 2003. Managing Carbon Monoxide Pollution in Meteorological and Topographical Problem Areas. Washington, DC: The National Academies Press. doi: 10.17226/10689.
×
Page 38
Suggested Citation:"1. Ambient Carbon Monoxide Pollution in the United States." Transportation Research Board and National Research Council. 2003. Managing Carbon Monoxide Pollution in Meteorological and Topographical Problem Areas. Washington, DC: The National Academies Press. doi: 10.17226/10689.
×
Page 39
Suggested Citation:"1. Ambient Carbon Monoxide Pollution in the United States." Transportation Research Board and National Research Council. 2003. Managing Carbon Monoxide Pollution in Meteorological and Topographical Problem Areas. Washington, DC: The National Academies Press. doi: 10.17226/10689.
×
Page 40
Suggested Citation:"1. Ambient Carbon Monoxide Pollution in the United States." Transportation Research Board and National Research Council. 2003. Managing Carbon Monoxide Pollution in Meteorological and Topographical Problem Areas. Washington, DC: The National Academies Press. doi: 10.17226/10689.
×
Page 41
Suggested Citation:"1. Ambient Carbon Monoxide Pollution in the United States." Transportation Research Board and National Research Council. 2003. Managing Carbon Monoxide Pollution in Meteorological and Topographical Problem Areas. Washington, DC: The National Academies Press. doi: 10.17226/10689.
×
Page 42
Suggested Citation:"1. Ambient Carbon Monoxide Pollution in the United States." Transportation Research Board and National Research Council. 2003. Managing Carbon Monoxide Pollution in Meteorological and Topographical Problem Areas. Washington, DC: The National Academies Press. doi: 10.17226/10689.
×
Page 43
Suggested Citation:"1. Ambient Carbon Monoxide Pollution in the United States." Transportation Research Board and National Research Council. 2003. Managing Carbon Monoxide Pollution in Meteorological and Topographical Problem Areas. Washington, DC: The National Academies Press. doi: 10.17226/10689.
×
Page 44
Suggested Citation:"1. Ambient Carbon Monoxide Pollution in the United States." Transportation Research Board and National Research Council. 2003. Managing Carbon Monoxide Pollution in Meteorological and Topographical Problem Areas. Washington, DC: The National Academies Press. doi: 10.17226/10689.
×
Page 45
Suggested Citation:"1. Ambient Carbon Monoxide Pollution in the United States." Transportation Research Board and National Research Council. 2003. Managing Carbon Monoxide Pollution in Meteorological and Topographical Problem Areas. Washington, DC: The National Academies Press. doi: 10.17226/10689.
×
Page 46
Suggested Citation:"1. Ambient Carbon Monoxide Pollution in the United States." Transportation Research Board and National Research Council. 2003. Managing Carbon Monoxide Pollution in Meteorological and Topographical Problem Areas. Washington, DC: The National Academies Press. doi: 10.17226/10689.
×
Page 47
Suggested Citation:"1. Ambient Carbon Monoxide Pollution in the United States." Transportation Research Board and National Research Council. 2003. Managing Carbon Monoxide Pollution in Meteorological and Topographical Problem Areas. Washington, DC: The National Academies Press. doi: 10.17226/10689.
×
Page 48
Suggested Citation:"1. Ambient Carbon Monoxide Pollution in the United States." Transportation Research Board and National Research Council. 2003. Managing Carbon Monoxide Pollution in Meteorological and Topographical Problem Areas. Washington, DC: The National Academies Press. doi: 10.17226/10689.
×
Page 49
Suggested Citation:"1. Ambient Carbon Monoxide Pollution in the United States." Transportation Research Board and National Research Council. 2003. Managing Carbon Monoxide Pollution in Meteorological and Topographical Problem Areas. Washington, DC: The National Academies Press. doi: 10.17226/10689.
×
Page 50
Suggested Citation:"1. Ambient Carbon Monoxide Pollution in the United States." Transportation Research Board and National Research Council. 2003. Managing Carbon Monoxide Pollution in Meteorological and Topographical Problem Areas. Washington, DC: The National Academies Press. doi: 10.17226/10689.
×
Page 51
Suggested Citation:"1. Ambient Carbon Monoxide Pollution in the United States." Transportation Research Board and National Research Council. 2003. Managing Carbon Monoxide Pollution in Meteorological and Topographical Problem Areas. Washington, DC: The National Academies Press. doi: 10.17226/10689.
×
Page 52
Suggested Citation:"1. Ambient Carbon Monoxide Pollution in the United States." Transportation Research Board and National Research Council. 2003. Managing Carbon Monoxide Pollution in Meteorological and Topographical Problem Areas. Washington, DC: The National Academies Press. doi: 10.17226/10689.
×
Page 53
Suggested Citation:"1. Ambient Carbon Monoxide Pollution in the United States." Transportation Research Board and National Research Council. 2003. Managing Carbon Monoxide Pollution in Meteorological and Topographical Problem Areas. Washington, DC: The National Academies Press. doi: 10.17226/10689.
×
Page 54
Suggested Citation:"1. Ambient Carbon Monoxide Pollution in the United States." Transportation Research Board and National Research Council. 2003. Managing Carbon Monoxide Pollution in Meteorological and Topographical Problem Areas. Washington, DC: The National Academies Press. doi: 10.17226/10689.
×
Page 55
Suggested Citation:"1. Ambient Carbon Monoxide Pollution in the United States." Transportation Research Board and National Research Council. 2003. Managing Carbon Monoxide Pollution in Meteorological and Topographical Problem Areas. Washington, DC: The National Academies Press. doi: 10.17226/10689.
×
Page 56
Suggested Citation:"1. Ambient Carbon Monoxide Pollution in the United States." Transportation Research Board and National Research Council. 2003. Managing Carbon Monoxide Pollution in Meteorological and Topographical Problem Areas. Washington, DC: The National Academies Press. doi: 10.17226/10689.
×
Page 57
Suggested Citation:"1. Ambient Carbon Monoxide Pollution in the United States." Transportation Research Board and National Research Council. 2003. Managing Carbon Monoxide Pollution in Meteorological and Topographical Problem Areas. Washington, DC: The National Academies Press. doi: 10.17226/10689.
×
Page 58
Suggested Citation:"1. Ambient Carbon Monoxide Pollution in the United States." Transportation Research Board and National Research Council. 2003. Managing Carbon Monoxide Pollution in Meteorological and Topographical Problem Areas. Washington, DC: The National Academies Press. doi: 10.17226/10689.
×
Page 59
Suggested Citation:"1. Ambient Carbon Monoxide Pollution in the United States." Transportation Research Board and National Research Council. 2003. Managing Carbon Monoxide Pollution in Meteorological and Topographical Problem Areas. Washington, DC: The National Academies Press. doi: 10.17226/10689.
×
Page 60
Suggested Citation:"1. Ambient Carbon Monoxide Pollution in the United States." Transportation Research Board and National Research Council. 2003. Managing Carbon Monoxide Pollution in Meteorological and Topographical Problem Areas. Washington, DC: The National Academies Press. doi: 10.17226/10689.
×
Page 61
Suggested Citation:"1. Ambient Carbon Monoxide Pollution in the United States." Transportation Research Board and National Research Council. 2003. Managing Carbon Monoxide Pollution in Meteorological and Topographical Problem Areas. Washington, DC: The National Academies Press. doi: 10.17226/10689.
×
Page 62
Suggested Citation:"1. Ambient Carbon Monoxide Pollution in the United States." Transportation Research Board and National Research Council. 2003. Managing Carbon Monoxide Pollution in Meteorological and Topographical Problem Areas. Washington, DC: The National Academies Press. doi: 10.17226/10689.
×
Page 63
Suggested Citation:"1. Ambient Carbon Monoxide Pollution in the United States." Transportation Research Board and National Research Council. 2003. Managing Carbon Monoxide Pollution in Meteorological and Topographical Problem Areas. Washington, DC: The National Academies Press. doi: 10.17226/10689.
×
Page 64
Suggested Citation:"1. Ambient Carbon Monoxide Pollution in the United States." Transportation Research Board and National Research Council. 2003. Managing Carbon Monoxide Pollution in Meteorological and Topographical Problem Areas. Washington, DC: The National Academies Press. doi: 10.17226/10689.
×
Page 65
Suggested Citation:"1. Ambient Carbon Monoxide Pollution in the United States." Transportation Research Board and National Research Council. 2003. Managing Carbon Monoxide Pollution in Meteorological and Topographical Problem Areas. Washington, DC: The National Academies Press. doi: 10.17226/10689.
×
Page 66
Suggested Citation:"1. Ambient Carbon Monoxide Pollution in the United States." Transportation Research Board and National Research Council. 2003. Managing Carbon Monoxide Pollution in Meteorological and Topographical Problem Areas. Washington, DC: The National Academies Press. doi: 10.17226/10689.
×
Page 67
Suggested Citation:"1. Ambient Carbon Monoxide Pollution in the United States." Transportation Research Board and National Research Council. 2003. Managing Carbon Monoxide Pollution in Meteorological and Topographical Problem Areas. Washington, DC: The National Academies Press. doi: 10.17226/10689.
×
Page 68
Suggested Citation:"1. Ambient Carbon Monoxide Pollution in the United States." Transportation Research Board and National Research Council. 2003. Managing Carbon Monoxide Pollution in Meteorological and Topographical Problem Areas. Washington, DC: The National Academies Press. doi: 10.17226/10689.
×
Page 69
Suggested Citation:"1. Ambient Carbon Monoxide Pollution in the United States." Transportation Research Board and National Research Council. 2003. Managing Carbon Monoxide Pollution in Meteorological and Topographical Problem Areas. Washington, DC: The National Academies Press. doi: 10.17226/10689.
×
Page 70
Suggested Citation:"1. Ambient Carbon Monoxide Pollution in the United States." Transportation Research Board and National Research Council. 2003. Managing Carbon Monoxide Pollution in Meteorological and Topographical Problem Areas. Washington, DC: The National Academies Press. doi: 10.17226/10689.
×
Page 71

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1 Ambient Carbon Monoxide Pollution in the United States INTRODUCTION Carbon monoxide (CO) has been central to the evolution of air quality management in the United States. CO is produced primarily by the incom- plete combustion of carbon-containing fuels, such as gasoline, natural gas, oil, coal, and wood. In a 1977 National Research Council (NRC) report, CO was declared "probably the most publicized and best known criteria pollutant" (NBC 1977~. The NBC attributed this recognition to the severe adverse health effects (including death) that result from acute exposure, which have been observed for centuries. Reducing human exposure to the products of incomplete combustion was an early objective of air quality management in the United States. CO was and still is the most recogniz- able indicator of incomplete combustion and has Tong been viewed as one of the most fundamental indicators of ambient air quality. When continu- ous monitors were first installed in some cities in the early 1960s, maxi- mum 8-hour average concentrations in excess of 30 parts per million (ppm) were not unusual (DHEW 1970, EPA 19791. National Ambient Air Quality Standards (N~AQS) for ambient con- centrations of CO (9 ppm for an 8-hour average and 35 ppm for a 1 -hour average) were instituted in ~ 97 ~ on the basis of studies linking ambient CO concentrations with neurobehavioral effects. Although neurobehavioral 16

Ambient CO Pollution in the United States 17 effects no longer serve as the basis for the standards, subsequent studies linking CO to increased risk of chest pain and hospitalization for persons with coronary artery disease have supported retention of the regulation. In the early 1970s, monitoring of CO indicated that exceedances ofthe 8-hour standard were common. The first comprehensive national report on emissions and air quality trends found that over 90% of monitors operating in 1971 recorded exceedances of the 8-hour NAAQS (EPA 1973~. How- ever, the situation improved fairly rapidly, primarily due to vehicle pollu- tion controls. By the year 2000, only four locations (Birmingham, Ala- bama; CaTexico, California; Lynwood, California; and Fairbanks, Alaska) reported exceedances ofthe 8-hour standard (Table 1-~. As ofthe end of 2002, both Lynwood and Fairbanks reported 2 years with no violation of the CO standard. The locations that continue to have high concentrations of CO also tend to have topographical and meteorological characteristics that exacerbate pollution (e.g., nearby hills that inhibit wind flow and temperature inver- sions that inhibit vertical pollutant dispersion). Attainment of the health- based NAAQS for CO has proved somewhat difficult under those condi- tions. The question arises whether unique approaches are necessary to manage CO in such problem areas or the current policies will ultimately achieve good air quality. An issue for areas that now meet the NAAQS is their vulnerability to future exceedances as a result of increases in vehicle- miles traveled (VMT) or unusual meteorological conditions favoring CO accumulation. STUDY BACKGROUND AND CHARGE in response to the challenges posed for some locations by the NAAQS for CO, a committee was established by the NRC to investigate the problem of CO in areas with meteorological and topographical problems. The com- mittee's statement of task was as follows: An NRC committee will assess various potential approaches to predict- ing, assessing, and managing episodes of high concentrations of CO in meteorological or topographical problem areas. The committee will con- sider interrelationships among emission sources, patterns of peak ambient CO concentrations, and various CO emissions control measures in such areas. In addition, the committee will consider ways to better understand

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20 Managing CO in Meteorological and Topographical Problem Areas relationships between episodes of high ambient CO, personal exposure, and the public-health impact of such episodes and alternative ways to measure progress in controlling ambient CO. An interim report dealing with Fair- banks, Alaska, as a case study was completed in May of 2002. A final report, including other CO problem areas, will be completed by the end of the study. The committee will address the following specific issues: · Types of emission sources and operating conditions that contribute most to episodes of high ambient CO. · Scientific bases of current and potential additional approaches for developing and implementing plans to manage CO air quality, including the possibility of new catalyst technology, alternative fuels, cold-start technol- ogy, as well as traffic and other management programs for motor vehicle sources. Control of stationary source contributions to CO air quality also will be considered. . Assessing the effectiveness of CO emissions control programs, including comparisons among areas with and without unusual toco~rachi- -r -en- ~r- cal or meteorological conditions. · Relationships between monitored episodes of high ambient CO concentrations and personal human exposure. The public-health impact of such episodes. Statistically robust alternative methods to assist in tracking prog- ress in reducing CO that bear a relation to the CO concentrations consid- ered harmful to human health. This study provides scientific and technical information potentially helpful in the development of state implementation plans (SIPs); however, the committee does not provide prescriptive advice on the development of specific SIPs for achieving CO attainment. In addition, the committee does not suggest changes in regulatory compliance requirements for areas in nonattainment of the NAAQS, and it does not recommend changes in the NAAQS for CO. Fairbanks, Alaska, was chosen as a case study for the interim report because its meteorological and topographical characteristics make it sus- ceptible to severe winter inversions that trap CO and other pollutants at ground level (NRC 2002~.

Ambient CO Pollution in the United States 21 SUMMARY OF INTERIM REPORT Fairbanks, Alaska, was chosen as a case study for the interim report because its meteorological and topographical characteristics make it sus- ceptible to severe winter inversions that trap CO and other pollutants at ground level. The committee's interim report, entitled The Ongoing Chal- lenge of Managing Carbon Monoxide Pollution in Fairbanks, Alaska, was completed in 2002 (NRC 2002~. The interim report allowed the committee to assess the characteristics ofthe CO problem in detail in one meteorologi- cal and topographical problem area. It provided the committee with general lessons applicable to other locations as well as characteristics of the prob- lem that were unique to Fairbanks. Because the committee devoted signifi- cant effort to assessing CO episodes in Fairbanks, the final report also draws on the Fairbanks case study. In the interim report, the committee found that Fairbanks has made great progress in reducing its violations of the 8-hour CO health standard, and has reduced the number of days annually with violations from over 130 during 1973 and 1974 to zero over the last 2 years (2001 and 2002~. De- spite this progress, the committee also concluded that Fairbanks will con- tinue to be susceptible to violating the 8-hour CO health standard on some occasions for many years to come because of its unfavorable meteorologi- cal and topographical conditions. Those adverse natural conditions might be compounded by future increases in the population of Fairbanks brought about by large pipeline or military construction projects. The findings associated with the progress on reducing CO violations and vulnerability to future violations are relevant to CO episodes in other meteorological and topographical problem areas. The committee recommended that there be improvements to local emis- signs controls, including vehicle emissions inspection and maintenance (I/M) programs, low-sulfur gasoline, and traffic control strategies. In par- ticular, the committee found that the Fairbanks North Star Borough is mak- ing substantial efforts to characterize and control cold-start emissions, despite the difficulty in quantifying emissions-reduction credits in its CO- attainment plan. The main method for controlling these emissions is through electrical heating devices known as plug-ins that preheat the engine coolant or lubricant of parked motor vehicles. Plug-ins substantially reduce CO emissions during the cold-start phase of engine operations by reducing the length of time needed for the catalyst to become fully operational. The committee recommended that the borough continue to expand the plug-in program by requiring or encouraging the equipping of more parking spaces

22 Managing CO in Meteorological and Topographical Problem Areas with electric outlets for plug-ins. The committee noted that the alert-day program, where the public is alerted on days when CO is forecasted to exceed the standard, was important in Fairbanks because it was part of a larger public information campaign to encourage motorists to use plug-ins. The emphasis on controlling cold-start emissions through plug-ins is gener- ally limited to Fairbanks and likely does not translate well to the lower 48 states. ~ its interim report, the committee noted that officials with the bor- ough argued that Fairbanks should be granted an exemption from the Clean Air Act with regard to the ambient CO health standards because of its ex- treme meteorological and topographical conditions. However, the commit- tee concluded that a similar argument could be made for other regions with regard to a variety of air pollutants. Furthermore, the ambient concentra- tions of CO observed in Fairbanks have exceeded the level that EPA identi- fied in the health-based standard for the protection of the general popula- tion and susceptible individuals. Thus, the committee concluded that ef- forts to control CO be continued and improved. TH:E COMMITTEE'S APPROACH TO ITS CHARGE In this final report, the committee addressed meteorological and topo- graphical conditions that foster pollution episodes, CO and related emis- sions from mobile and other sources, and air quality management options. The committee also examined monitoring data for ambient CO, including episodes when 8-hour average concentrations exceeded the NAAQS. In general, although monitoring of CO at locations with problems has oc- curred for an extended period of time, data and modeling to assess the spatial end temporal extent of high-CO events are limited. The limited data reduced the committee' s ability to assess human exposures during high-CO episodes. In addition, little exposure or epidemiological data specific to locations with high-CO episodes resulting from meteorological and topo- graphical conditions were available to assess the public health effects of those episodes. In the absence of such data, the committee reviewed rele- vant clinical and epidemiological studies presented in the scientific litera- ture and considered by EPA in assessing the health effects of exposure to ambient CO at concentrations and durations exceeding the NAAQS. Thus the committee did not conduct a comprehensive examination of the health effects of CO and did not examine the likelihood or actual risk of adverse health effects from exposure to ambient CO concentrations in these meteo-

Ambient CO Pollution in the United States 23 rological and topographical problem areas. In addition, the committee did not consider other CO sources such as cigarettes and therefore did not attempt to put the risks of ambient CO exposure into the context of other CO sources. The technical feasibility and potential for emissions reductions of a number of air quality management options are also discussed in the body of this report. Promising options available for controls at the federal, state, and local levels are presented in the summary. The committee found that light-duty vehicles are the primary source of emissions and potential emis- sions reductions, so most options focused on control of emissions from those vehicles. The committee's recommendations follow the relevant supporting evidence. REPORT CONTENTS This final report documents the committee's response to the charge described above. The report consists of four chapters and a summary. Chapter 1 provides background information on the regulation and sources of ambient CO pollution in the United States necessary to characterize the key issues stated in the committee's charge. The main topics include the NAAQS and areas exceeding CO standards, sources of CO emissions, health effects of CO, relationship of CO to other air pollutants, and issues relating to the spatial distribution of CO. Chapter 2 describes the meteoro- logical and topographical conditions that foster pollution episodes in CO problem areas. Temperature inversions, Tong-term meteorological trends, temporalpatterns of CO concentrations, end vulnerability of areas to future exceedances are discussed in detail. Chapter 3 discusses CO management and the tools needed to implement CO standards, such as emissions control strategies and monitoring and modeling tools. Some control strategies de- scribed include emissions standards, I/M programs, fuels, and transpor- tation-conkol measures. Finally, Chapter 4 focuses on long-term issues related to exposures, controls, and management of CO. NATIONAL REGULATORY SETTING FOR AMBIENT CO National Ambient Air Quality Standards To control adverse health effects from CO exposure, the U.S. Environ-

24 Managing CO in Meteorological and Topographical Problem Areas mental Protection Agency (EPA), acting per Sections 108 and 109 of the Clean Air Act (CAA), established the NAAQS for CO in 197 ~ . Recogniz- ing that exposure can have both acute and longer-term effects, the NAAQS for CO have two criteria with different averaging periods: 35 ppm averaged over ~ hour, and 9 ppm averaged over ~ hours.) Each criterion is not to be exceeded more than once per year; the second and subsequent exceedances within a year are considered violations of the standard. The 8-hour stan- dard has proven to be more difficult to meet than the 1-hour, especially for a handful of cities. The standard has been periodically reviewed on the basis of new scientific findings, as mandated by the CAA. The most recent review was published in 2000 (EPA 2000a). EPA originally designated an area as being in "nonattainment" of the S-hour standard if the second-highest 8-hour average CO concentration measured during a calendar year (known as the "design value") was greater than 9 ppm. After the Clean Air Act Amendments of 1990 (CAAA90), EPA designated areas that had previously been in nonattainment as "seri- ous" if the design value was ~ 6.5 ppm or greater, "moderate" if the design value was 9.1-16.4 ppm, and "not classified" if recent data were insuffi- cient to determine whether the standard was met. Moderate areas that did not reach attainment by July 1996 could be reclassified by EPA as serious. Nonattainment areas are required to submit a state implementation plan (SIP) to EPA that includes a characterization of pollutant concentrations and emissions, a description of the emissions reductions the area plans to make, and an "attainment demonstration" showing how the emissions re- ductions will enable the area to attain and maintain compliance with the NAAQS. To be eligible for reclassification from nonattainment to attain- ment status for CO, an area must have air quality monitoring data indicating that it did not violate the NAAQS during the previous 2 years. Though areas typically apply forreciassification immediately, Smith and Woodruff (200~ discussed how Fort Collins, Colorado, has delayed their application for reclassification since ~ 994 to pursue wider air quality goals. To meet the city's goal of continually improving air quality as described in the City of Fort Collins Air Quality Action Plan (2001), the city used their non- attainment status to pursue emissions control strategies that are not tradi- tionally implemented in attainment areas. One example is the vehicle emis- sions FM program, which currently is not required when a city is in attain- ~Thoughthe standard is 9.0ppm, in practice anexceedance does not occur until the 8-hour average is greater than 9.4 ppm. Values between 9.1 ppm and 9.4 ppm are rounded down to 9.0 ppm.

Ambient CO Pollution in the United States 25 meet. However, Fort Collins has recently applied to be reclassified and may discontinue some emissions control programs. Officials in Fort Col- lins are undertaking a feasibility study to explore both the voluntary and/or mandatory control programs after the state mandatory I/M program is elimi- nated. Throughout the report, the terms exceedance, exceedance days, and violation are used. They are defined as follows: an exceedance of the CO standard is any CO concentration measurement of 9.5 ppm or above for an 8-hour average;2 exceedance days are days on which one or more nonover- lapping 8-hour average CO concentration was 9.5 ppm or greater; and a violation is two or more exceedances within a calendar year. Note that more than one exceedance can occur in a day if there is more than one nonoverIapping S-hour period with an average CO concentration of 9.5 ppm or greater. Vulnerable Areas and the Form of the CO Standard The form ofthe CO standard, where a violation occurs upon the second and all subsequent exceedances in a calendar year, contributes to the diff~- culties that meteorological and topographical problem areas have in attain- ing the standard. A significant probability of an exceedance exists with the current attainment test because of the stochastic nature of ambient air pol- lutant concentrations (Gibbons 2002~. Areas must control CO under very infrequent, though not uncommon, meteorological conditions. The for ~~ of the new 8-hour ozone standard (the 3-year average of the fourth highest annual value) was changed from the form ofthe 1 -hour standard (the fourth highest value over 3 years) in part because the latter form is more suscepti- ble to extreme meteorological conditions. Conformity Requirements Transportation conformity requirements were originally developed to ensure that federal funding and approval was given to those transportation Although the 8-hour NAAQS for CO is 9 ppm, because early monitoring instruments had limited precision of about 1 ppm it has been the practice to consider an 8-hour average an exceedance only if it is 9.5 ppm or greater.

26 Managing CO in Meteorological and Topographical Problem Areas activities consistent with air quality goals. Transportation conformity was first introduced in the CAA Amendments of ~ 977, which included a provi- sion linking air quality to transportation planning by ensuring that transpor- tation investments conform with STPs (DOT 2000~. Conformity require- ments were made more rigorous in the CAAA90 and in the regulations EPA issuedin 1993 to implement the requirements (40 CFR § 51 and 93 tI9933~. The CAAA90's conformity mandate requires that transportation plans, programs, and projects in nonattainment or maintenance areas funded or approved by the Federal Highway Administration (FHWA) or the Federal Transit Agency (FTA) do not: (1) create new violations of the federal air quality standards; (2) increase the frequency or severity of existing viola- tions; or (3) delay timely attainment of CAA standards. Metropolitan planning organizations (MPOs) are responsible for per- forming air quality conformity analyses. MPOs must have transportation plans in place that present a 20-year perspective on transportation invest- ments for their region as well as a short-term transportation improvement program (TIP). The TIP is a multi-year prioritized list of projects (3 years at a minimum) proposed to be funded or approved by FHWA or ETA. The conformity analysis is done for the system of projects contained in a re- gion's TIP and transportation plan, and must show emissions consistent with those allowed in the SIP. Conformity determinations must be made at least every 3 years, or as changes are made to plans, TIPs, or projects. A formal interagency process is required to establish procedures for consultation between MPOs, EPA, FHWA, ETA, and state and local transportation and air quality agencies. These procedures apply to the development of the SIP, the transportation plan, the TIP, and conformity determinations. The sIP must establish interagency consultation procedures for the coordinating agencies and include schedules for implementation of all strategies. Once EPA approves the part ofthe SIP that describes the interagency consultation process (the conformity SIP), it is then enforceable by EPA as a federal regulation. One of the key components of the conformity determination for CO is the application of project-level emissions analysis and, on occasion, hot- spot analysis. During SIP preparation, emissions budgets are created for nonattainment areas. These budgets set limits on the mass of the criteria pollutant that can be emitted in the area and are usually broken into general and transportation budgets. For an area to be in conformity with the SIP, the sum of the emissions from all transportation projects may not exceed the transportation budget., unless reductions in the general budget are made to compensate In cases where a project could create a local violation or

Ambient CO Pollution in the United States 27 exacerbate pollution in an existing problem area, hot-spot analysis also might be needed. In hot-spot analysis, microscale CO concentrations resulting from an individual roadway project are modeled to investigate whether the project will cause a localized CO problem. Analysis of CO hot spots can be done quantitatively, typically through the use of Gaussian dispersion models, which are described in Chapter 3 of this report. Dispersion modeling may be needed to identify possible violations during SIP preparation. Also, traffic-simulation models can be combined with instantaneous emissions at the microscale level to predict emissions inventories and to assess queu- ing and traffic flow along specific roadway segments or at specific intersec- tions. Proposed projects to change traffic patterns are often analyzed by starting with the three intersections with the highest traffic volumes and poorest level-of-serv~ce to determine if CO problems exist, and then model- ing other intersections where capacity is equaled or exceeded. With EPA approval, areas also can establish their own thresholds for quantitative analysis (Guensler et al. 1998~. Alternative screening methods can be used for CO project-level hot- spot analysis (40 CFR § 51 and 93 t19931~. Screening tools are simple estimation techniques that determine whether transportation projects are in need of more rigorous testing and additional analysis. They are used to provide conservative estimates of the air quality impacts of a specific source, with the assumption that if a project passes the conformity criteria using the screening tools then it would also pass more rigorous analysis. The benefit of screening tools is that they reduce the number of transporta- tion projects requiring more detailed quantitative CO modeling and elimi- nate the need for more detailed modeling for those sources that clearly will not cause or contribute to an exceedance of the CO NAAQS. AREAS WITH RECENT EXCEEDANCES OF THE CO STANDARD Nationally, CO concentrations have declined significantly over the past 30 years. In the early 1970s, when CO monitoring in the United States became widespread, many cities reported numerous exceedances of both the 1-hour (35 ppm) and 8-hour (9 ppm) NAAQS for CO. In 1971, 53 of 58 monitoring stations (91%) recorded exceedances ofthe 8-hour standard, and 7 of 58 stations (12%) recorded exceedances of the 1-hour standard (EPA 1973~. Improvements occurred rapidly, primarily resulting from

28 Managing CO in Meteorological and Topographical Problem Areas advances in motor-vehicle emissions control technology. EPA (1976) noted that, although ambient concentrations of many other pollutants showed few signs of improvement, "there was an evident decline in the proportion of stations at which the 8-hour CO standard was exceeded." In 1974, the number of stations reporting exceedances of the 8-hour standard fell to 56% (211 of 377 stations). Since then, the number of areas showing exceedances ofthe 8-hour standard has continued to decrease, and no moni- tor has shown an excedance of the 1-hour standard since 1995. EPA (2002a) reports that the national average ambient CO concentration in 2001 was 62% lower than it was in 1982 and 38°/O Tower than it was in 1992. Table 1-1 shows the ~ ~ cities that have had the most difficulty meeting the 8-hour NAAQS since ~ 995, ranked by the total number of exceedance days at the monitor recording the highest number of exceedances during the 7-year period from ~ 995 to 2001. The table shows the aerometric informa- tion retrieval system identification number (AIRS ID), latitude, longitude, and elevation above mean sea level of each monitor. Second monitors are listed for Spokane, Washington, and Las Vegas, Nevada, because the first monitor ceased operating before the end of the 7-year period. A second monitor is listed for Anchorage, Alaska, because the second monitor is located in a residential neighborhood, rather than in a downtown area, providing an interesting diurnal comparison, and because it recorded the most recent exceedance in Anchorage. Birmingham, Alabama, stands out in Table 1-1 because of the large total number of exceedance days during the ~ 995-200 ~ period and because the annual number appears to be increasing with time. This is a special case involving CO emissions from an industrial source a mineral-wool facility. This industry is not regulated for CO emissions; however, a moni- tor placed close to the facility by the Jefferson County Public Health De- partment detected frequent exceedances of the CO standard. To rectify the problem, the facility has agreed to changes in their stack height and operat- ing procedures. The committee did not become aware of the Birmingham exceedances until after its last meeting. The committee relied on EPA's listing of locations that have recently violated the CO standard in their annually updated air quality update (EPA 2002b). Birmingham, Alabama, was not listed as violating the CO standard in the air quality update until 2002, despite the fact that violations ofthe CO standard dated back at least 4 years. Because this issue came up so late in the study and because the exceedances are due to problems in the facility's operations, not meteoro- Togical and topographical features, Birmingham will not be discussed in detail in this report. However, this case indicates that other areas may be

Ambient CO Pollution in the United States 29 experiencing exceedances of the CO standard that are not being detected by the fixed monitoring site network. Trends in national average ambient CO concentrations do not always mirror trends in nationwide emissions. One reason might be that most monitors are located in urban areas, so changes in air quality are most likely to track changes in urban air emissions rather than in total emissions. Be- cause light-duty vehicles dominate urban emissions and air quality monitor sites are located near roadways, the improvements in ambient CO concen- trations disproportionately reflect reductions in emissions from these vehi- cles, while emissions from most other sources remain basically unchanged. Characteristics of Exceedances Thirty years ago exceedances of the 8-hour NAAQS for CO occurred in all months of the year, but now they are a winter phenomenon in most areas.3 Figures 1-l and 1-2 show the number of days with exceedances by month and year (the year is defined as July through June centering on the winter season) for Lynwood, California, and Fairbanks, Alaska. There are two reasons for the pattern: reduced solar heating during winter, which favors a more stable atmosphere with less vertical mixing and Tower wind- speeds; and increased emissions in winter. CO Trends in Problem Areas Table ~-~ shows a generally downward trend in the number of exceed- ance days recorded in the 10 cities that are the focus of this report for the years since 1995. A downward trend in concentrations also can be seen in Figure 1-3, which shows the decline in the nationwide composite average of annual second-highest 8-hour CO concentrations from 1978 to 1997. The smoother decline of the composite average is a result of the large num- ber of sites included. The annual second-highest nonoveriapping 8-hour average CO concen- tration is a statistic that shows a great deal of variability at any one site. In its interim report ARC 2002), the committee looked at trends in other 3Exceedance days occulted in Birmingham in June and July of 2001.

30 Managing CO in Meteorological and Topographical Problem Areas 30 26 : c, in 16 10 5- .T - tar ~ I' ill ~~IIINIIINIIINIII~IIII ~ , . rid _.. l ~81 ~~° ~~ ~ ~ ~ ~ ·-,~J~>~ I ISIS ~1~[ -~..~¢ ~ ' W:~.~t' - ~C. · M0~H -30 25~ rid to o FIGURE 1-1 Number of days with exceedances of He 8-hour NAAQS for CO per month and per year in Lynwood, California. statistics (including the 75th, 90th, 95th, and 99th percentiles) for the three monitoring sites in Fairbanks, Alaska, for the six winter seasons from 1995- ~ 996 to 2000-2001. Although these other statistics showed less variability, they could all be fit over the 6-year period with straight lines. The slopes ofthe lines showed that the statistics declined at about 7% per year, consis- tent with steadily declining CO concentrations. A recent study (Eisinger et al. 2002) looked at whether the downward trend in CO concentrations was also occurring at microscaTe monitoring sites—sites located in close proximity to high traffic density. The study concluded that, although CO concentrations at the microscale sites are often higher than concentrations found at larger-scare monitoring sites (sites located in extended urban areas and more rural areas), CO concentrations at microscaTe sites are declining at the same rate as concentrations recorded at monitors representing larger regions.

Ambient CO Pollution in the United States 31 CZE~ 15 10- 5- 11~1 ~1~1 If; :~ to '15 ~ rn -10 ;~ Ad _5 ~ rll CD O FIGURE 1-2 Number of days win exceedances of the 8-hour NAAQS for CO per monk and per year in Fairbanks, Alaska. The numbers include days on which exceedances occurred at any of the three monitoring sites. SOURCES OF CO EMISSIONS National Inventory The major categories of CO emissions sources include transportation (mobile sources), industrial processes, nontransportation fuel combustion (which includes stationary and area sources), and miscellaneous sources. Figure 14 presents an estimate of national CO emissions over the 1982- 2001 time period. The figure gives a general indication of the dominant fraction of mobile-source emissions compared with other major source categories. The mobile-source emissions referred to in Figure 1 - can be separated into on-road, off-road, and nonroad emissions. On-road emissions come from both light-duty vehicles (LDVs) and heavy-duty vehicles (HDVs).

32 Managing CO in Meteorological and Topographical Problem Areas ~2 i 10 to. 8- - o ._ lo 2 o 6- - 4- 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 Year FIGURE 1-3 Nationwide composite average of the annual second-highest 8- hour CO concentrations, 1978-1997. Note that there were 184 mon~tonng sites from 1978 to 1987 and 368 sites from 1988 to 1997. The boundary separating LDVs from HDVs historically has been 8,500 pounds gross vehicle weight (GVW) (the weight of the vehicle plus the weight ofthe rated Toad-hauling capacity). LDVs, which include passenger cars and light trucks, are fueled primarily by gasoline; HDVs use both diesel fuel and gasoline. The heavier HDVs (those with a GVW greater than 26,000 pounds) use diesel fuel almost exclusively. Diesel engines emit much less CO overall compared with LDVs; thus, on-road CO inven- tories are dominated by LDV emissions. Nonroad emissions come from nonroad engines including construction, Togging, mining, and farm equip- ment and lawn and garden equipment. Off-road vehicles include marine vessels, recreational vehicles, locomotives, and aircraft. Table 1-2 provides an estimated inventory of CO emissions in the United States in 1999 (EPA 2001a). An estimated 77% of the anthro- pogenic CO emissions come from mobile sources, including on-road vehi- cles (51%) and nonroad engines and vehicles (26%~. The remaining CO emissions are from area and point sources, including fuel combustion and industrial processes. It should be noted, however, that this inventory has significant uncertainties. For example, an N~C report (2000) reviewing EPA's Mobile Source Emissions Factor (MOBILE) mode! discusses the

Ambient CO Pollution in the United States 33 Co ~Em;~sion~, 19~0011- · ~ c~ ~ t~' In. O To ~ Bile ,~ ~1 .~ ..~,. ~ ~,0 ~ ~ :~, ~~ .~.= ~ ~~ O5~ - ~~= !i .~ StS, ~ ~ ge 977 ~ ~ ~ .~! 198~. 0% -A 1'se:~. -~% mc FIGURE 1-4 Nationwide CO emissions from 1982 to 2001. Note that from 1982 to 2001 there was 0% change, and from 1992 to 2001 there was a 6% increase in emissions. Emissions are shown in thousands of short tons. One short ton is equivalent to 2,000 pounds (lb) or 0.9072 metric tons. A long ton is a measurement weight equivalent to 2,240 lb or 1.0 metric tons. Source: EPA 2002a. substantial inaccuracies in estimates of fleet emissions and effectiveness of control strategies for on-road vehicles. Regional Inventories In urban areas, mobile sources tend to contribute more to the mix of emissions than indicated by the national average. On the basis of its MO- BILE model, EPA suggests that vehicles may contribute 95% or more of CO emissions in cities classified by EPA as having serious air pollution (EPA 2001a). Table 1-3 shows emissions inventories for five cities that have had CO exceedances in the past. Mobile sources contribute most of the CO emissions, ranging from 78°/O for Fairbanks to 96% for Phoenix. On-road vehicles contribute the majority of mobile-source emissions, rang- ing from 62% to 84%.

34 Managing CO in Meteorological and Topographical Problem Areas TABLE I-2 National CO Emissions Inventory Estimates for 1999 Source Category Thousands of Short Percent of Total Tons (DO) Point or area fuel combustion 5,322 5.46 Electric utilities 445 0.457 Industry 1,178 1.21 Residential wood burning 3,300 3.39 Other 399 0.409 Industrial processes 7,590 7.79 Chemical and allied product 1,081 1.11 manufacturing Metals processing 1,678 1.72 Petroleum and related indus- 366 0.376 tries Waste disposal and recycling 3,792 3.89 Other industrial processes 599 0.615 _ _ . On-road vehicles 49,989 51.3 Light-duty gas vehicles and 27,382 28.1 motorcycles Light-duty gas trucks 16,115 16.5 Heavy-duty gas vehicles 4,262 4.37 Diesels 2,230 2.29 Nonroad engines and vehicles 25,162 25.8 Recreational 3,616 3.71 Lawn and garden 11,116 11.4 Aircraft 1,002 1.03 Light commercial 4,259 4.37 Other 5,169 5.30 Miscellaneous 9,378 9.62 - Slash or prescribed burning 6,152 6.31 Forest wildfires 2,638 2.71 Other 588 0.615 . Total 97,441 Source: EPA 2001a.

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36 Managing CO in Meteorological and Topographical Problem Areas Regional inventories are important because they form the basis for SIPs and are used in assessing local projects. However, the use of regional emissions inventories for analysis of localized CO exceedances can be problematic because these inventories might include sources that do not contribute to exceedances at specific locations. Air intakes at CO monitor- ing sites are a few meters above street level, so sources at higher elevations (e.g., smokestacks) might emit CO at an elevation higher than the inversion level and contribute little to measured CO concentrations. For example, in its interim report (NRC 2002) the committee noted the presence of very strong ground-level inversions in Fairbanks, Alaska. Such conditions mean that local power-plant emissions released well above the inversion height likely do not mix with ambient air at the monitor height and, thus, do not contribute to the high CO concentrations recorded at monitoring sites. In addition, some CO sources included within these regional inventories might be located at great distances from monitors and might not contribute to local exceedances. These issues are discussed further in Chapter 4. Vehicle Sources of CO Emissions The primary cause of CO emissions from vehicles is the incomplete combustion of gasoline. The fuel-oxidation process (combustion) is the conversion of the fuel to lower-molecular-weight intermediate hydrocar- bons (including olefins and aromatics). These hydrocarbons are converted to aldehydes and ketones, then to CO, and finally to carbon dioxide (COO. The initial reactions are faster than the final conversion from CO to CO2. Incomplete conversion of fuel carbon to CO2 results in part from insuffi- cient oxygen in the combustion mixture known as fuel-rich4 condi- tions and insufficient time to oxidize fuel carbon to CO2. CO emissions from diesel-powered vehicles are minimal compared with emissions from gasoline-powered vehicles, primarily because excess air is used in the diesel combustion cycle. The excess air, combined with high temperatures and pressures involved in the diesel cycle ensures more complete combus- tion. Hence, the following discussion is limited to gasoline-powered vehi- cles. 4The ratio of air to fuel mass that provides just enough O2 to convert all the carbon and hydrogen in gasoline to CO2 and water (termed the stoichiometric ratio) is about 14.7 to 1. Ratios less than 14.7 to 1 have more fuel than is optimal and are called fuel-rich.

Ambient CO Pollution in the United States 37 Trends in Vehicle Emissions Vehicles produce excessive CO when cold starts, increased Toad (e.g., climbing a hill), rapid acceleration, or engine malfunctions induce fuel-rich conditions (NRC 2000~. CO and unburned fuel can be greatly reduced by a variety of techniques, including additional oxidation in a catalytic con- verter mounted between the engine and the muffler. Box 1-l lists some of the major milestones in the control of emissions from automobiles starting with the Clean Air Act (CAA) of 1970 (EPA 2001a). These milestones, which have led to large reductions in CO and other pollutant emissions, include national standards for tailpipe emissions, new vehicle technologies, and clean fuels programs as well as state and local vehicle emissions I/M programs and transportation management programs. According to EPA, overall CO emissions were reduced by 25% be- tween 1970 and ~ 998 (EPA 2000b). Light-duty gasoline-vehicle emissions have shown a 57% decrease. Per-vehicle emissions have been reduced even more. Substantial reductions in light-duty gasoline-vehicle emissions over the past 30 years have offset increases in CO emissions from other sources; however, light-duty gasoline-powered vehicles continue to domi- nate CO emissions inventories. In addition to per-vehicle emissions, other factors such as vehicle- miles traveled (VMT), population growth, trip making, and the rapid growth of sport utility vehicles (SWs) in the vehicle population impact total CO emissions. The improvement in air quality discussed earlier in this chapter occurred despite an approximate 35°/O increase in VMT in the United States during the 10-year period from 1992 to 2001 (EPA 200Ib). Figure 1-5 illustrates the increases in VMT and the number of licensed drivers in the United States over the past 20 years and the decline in CO emissions from LDVs over the same period. Eventually, continuing in- creases in VMT might result in an increase in net CO emissions unless new emissions control technologies continue to reduce CO emission rates from new or in-use vehicles. Future CO emissions also might be affected by changes in the vehicle subfleet population as higher-emitting Tier 1 vehi- cles, such as SUVs, begin to make up a greater fraction of the overall fleet. Other Sources of Ambient CO Emissions The remaining CO emissions come from point and area sources, in-

38 Managing CO in Meteorological and Topographical Problem Areas eluding wood burning, lawn and garden equipment, and natural sources, such as wildfires. It should be noted that CO is produced not only through incomplete combustion but also from the thermal decomposition of CO2,

Ambient CO Pollution in the United States 39 ~ Vehicle miles traveled Licensed Diners ~ CO light~uty Chicle emissions ~ 3'000'000 1 ~ ~ . 1 80,000 ° O ~ > ,,, 2 2,500,000 u, 3 s 2,000,000- E ~ `,, 1,500,000- t, _ ~ s ~ ~ 1,000,000- o E ' 500.000- ~ ._ o- _ I. r ' I ' ' I r I ' I I I I ' ' ' I 9~ BOW COW 9~ 9~ 9o<3 coo 9~ too 909 99° 99~ 99~ 99~ 99> 99~ 9900 9~ Woo Time (years) 70,000 60,000 , - 50 000 s ' in 40,000 ~ 30,000 be 0 - 20,000 =~ 10,000 8 - o FIGURE 1-5 Nationwide trends in total vehicle-miles traveled, number of li- censed drivers, and CO light-duty vehicle emissions Tom 1980 to 1998. a process that occurs mainly in power plants. The contribution of CO pro- duced through thermal decomposition to emissions inventories however is likely to be small. Table 1-2 shows nationwide CO emissions in ~ 999 from various sources. Area and point sources make up a smaller fraction of CO emissions inventories than mobile sources do, but are still important contri- butors to total CO emissions. For example, point and area sources can play a dominant role in localized exceedances, such as those that have occurred recently in Birmingham, Alabama. In addition, operators of lawn and garden equipment may experience high personal exposures to CO. HEALTH EFFECTS OF CO Clinical and Epidemiological Studies of CO Effects The health effects of CO have been assessed through controlled expo- sure of human volunteers and a growing body of community epidemiologi- cal studies (EPA 2000a). Exposure studies executed under various experi- mental protocols have produced substantial information on the toxicity of CO, its direct effects on blood and tissues, and how those effects are ex- pressed in terms of changes in organ functions. Community epidemiologi-

40 Managing CO in Meteorological and Topographical Problem Areas cat studies attempt to extend these results to understand the potential com- munity health effects of ambient CO exposures. CO affects human health by impairing the ability of the blood to carry oxygen (02) to body tissues. When CO is inhaled, it rapidly crosses from the lungs into the blood, where it binds to hemoglobin to form carboxy- hemoglobin (COHb), a useful marker for predicting CO health effects. Because CO has an affinity for hemoglobin more than 200 times that of 02, the presence of CO in the lungs will displace O2 from the hemoglobin. In other words, when CO is present in the lungs, hemoglobin is unable to reach complete O2 saturation. In addition, the binding of CO increases hemoglobin's binding of 02, thereby inhibiting the release of O2 from he- mogiobin to body tissues. The effect of COHb is illustrated by a leftward shift in the O2-hemogiobin dissociation curve (Figure I-6~. This effect continues until the COHb dissociates, typically several hours after CO exposure. CO also may affect O2 transport to muscle (EPA 2000a). CO has been shown to bind to myogiobin, which supplies oxygen to muscles during strenuous exercise, when muscle demand for oxygen is greater than the supply of oxygen available from the blood. COHb levels in healthy individuals not exposed to high concentrations of ambient CO are typically 0.3% to 0.7%. CO is formed endogenously through normal metabolism of heme leading to approximately these levels of COHb (i.e., this occurs irrespective of ambient exposure to CO). Expo- sure to high ambient CO concentrations can result in concentrations of COHb at 2% or higher if the exposure lasts long enough (hours). The exposure time is critical as there is an S-12 hour period necessary for equi- librium between ambient CO and blood COHb concentration. For people who smoke, cigarettes are usually the most significant source of personal CO exposure. COHb concentrations average about 5% in smokers and are up to 10% or even higher in some very heavy smokers (Beckett 1994~. The CO ambient-air health standards set by EPA are intended to keep COHb concentrations for nonsmokers below 2% and protect the most sus- ceptible members of the population. The goal of both the 1-hour and 8- hour standards is related to maintaining COHb concentrations below this level. Because of the time required for ambient CO to affect COHb levels, it requires exposure to 35 ppm of CO for 1 hour to achieve approximately the same level of COHb as exposure to 9 ppm of CO for 8 hours. EPA (2000a) provides a comprehensive review ofthe literature pertaining to the health effects of CO for typical environmental exposures that would be associated with COHb levels less than 10%. The major findings presented by EPA (2000a) are summarized below; the committee accepts these

Ambient CO Pollution in the United States 41 loo 80 .o <'s 60 oh Cat o ._ o 40- I 20 - O— 50% CON ~ -,, ~ /,.# ;< /CO: l 2 4 6 8 10 12 14 16 18 20 Blood O2 Concentration (%) FIGURE 1-6 Diagram of hemoglobin response to the presence of COHb. The concentration of O2 in the environment surrounding the hemoglobin is shown on the x-axis. The O2 saturation, or how much of the hemoglobin's capacity for storing O2 is used, is shown on the y-axis. At higher O2 concentrations, as are found in the lungs, the hemoglobin can be more O2 saturated. Likewise, at lower O2 concentrations, as are found in other parts of the body, O2 will dissociate from the hemoglobin to achieve O2 percent saturations as indicated by the curve. The presence of COHb shifts this curve to the left. For a given O2 concentration, the hemoglobin will require a higher O2 percent saturation and allow less O2 to be released to the body's tissues. Source: Adapted from Shephard 1983. findings as sufficient evidence of the health effects caused by exposure to CO at concentrations of 9 ppm and above for extended periods of time. However, the committee has noted that the public may not be sufficiently aware that CO poses a health threat. The acute affects of CO poisoning are well understood (Raub et al. 2000~. Generally, in otherwise healthy people, headache develops when COHb concentrations reach 10%; tinnitus (ringing in the ear) and light- headedness at 20%; nausea, vomiting, and weakness at 20-30%; clouding of consciousness and coma at around 35%; and death at around 50% (Coburn ~ 970~. However, the outcomes of Tong-term, low-concentration CO exposures are not as well understood. Because of the critical nature of blood flow and O2 delivery to the heart and brain, these organ systems, as well as the lungs (the first organ to come into contact with the pollutant), have received the most attention.

42 Managing CO in Meteorological and Topographical Problem Areas The most well-documented effect in controlled-exposure studies is that of CO on reproducible exercise-induced angina. Angina is a type of chest pain that occurs when there is not enough blood flow to the heart muscle, and it is a symptom of coronary artery disease. In patients with known coronary artery disease, COHb concentrations as Tow as 3% exacerbate the development of exercise-induced chest pain (AlIred et al. l 989a). Concen- trations as Tow as 6% are associated with an increase in the number and frequency of premature ventricular contractions ofthe heart during exercise inpatients with severe heart disease (Alfred et al.1989b; Sheps et al.1990~. These results have provided support for epidemiological studies associating ambient CO with heart-disease exacerbation. Large cohort studies of envi- ronmental exposures have confirmed that high daily ambient CO concentra- tions are associated with statistically significant increases in the numbers of hospital admissions for heart disease (Poloniecki et al. 1997; Schwartz 1999) and congestive heart failure (Morris et al. 1995) and with increases in deaths from cardiopulmonary illnesses (Prescott et al. 1998~. Neuropsychiatric (neurological end psychiatric) disorders and cognitive impairments due to long-term, Tow-concentration CO exposures have been hypothesized, in part on the basis of extrapolation from the known acute effects of high-dose CO poisoning. In clinical experiments on healthy volunteers, controlled CO exposure was associated with subtle alterations in visual perception when COHb concentrations were above 5% (McFarland 1970; Horvath et al.1971~. However, the significance ofthis finding remains unknown. Similar studies have shown measurable but small effects on auditory perception, driving performance, and vigilance (Beard and Wertheim 1967; McFarland 1973; Benignus et al. 1977~. The neurobehavioral effects described in Beard and Wertheim (1967) served as the scientific basis for the original CO standard promulgated in 1971. However, later studies questioned those results, so the current standard is based on the aggravation of angina pectoris and other cardiovascular dis- eases. The role of CO in pulmonary disease is unclear. In the Seattle area, a single-pollutant model showed a 6% increase in the rate of hospital admis- sions for asthma with each 0.9-ppm increase in ambient CO, but CO in- creases were concomitant with increases in other air pollutants (Sheppard et al. ~ 999~. In Minneapolis and Toronto, CO concentrations showed weak and inconsistent associations with total admissions for respiratory diseases (Burnett et al. 1997; Moolgavkar et al. 1997~. EPA (2000a) cautions that the biological plausibility of CO's association with respiratory illness is

Ambient CO Pollution in the United States 43 tenuous, because the mechanism by which ambient CO exposures could produce or promote harmful respiratory effects has not been demonstrated. As described in subsequent sections, CO is closely associated with copollutants, including hydrocarbons (Husk and fine particulate matter (PM2 s) from motor-vehicle emissions. The respiratory effect attributed to CO might be the result of exposure to HCs (Pappas et al.2000) or motor- vehicle-related PM2 s (Buckeridge et al. 2002~. A fetus is more susceptible to CO than an adult; the O2-hemogiobin dissociation curve is to the left of that in the adult and is shifted even fur- ther to the left by CO exposure. Also, because the half-life of fetal COHb is longer than that in adults, it may take up to five times longer for its con- centrations to return to normal. Studies have shown that exposure to high concentrations of CO during the last trimester of pregnancy may increase the risk of Tow birth weights and that exposures to CO and airborne particu- late matter (PM)6 during pregnancy may trigger preterm births (Ritz and Yu 1999; Ritz et al. 2000~. A recent study correlated CO and O3 exposure during pregnancy to birth defects such as cleft lip and defective heart valves (Ritz et al.2002~. The correlation of these effects with ambient CO occurred at concentrations below the NAAQS. The study was inconclusive regarding the effects of PM~o and nitrogen dioxide; however, the lead au- thor of the study cautioned that the real culprit might be other pollutants, such as PM and some air tonics, that are coemitted with CO in tailpipe emissions (Ritz 2002~. Public-health laws are designed to protect the most susceptible mem- bers ofthe population. People with coronary artery disease or other cardio- pulmonary diseases, fetuses, infants, and athletes who exercise heavily in high-CO atmospheres are particularly susceptible to adverse health effects sThe terms volatile organic compounds (VOCs) and HCs are used to denote organic compounds that are emitted as vapors under typical atmospheric conditions. Unless quoting a source or a regulation that uses another term, the report uses the term HC exclusively. Appendix B describes the differences among the terms used to refer to HCs. 6Airborne particulate matter (PM) refers to a broad class of discrete solid particles and liquid droplets of varied chemical composition and size. PM~Orefers to the subset of PM with an aerodynamic diameter less than or equal to a nominal 10 micrometers. PM2 Refers to the subset of PM with an aerodynamic diameter less than or equal to a nominal 2.5 micrometers.

44 Managing CO in Meteorological and Topographical Problem Areas from CO. The evidence summarized above, and described more fully in EPA (2000a), indicates that attainment of the ambient-CO standards can decrease morbidity and mortality from atherosclerotic heart disease. Although less conclusive, there is evidence that attainment of the CO stan- dards will also decrease fetal Toss and childhood developmental abnormali- ties. These health benefits translate into economic savings associated with avoided health care and avoided work-time Tosses as well as intangible savings in quality of life. Control of CO through new-vehicle emissions standards has also had a significant collateral public-safety benefit through the reduction of acci- dental CO poisoning (Cobb and Etzel 1991; Shelef ~ 994; Marr et al. 1998~. Mott et al. (2002) recently used computerized death-certificate data main- tained by the Centers for Disease Control and Prevention to evaluate the influence of national vehicle emissions controls on unintentional motor- vehicle-related CO deaths between 1968 and 1998. They estimated that over ~ i,000 deaths were avoided because ofthese standards, a reduction in unintentional motor-vehicle-related CO mortality from 4.0 to 0.9 deaths per . million person-years. Summary of CO Benefits and Costs from the Clean Air Act Although there have been no comprehensive assessments of health benefits from controlling CO at individual locations, including meteorolog- ical and topographical problem areas, EPA has estimated nationwide bene- fits attributable to the Clean Air Act in two reports: Final Report to Con- gress on the Benefits and Costs of the Clean Air Act, 1970 to 1990 (EPA

Ambient CO Pollution in the United States 45 1997a) and Final Report to Congress on the Benefits and Costs of the Clean AirAct, 1990 to 2010 (EPA 1999~. However, these documents do not separate the health benefits of CO control from other criteria pollutants. In the 1997 report, the control of PM and CO under the Clean Air Act is estimated to reduce the mean number of hospitalizations for congestive heart failure by 39,000 annually in 1990 compared with a no-control sce- nario (EPA 1997a). The no-control scenario assumes that no air pollution controls were established beyond those in place prior to the enactment of the 1970 amendments to the Clean Air Act. In the ~ 999 report, the control of PM, CO, NOX, sulfur dioxide, and ozone under the CAAA90 is estimated to reduce the mean number of hospitalizations for respiratory ailments and congestive heart failure by 64,000 annually in 2010 compared with a pre- CAAA90 scenario (EPA 1997a). The pre-CAAA90 scenario assumes that no air pollution controls were established beyond those in place prior to the enactment of the CAAA90. It is clear from these reports why the emphasis in air quality management in the United States is on PM and ozone. For example, the 1999 document estimates that controlling PM under the CAAA90 will reduce premature mortality in 2010 by a mean value of 23,000 annually. No reduction in premature mortality is attributed to CO control. CO Exposure Exposures in Vehicles An issue unique to motor vehicles is the proximity of emissions sources to receptors. Automobile air vents can take in exhaust emissions from other vehicles, thereby accumulating CO in the interior compa~l~ent. Studies have shown that when CO concentrations near roadways average 3~ ppm, the average concentration in rider compartments is typically 5 ppm (Akland et al. 1985; Flachsbart et al. 1987~. A study released by the California Air Resources Board (Rodes et al. ~ 998) reported that CO levels were between 2 and 10 times higher inside vehicles than at roadside or fixed monitoring stations due to simple dispersion of the pollutant. Re- searchers found similarly high concentrations of HCs and toxic compounds such as benzene and 1,3-butadiene. The relationship ofthese pollutants to CO is discussed further in a later section of this report. Flachsbart (1999) summarizes exposures to mobile-source CO emissions in various micro-

46 Managing CO in Meteorological and Topographical Problem Al reas environments and shows how congested roadways, street canyons, tunnels, underpasses, drive-up facilities, end perking garages canproduce exposures well above ambient conditions. Relationship of Indoor to Ambient Concentrations Most people spend a majority oftheir time indoors; this is particularly true in cold climate areas during winter, when ambient CO concentrations tend to be highest. That leads to the question of the relationship between indoor and outdoor concentrations. Air pollution in buildings can come from indoor sources and from air exchange with outdoor ambient pollution. Air exchange may be active, as in the case of a mechanical ventilation system, or passive, as in the case of infiltration associated with temperature or pressure differences between the outside and the inside of a building. Though homes in northern climates may be tighter, air exchange through leaks is controlled by the temperature difference, with a large temperature gradient producing a greater infiltration rate. Thus, CO penetrates freely with infiltration air from the outside, even in winter in Fairbanks, and is not removed by building materials or ventilation systems. Furthermore, there are no effective indoor chemical or physical processes for lowering CO on the time scales of interest for exposure and toxic effects. Hence, being indoors offers little protection from outside CO levels. The relationship between indoor and outdoor CO concentrations can be evaluated with a simple differential mass-balance mode] (Shair and Heitner 1974) that has the following steady-state solution when we combine active ventilation and passive infiltration into a single air-exchange term: pa CO 5 a + k (<a + k)V C - ~ + where Ci= indoor concentration, mg/m3; CO = outdoor concentration, mg/m3; p = penetration coefficient, 0-1; a = air exchange rate, hours; k= decay rate, hours; S = mass flux of the indoor source, mg/h; and V= building volume, m3 .

Ambient CO Pollution in the United States 47 For CO, the relationship is simpler because the penetration coefficient (p) is unity and the decay rate (k) is effectively zero. Therefore, the solution iS Ci = Co+ S ad in the absence of indoor sources (5), the steady-state indoor concentra- tion of CO will equal the average outdoor concentration. When a source of CO is present indoors (e.g., from a faulty furnace, an underground park- ing garage, a kerosene heater, or a tobacco smoker), the indoor source adds to the background concentration from the outdoor air (EPA 2000a). There- fore, buildings do not provide protection from high outdoor concentrations of CO. The idea that buildings provide protection from high outdoor CO concentrations is a common misconception.7 Other Exposures The contribution to personal exposures from certain sources, such as gasoline-powered lawnmowers, snowmobiles, recreation boats, generators, and garden equipment, can be substantially greater than fixed site monitor- ing data suggest. These sources contribute considerably less to the regional inventory than do mobile sources, and the exposed operators of lawn and garden equipment are working in close proximity to the CO emissions source. in combination with urban background concentrations, localized sources may subject some individuals to very high CO concentrations. Spatial Distribution of CO Studies that demonstrate the spatial end temporal distribution of CO are beneficial in assessing the potential human exposure to CO and other pol- Jutants from vehicle emissions. Saturation studies are one method. They typically rely on portable monitors that "saturate" a geographical area with samplers to assess the air quality in places where high concentrations of fin this regard, CO is different from 03, which is highly reactive and is de- stroyed when infiltrating inside from outdoors.

48 Managing CO in Meteorological and Topographical Problem Areas pollutants are possible. Monitors can be deployed at temporary fixed-site locations or in mobile sampling vehicles. These studies are helpful to air pollution control agencies for evaluating their ambient air monitoring net- works, characterizing pollutant concentrations over the entire saturation study area, and locating hot spots or high pollutant impact points. Personal and indoor monitoring could be incorporated into such studies to relate ambient concentrations to personal exposure. Saturation studies typically involve deploying temporary mobile or stationary monitors throughout a wide study area to characterize the spatial extent of CO. The committee considered several of these studies (Morris 2001; Guay 2001; Lawson 2002; Ranse] 2002) during its deliberations. On particular example is the study that was carried out in the Las Vegas Valley during the winter of 2001-2002 (Ranse! 2002~. The objectives ofthe study were: (1) to evaluate the adequacy of the monitoring network to measure the spatial distribution of CO and (2) to ensure that no areas of higher concentrations were missed with the existing network. The study collected data from 64 temporary fixed monitoring sites operating continuously for six weeks and from ~ ~ episodes using a mobile sampling van that collected 1-minute average CO concentrations. These measurements were in addi- tion to those made at 14 already existing permanent monitoring stations. The mobile sampling van was equipped with two samplers capable of mea- suring average CO concentrations and a global positioning system (GPS) to determine location and provide real-time mapping and display. Figure 1-7 shows the locations of CO measurements made with the van. The study concluded that current permanent monitoring sites are suitably located to identify peak CO concentrations and to map areas where relativeiv higher CO levels may occur. Another saturation study reviewed by the committee was performed during the winter of 1997-1998 in Anchorage, Alaska (Morris and Taylor 1998; Morris 20011. The objective ofthe study was to determine whether the permanent monitoring network adequately characterized CO exposures in neighborhoods, near major roadways, and in parking lots. The study used 16 temporary fixed monitors to supplement the 4 permanent monitors. The study concluded that the current permanent network adequately charac- terizes CO concentrations at roadway sites but might not characterize the upper range of CO concentrations in neighborhoods. The high CO concen- trations observed at most residential monitoring sites, often in the morning, indicated that cold-start and/or warm-up idling of vehicles by commuters is a significant source of CO in those locations.

49 ! ., i .. If Ail, ~1 ADS it.< ,,;~61~ o, o O N h ., ° ., N o ' ~ ,`~3,W, ,,' d 0 ~ ~ (Fen apache Ed ~ }. ii , '\ ~.S 9J~p~Lllb6 .- i .."''^"" ~ . . .. a. .~ ' ''''''''''''''''''pA[8 date S ';. ,, <>, a,.... Cal Ce · _ Ce - Ce Cal - ·_ ~0 Ce = O O ~ Can au — , - . . O V: U. Ce ~ g ·~ =^ ~ _ Cal O O ~ O Ce O at ~ ~ O^ Ce ·= ~ o o Cal It O Cd C) O 1 _4 _ ~ Ct

50 Managing CO in Meteorological and Topographical Problem Areas - ;~VENTURA ~£ °~1 Jo mice LOS ANQEL~S COUNTY .SAN FERNANDO \ \YA L LEY 35 30. ; 7 _ i 147\ l ar ·37 J I ~ / ~8 HOLL~WOO0~/ tfA-~=NA \~2 - 31,/~0WNTOWN I== ~L I- ~ _ _' END i. 2 or ~ A ~ R POR T l 1 ~ | / lS SAN a~P`NARDINO | 1 R EDL AN DS I .~4 1' ~ _ 16,' Rl VERSIDE FIGURE 1-8 Maximum 8-hour average CO concentrations in We South Coast Air Basin for 1956-1967. Source: DHEW 1970. Finally, a comparison of the change in the spatial distribution of the maximum 8-hour CO concentration over time in the Los Angeles area provides a qualitative description of the reduction in exposure to high CO. Figure 1-S shows that the 8-hour CO concentration was exceeded 0.~% of the time averaged over the period from 1956 through 1967. Since there are 1,095 discrete 8-hour time periods in a year, the 0.1 percentile value ap- proximates the 8-hour concentration likely to be exceeded an average of once per year (DHEW 1970~. Figure 1-9 shows the maximum 8-hour CO concentration for 2000 (SCAQMD 2000a). Although 2000 may have been a favorable year in terms of meteorology, it is clear that the 8-hour peak concentrations and the spatial extent of CO pollution have decreased greatly since the 1956-1967 time period. Roadway Health Effects The correlation between CO and other motor-vehicle-related emissions is important because of studies linking health impacts and proximity to

Ambient CO Pollution in the United States 51 .~ o . ~.~ :~. .,: ~ ./ .::~, . .; Hi,, :. me*_ ~ ~ ~ : San Bem:ard~no ~ - . ~~ Los i4ngete`. ~ = ~: ~ ~ ~0 \ ~ ,. i) Long Beach ~ 1 ~ ~ Scot AIR I~Dl~i.~i,,~= if.- ~~,r . 'ail ~ Aiii 3li5~' tSC=i —-— Cowl r me AiR Y~N'lTORt. S1A'~M ~ Los i4ngetec ~ ~ _:J 4>~ ~ Anaheim ~ : , . , , , ~ ., A ':~ ;.~. ' ~ Am .: . ~ A. ~ &, 'I I'd'' A .,'2;,:~. e~:~<#'sa;~- ~ ~ . at -~ ~ . . . A a_, . .. t .~ I' F~- '' ~', . . , . alF—~~~ .: ~ . ~ ~~~~ Itlll ~I1Il~~~t14~1~~~ Jo— - . i''. FIGURE 1-9 Maximum 8-hour average CO concentrations in the South Coast Air Basin in 2000. Source: SCAQMD 2000a. major roadways. CO is a relatively easy pollutant to measure and thus can be an indicator of roadway emissions. As shown in Figure 1-10' CO is highly correlated with black carbon and ultrafine particles in close proxim- ity to highways (Zhu et al. 2002~. Brunekreef (1997) found a reduction in lung function in children living near a major highway; Hoek et al. (2002) found an association between cardiopulmonary mortality and proximity to a major roadway; and Buckeridge et al. (2002) reported the effects of motor-vehicle emissions on respiratory health. These studies attribute most health impacts to PM and HCs. Ritz et al. (2002) showed a correlation between traffic-related CO emissions and birth defects in southern Califor- nia. Recently, Wilhelm and Ritz (2003) reported an association between residential proximity to traffic and adverse birth outcomes such as low birth weight and preterm birth. RELATIONSEOP OF CO TO OTHER AIR POLLUTANTS As discussed earlier in this chapter, mobile sources, both on-road and off-road contribute 75-95% of CO emissions in selected urban areas. There-

52 Managing CO in Meteorological and Topographical Problem Areas .o .e .0 ~ 1.6 .> 1.4 cry ED 0.0 Upwind Particle Number Be - —— — ~ CO , ~ ~ 1 ~ ~ I - 00 - 200 - 100 0 100 200 300 Distance to the 405 Freeway (m) Downwind FIGURE 1-10 Relative mass, total particle number, black carbon, and CO concentrations versus downwind distance from a freeway. Source: Zhu et al. 2002. Reprinted with permission; copyright 2002, Air & Waste Management Association. fore, CO may be an indicator of other, less well-characterized pollutants emitted from vehicles, such as fine particulate matter (PM2 s) and air toxics associated with HCs. However, CO has some substantial shortcomings as an indicator of othermobile-source emissions. The correlation between CO and other pollutants, and CO's role in tropospheric ozone, are discussed below. The correlation between CO and other motor-vehicle-related emissions is important because of the studies linking health impacts of air pollution to proximity to major roadways. Those studies are also discussed in this section. Association of CO Emissions to Other Emissions Automobile exhaust is a complex mixture of compounds, some of which are classified as criteria air pollutants and others as hazardous air pollutants (HAPs) or "air tonics." The correlation of CO with PM2 s and some air taxies is especially strong for gasoline-powered light-duty vehicles (LDVs) operating under fuel-rich conditions. As discussed more exten-

Ambient CO Pollution in the United States 53 -1.3% \NOx 8.3% \ WOW CO\: 0.2% 0.2% FIGURE 1-11 Degree of overlap among the highest 10% of emitters of CO, HC, and NOX in He light-duty vehicle fleet, based on the results of emissions tests on 12,977 vehicles administered during random roadside inspections in California, from June 9, 1998, until October 29, 1999. Note that the sizes of the overlapping areas are not drawn to scale. Of the vehicles tested, 78% did not fall in the top 10% for CO, HC, or NOX. Source: Diagram prepared by Gregory S. Noblet, University of California, Berkeley. Reprinted with permission. sively in Chapter 3, fuel-rich conditions exist when excess fuel is intro- duced into the engine combustion process, greatly increasing the production of CO, unburned HCs, and PM2.s. Virtually all gasoline-powered vehicles are designed to operate under fuel-rich conditions during cold-start opera- tion, leading to a significant proportion of total emissions. In Fairbanks, Alaska, winter cold-start and initial-idle emissions contributed an estimated 45% of overall on-road emissions (NRC 2002~. Fuel-rich conditions also occur during hard accelerations and climbing up grade, when the fuel-me- tering system injects extra fuel to improve vehicle performance, or because of malfunctions in fuel-metering and other system components. Substantial evidence demonstrates that a large fraction of emissions is from a relatively small percentage of LDVs~ that have a disproportionate impact on total air pollution from mobile sources. Figure 1-11 (above) shows the overlap among the highest-emitting 10% of vehicles randomly Typical numbers reported in the literature (usually obtained from measure- ments of in-use vehicles) show that 50-60% of on-road LDV exhaust emissions are produced by about 10% of LDVs (NRC 2001~.

54 Managing CO in Meteorological and Topographical Problem Areas pulled over and tested for CO, HCs, and nitrogen oxides (NOX) in Califor- nia. The figure indicates that there are significant similarities in the high- emitting subset of vehicles, especially regarding CO and HCs. EPA lists 21 mobile-source (on-road and nonroad) air tonics, shown in Table 1-4. For some of these (e.g., arsenic and dioxin), emissions invento- ries show that mobile sources contribute only a small fraction to their over- all emissions, but for most, mobile sources are significant if not dominant among contributors. However, it should be noted that the uncertainties associated with emissions inventories for air toxic s and PM are likely greater than for CO and HCs. Inventories for these pollutants piggyback on estimates for CO and HC, introducing another level of uncertainty. In addition, less ambient monitoring and emissions data are available to de- velop and evaluate these emissions. Table 1-5 lists direct emissions by source category for five important mobile-source air tonics. All ofthese air tonics are either known or probable human carcinogens, and some have additional noncancer health effects. The top four show a sizable fraction of emissions associated with LDVs, which are the largest source of CO emissions. However, both formaldehyde and acetaldehyde have significant secondary sources (from atmospheric chemical reactions of VOCs, includ- ing VOCs emitted from LDVs) as well as direct emissions that contribute to their ambient concentrations. The relationship between CO and PM2 s emissions is highly uncertain. Ambient observations in urban areas tend to show that a significant fraction of PM2 s emissions come from mobile sources (NARSTO 2003~. PM diesel emissions have garnered particular attention and have been classified as an air toxic (EPA 2002c). The Multiple Air Toxic s Exposure MATES-I study in the South Coast Air Basin of California (SCAQMD 2000b) esti- mated that PM diesel emissions have a much higher cancer risk compared with all other air tonics combined. PM diesel emissions are primarily from heavy-duty diesel vehicles (HDDVs) and off-road diesel engines, which tend to have very low CO emissions compared with LDVs. The emissions from HDDVs and LDVs are correlated because both sources travel the same roadways, but the spatial and temporal patterns of the emissions from these two vehicle classes may differ greatly. For example, Lena et al. (2002) found that site-to-site variability in the number of large trucks9 was much greater than that for light-duty vehicles, and that the ratio of passen- 9Lena et al. (2002) defined large trucks as those with two axles that had four tires on the rear axle, or trucks with more than two axles.

Ambient CO Pollution in the United States 55 TABLE 1-4 Mobile-Source (On-Road and Nonroad) Air Toxics T6entif~ed by EPA and the Percent of National Emissions from Mobile Sources Mobile- Mobile- Source Source Emissions Emissions Air Toxic (%) Air Toxic (%) Acetaldehyde 70 Lead compounds 23 Acrole~n 39 Manganese compounds 1.5 Arsenic compounds 0.6 Mercury compounds 4 Benzene 76 Methyl tertia~y-butyl 1,3-Butadiene 60 ether (MTBE) 86 Chromium compounds 4.2 Naphthalene ur~own Dioxin/fi~rans 0.2 Nickel compounds 8.5 HDDV diesel particulate Polycyclic organic matter and diesel matter (POM) 6 exhaust organic gases 100 Styrene 40 Ethylbenzene 84 Toluene 74 Formaldehyde 49 Xylene 79 n-Hexane 44 Source: EPA 2000c. ger cars to large trucks varied greatly by site. Further, CO-related regula- tions will have little impact on diesel PM concentrations. The contribution of LDV emissions to PM2 s concentrations is an area of active research. A study of PM2 s in Denver, Colorado, found that LDVs contributed a much larger fraction of PM2 s emissions than did diesel vehi- cles, although it is not clear whether that result is unique to the location (Fujita et al. 1998; Norton et al. 1998~. This is in contrast to a study from southern California that found diesels to be the dominant contributor of mobile-source-emitted PM2s (Schauer et al. 1996~. ARCADIS G&M (2003) recently studied seven cities and found that LDVs were the domi- nate contributor to mobile-source-emitted PM2 s in Birmingham, Alabama, and Westbury, Connecticut, and they contributed approximately the same amount as diesels in Las Vegas, Nevada. This study found diesels to be the dominant source in Albany, New York; Houston, Texas; Long Beach, California; and El Paso, Texas. In terms of toxicity, a recent study found no difference in the toxicity of particles emitted from diesel and LDVs, but particles from diesel and LDV high-emitters were much more potent on an equivalent mass than those from normal-emitters (Seagrave et al. 2002~.

56 U. C) X ~ ¢ .o 4) ~ o ~o ~ ~ o C~ V: o C~ O. · _. C) au 50 o ~o V: ~ ~ V U. o V) o o C~ o - CO o Cq - o .~ CO C~ .~ 3 V, o o o ~ C~ _ C) .~ o E~ . - ¢ \o \ ~ ~ 0 0 ~o ~o a~ O~ O~ ~ ~ o o _ _ t_ =, o o __ o o o o o o o o [_^ =~^ o o o ~ ~o o~ o o - - o o o o o o _ ~^^^\ ~o ~o ~o ~o o o~ o~ o~ o~ oo ~ ~ oo ~ ~ ~ ~t _d — ____o o o o o o o o o o o o o o o o o o ^ ^ ~ o a~ ~ ~ 0 ~ ~ ~ o \ \ ^ ^ o o ~o ~o ' o' - - ~ ~ o o o - - o o o o o o o o o o ^ - ~^ o^ o~ o~ ') ~ ~ O \ o ~o \o \ ' o o - ~ ~ ~ o - - - o o o o o o o o t^o o o ~ ~ ~ o g o o o o g o o o o o o~ o o o o o ~ —^ =^ O~ O d-^ ax ~ ~ ~ ~ ~ oo ~ ~ ~ ~ o ~ m ~ ¢ Ct ~_ .= .= CC au ~, o^ au Ct C~ a' =0, _ U, O O O ~ ¢ O C,)^ ~ ~0 O ¢ U, ao . ~ C~ O O O ~ O Ct - ; ° LU a~ .. O . . U, o ~Q

Ambient CO Pollution in the United States 57 a I ~ - C) Q a' a) N a) m Q o., - 8 Ol.f ~7 ~0 05 04 ~2 ~1 . _ _ _ it;. ~ ~ _ _ _ ~~.__~ \\ , . . - - - - - - 777J:- ~ ~ ' 0 _ 1 2 ~ ~ 5 G 7 t 9 10 11 12 1} 14 IS 16 17 11 19 ~ 21 ~ 2.t Hour of the Day Benzene/3 CO FIGURE 1-12 Diurnal average CO and benzene in London Bloomsbury in 1996. Source: Williams 2000. Reprinted with permission; copyright 2000, Elsevier SAS. Ambient CO Concentrations and Other Pollutants The relationships among ambient concentrations of CO, air tonics, and PM are complex and are affected by differences in direct pollutant sources and by atmospheric processes that create chemical sinks and secondary products. Figure 1-12 (above) indicates that benzene and CO concentra- tions have similar diurnal patterns. Benzene concentrations also show a seasonal pattern similar to that of CO, with maximum concentrations occur- ring during winter. The correlation between ambient CO and benzene con- centrations stems from a similarity in emissions sources and benzene's fairly long atmospheric lifetime, which allows it to be dispersed with CO.~° Ambient measurements in the Los Angeles area have shown strong correla- tions between ambient levels of benzene and CO (r2 = 0.76) (Figure 1-13), and an even stronger correlation was observed between CO and ambient levels ofthe relatively short-lived species I,3-butadiene (/ = 0.84) (Figure RENA (2002c) estimates that the atmospheric lifetime of benzene is 11 days. That means that benzene is removed from the urban environment by meteorological processes (as opposed to a chemical sink), which is how most CO is removed.

58 Managing CO in Meteorological and Topographical Problem Areas 4.0 3.0 Q Q 2.5 S:L - ~ 20 so a, 1.0 o.5 0.0 % wo sex ~2,0 g6 . ,_ ~,,,~ ~ .' __~, ~ - ~d - 'S >-id.'' Burbank ·LA LB x Riverside O.0 1.0 2.O 3.0 40 ~ 0 CO (ppmv) FIGURE 1-13 Benzene versus CO for four sites (Burbank, Los Angeles, Long Beach, and Riverside) in California's Soup Coast Air Basin in 1996. Source: CARB 1999. 1-14) (CARB 1999~. MATES-B found that benzene, I,3-butadiene, and other air tonics (methylene chloride, perchioroethyTene, lead, and elemental carbon) have seasonal concentrations that peak during late fall and winter in the South Coast Air Basin (SCAQMD 2000b). This was ascribed to local seasonal meteorological conditions light winds and surface inver- sions inhibiting vertical dispersion of pollutants. Table 1-6 shows atmo- spheric lifetimes for selected air tonics. Formaldehyde and acetaldehyde are reactive in the atmosphere. They have lifetimes of a few hours during daylight (Atkinson 2000~. Secondary emissions sources greatly influence the concentrations ofthese compounds. The chemical reactions that lead to the formation of additional formalde- hyde and acetaldehyde from other HCs depend on solar radiation; therefore, higher concentrations occur during months with greater solar radiation. The MATES-B study (SCAQMD 2000b) found that these concentrations peak in the summer and fall in the South Coast Air Basin. The peak is delayed because increased vertical mixing and dispersion occurs during the sum- mer, which reduces the concentrations of these pollutants. Ambient concentrations of PM2.s are influenced by widely varying emissions and atmospheric processes. A recent assessment found that Mexico City and many areas in the western United States have their highest

Ambient CO Pollution' in the United States 59 Yet _ 2.~- _ 8 - a_ or ~ ~ ~ .~ - ~ ,_ ,.,,, a: .._.~r rant .1 01'0 ~ ~ EM I La . .. ~ i — - : ash - :AV 0,0 LO =D ~ ~ ~ ~ 50 60 70 Bo CO (pP~) FIGURE 1-14 1,3-Butadiene versus CO for four sites (Burbank, Los Angeles, Long Beach, and Riverside) in California's South Coast Air Basin in 1996. Source: CARE 1999. concentrations of PM during winter because of limited dispersion during winter months(NARSTO2003~." Figure1-15showsthecorrelationbe- tween daily average CO and PM2.s concentrations for the winter of 2000- 2001 in Fairbanks, Alaska. This limited data set shows a correlation co- efficient (A-squared) of 0.70. The meteorological conditions that lead to CO buildup may also play a role in episodes of high PM. Thus, cities that have had problems coming into compliance with the 8-hour NAAQS for CO because of their meteorology and topography may also be susceptible to violations of the 24-hour NAAQS for PM2s. However, Figure 1-15 makes clear that high CO levels do not necessarily coincide with high lev- els of PM2 s. Changes in emissions-producing activities, for example, a weekend when more people are home using a fireplace instead of out driv- ing, might shift the result of an inversion episode from high CO to rela- tively higher concentrations of PM2.s. In addition, the meteorological con- ditions that produce CO exceedances may be slightly different from those that produce high PM2 s concentrations. Also, secondary formation of PM2 s (from gas-to-particle conversion of nitrates, sulfates, and organics) This is in contrast to most areas in the eastern U.S. that have peak PM con- centrations in the summer.

60 Managing CO in Meteorological and Topographical Problem Areas TABLE 1-6 Calculated Atmospheric Lifetimes for Selected Compounds Compound Acetaldehyde Atmospheric Lifetimea (daylight hours) Benzene 1,3-Butadiene Carbon monoxide Formaldehyde aThe atmospheric lifetime for each compound was calculated based on the rec- ommended OH rate constants and a 12-hour average OH radical concentration of 3.0 x 106 molecule/cm3. Source: Atkinson 1994. 5.9 75.3 1.4 440.9 9.9 can occur, especially during summer when photochemistry is most preva- lent, and further obscure the relationship between CO and PM2 s. For example, Salt Lake City, Utah, was originally in nonattainment for the CO NAAQS. In addition, the area had exceedances~2 of the 24-hour PM2 s standard of 65 micrograms/cubic meter four times: January I, 2000, at the Cottonwood site (24-hour value = 71.3 ~g/m3~; December 30 and 31, 2000, at the Hawthorne site (24-hour values = 72.4 and 66.3 ~g/m3, respec- tively), and December 30, 2000 at the North Salt Lake site (24-hour value = 68.7 ~g/m3~. CO measurements were not particularly high on those days. The highest 8-hour average CO concentration at the Cottonwood site on January 1, 2000, was 2.7 ppm. The highest 8-hour average CO concentra- tion at Hawthorne on December 30,2000, was 3.4 ppm, and on January 31 it was 2.0 ppm. (There is no CO analyzer located at the North Salt Lake site.) According to Robert Dailey of the Utah Depart lment of Environmen- tal Quality (personal communication, September 20,2002), high PM2 5 and high CO concentrations occur in response to prolonged winter temperature inversions. The inversions can last 2 to 3 weeks without a break, as was the case during these PM2s exceedances. When inversions have some fog associated with them, PM2 5 values are high, but CO values remain rela- tively Tow. When clear skies accompany inversions, PM2 s concentrations ]2The PM2 5 standard is written as an 95th-percentile exceedance; thus, a single exceedance of this standard does not mean the area is out of attainment.

Ambient CO Pollution in the United States 61 3.0 - . Correlation Coeffecient = 0.70 2.5 E Q 2.0- ._ ._ `,, 1.5 - In s ,'5 1.0- o . · ~ · ~ . . 1 · ~ . . . · . ~ . . . · . , · ~ · _ ~ ·. . . o 5 10 15 20 25 30 35 40 45 50 PM2.5 ,ug/m3 FIGURE 1-15 Correlation of daily average CO and PM2 5 concentrations at the state building, Fairbanks, Alaska, November 2000 to February 2001. are lower, and CO concentrations are high. A hypothesis explaining these observations might be that during inversions with fog, aqueous reactions in the fog form secondary PM2 s more quickly, and during clear inversions stratification in the inversion traps CO closer to the ground. However, this conclusion has not been confirmed. Roles of CO in Tropospheric Ozone and Climate Change Tropospheric Ozone In the atmosphere, the only chemical loss process for CO is by reaction with the hydroxy! (OH) radical. The overall reaction is OH + CO (+ 02) = HO2 + CO2. Using a global average tropospheric OH radical concentration of 9.4 x 105 molecule cm~3 (Prinn et al. 2001), the average CO lifetime is calculated

62 Managing CO in Meteorological and Topographical Problem Areas to be 2 months, which is sufficiently long for CO emitted in the United States to be mixed throughout the northern hemisphere. When enough NO is present that HO2 radicals react only with it, HO2 + NO = OH + NO2, the photolysis of NO2 and the rapid reaction of the oxygen, CHOP), atom with 02, NO2 + sunlight = NO + 0~3P) 0~3P)+O2+M=O3+M(M=air), leads to net formation of ozone from the reaction of OH radicals with CO (in the presence of NO such that HO2 radicals react dominantly with NO), CO+2O2=CO2+O3 Therefore, CO can be viewed as the simplest ozone-forming "hydrocar- bon." Because CO reacts rather slowly in the atmosphere, and its photo-oxi- dation results in the conversion of only one molecule of NO to NO2 per molecule of CO oxidized (see above), CO has a significantly lower ozone- forming potential (grams of O3 foe per grams of reactant emitted) than the HC mix in vehicle exhaust (Carter ~ 998~. The ~ 997 maximum incre- mental reactivity (MIR) scale (Carter 1998) of CO is 0.065 g of O3 per gram of CO emitted, which can be compared to the MIR of exhaust emis- sions from vehicles fueled with two gasolines representative of California reformulated gasoline (testing conducted during the Auto/OiT Air Quality Improvement Research Program) of approximately 3.5 g of O3 per gram of HO emitted (NRC ~ 999~. However, because of the amount of CO and HCs emitted in vehicle exhaust, NBC (1999) concluded that CO from LDVs contributes 15-25% of the total ozone-forming potential of exhaust emis- sions. Therefore, despite its Tow ozone-forming potential, CO contributes to ozone formation in polluted atmospheres. Keep in mind however that ozone formation requires sunlight and is strongly temperature dependent, so it tends to be more of a problem during the summer months and not during the winter months when CO exposure tends to be a problem.

Ambient CO Pollution in the United States 63 Climate Change CO contributes to climate change in four ways: (~) it is itself a green- house gas (GHG), though its warming potential is much less than that of CO2; (2) CO is oxidized to CO2, as shown above, noting that direct emis- sions of CO from vehicles are an order of magnitude or more lower than those of CO2; (3) at low NOx concentrations, reaction of CO with OH radi- cals results in loss of OH radicals, and by removing OH from the atmo- sphere CO tends to increase the lifetime of methane (CH4), a powerful GHG; and (4) CO contributes to the formation of ozone (03), another GHG. Using CO As an Indicator of Other Pollutants In urban environments, CO can serve as an indicator of motor vehicle emissions from gasoline-fueled vehicles. The observed spatial and tempo- ral variability of CO shows that the effects of motor-vehicle pollution are heterogeneously distributed in urban areas and that CO can be a useful gauge of long-term human exposure to other pollutants of concern, includ- ing certain mobile-source air tonics. CO levels also demonstrate the exis- tence of"hot spots" in urban environments where high concentrations of CO and other products of automobile exhaust occur. However, CO is not a perfect indicator. CO does not react on the time scales of concern for urban pollution, and it is not representative of the chemical reactivity of other pollutants. The weather conditions that pro- duce high CO concentrations are generally unrelated to those that produce ozone pollution, which is most severe during the summer months. Because CO pollution is primarily due to exhaust emissions from LDVs in the urban environment, it is not strongly correlated with evaporative toxic emissions, diesel PM emissions, or stationary- and area-source emissions. Because CO is formed with other products of incomplete combustion (including unburnt fuel), CO emissions from a specific vehicle often corre- late with emissions of HCs and organic compounds (including the air toxics formaldehyde, acetaldehyde, 1,3-butadiene, and benzene). Emissions of formaldehyde and acetaldehyde depend on the fuel used and, more specifi- cally, on the presence of MTBE or ethanol in the fuel. MTBE leads to higher emissions of formaldehyde, and ethanol leads to higher emissions of acetaldehyde ARC 1999~. The exhaust emissions of benzene and other

64 Managing CO in Meteorological and Topographical Problem Areas higher emissions of formaldehyde, and ethanol leads to higher emissions of acetaldehyde (NRC 1999~. The exhaust emissions of benzene and other aromatic HCs depend on the fuel aromatic content, because benzene and other aromatic HCs are emitted as unburnt fuel and are formed in the com- bustion process. The precise correspondence of CO and other organic vehicle emissions depends on the fuel used (including the presence or absence of a fuel oxy- genate, the actual oxygenate used, end the aromatic content ofthe fuel) and onboard emissions control technologies and engine conditions (i.e., cold start, hard acceleration, etc.~. Furthermore, CO's relationship with other organic emissions varies as a function of time after emission. While CO is essentially nonreactive on day-Ion" time scales, most other organic com- pounds in vehicle exhaust are significantly more reactive than CO, and certain organic compounds (e.g., carbonyl compounds, alkyl nitrates, and peroxyacyl nitrates) are later formed in the atmosphere from atmospheric reactions of other HCs (Atkinson 2000~. For example, formaldehyde is removed rapidly by photolysis (and less rapidly by reaction with OH radi- cals) and has a lifetime of about 4 hours in overhead sun. It is also formed in the atmosphere from the photooxidation of almost all other HCs (Atkinson 2000~. Therefore, although CO can serve as a general indicator of motor-vehi- cle exhaust (and hence of exposure to vehicle exhaust and/or to photochem- ically processed vehicle exhaust), CO concentrations alone are uncertain estimators for concentrations of other organic compounds in the same air

Ambient CO Pollution in the United States 65 mass, except for other long-lived vehicle exhaust components such as ben- zene (and even then, only in the absence of other sources). However, rea- sonably strong correlations between CO and the shorter-lived volatile or- ganic compounds (VOCs) emitted from LDVs will still be observed over distance scales corresponding to travel times of the pollutants of approxi- mately a half-Tife or less (see Figure 1-14~. EQUITY CONSIDERATIONS IN THE SPATIAL DISTRIBUTION OF AMBIENT CO Because CO levels are not evenly distributed, exposure to CO within the population will vary. Individuals living in or near areas of high CO ("hot spots") are exposed to higher concentrations of CO and other mobile- source-related pollutants. Although the network of CO monitors is too sparse to identify all hot spots, the characteristics of the residents living near known hot spots can be examined. An analysis of data from the 2000 U.S. Census shows that the individual CO monitors that registered exceedances of the 9-ppm 8-hour average CO standard during the period 1995-2001 are often found in areas that have greater percentages of low- income and minority residents than their surrounding regions (Table 1-7~. All but six of the monitor areas, as defined by the census tract or tracts immediately surrounding each CO monitor, had higher percentages of nonwhite residents in 2000 than the region as a whole. In the area around the Sunrise Avenue monitor in Las Vegas, for example, 49.6% of residents are nonwhite, compared with 26.2% for the Las Vegas Metropolitan Statis- tical Area (MSA). Three monitor sites in Los Angeles have a Tower per- centage of nonwhite residents than the region as a whole, but even in those areas, over one-third of residents are nonwhite. The percentage of residents who are of Hispanic origin is higher than the regional share for all but three monitors. The differences are often dramatic. The population in the Las Vegas-Sunrise Avenue area is 68.7% Hispanic compared with 5.3% for the region, and percentages of Hispanic residents for the Phoenix monitor areas are over twice the percentage for the region. Per capita incomes were lower for residents in the monitor area than on average for the region (for all but four of the monitor areas) and were less than half of the regional average for the Las Vegas-Sunrise Avenue, Phoenix-Indian School Road, and Lynwood-Long Beach Avenue monitor areas. The monitor area in Denver at Speer and Auraria Parkway, where the per capita income is over twice

66 of To To o .= U) U) au · _ no ·_. Cal C) · _ VO · _ C) Ct V o · _ Cd - o 1 EM a, ~ C A V Cal, crJ: ·C :t .= O a, O ~ O .0 Ct .= 'C ._ .= ~ ~ 'O .= .0 3 o Z 'C C) Cal ~ 5= o Ct o C) ~ ~ X ~ =\ Go Do ~4 Do o Go Do Go - Cal - lo Do lo oo c~ ~ cr cr~ o ~o _4 ~ 0 . . O 0 ~o oo oo o~ o ~o o cr~ _4 o - C~ (~) t_ oo ~ oo oo . . . ~ 0 ~ ~n ~ ~ ~ 0 . . . . ~ 0 ~ ~ ~ ~ ~ , ~ oo rn oo _ 0 0 0 0 0 , 0, O O — O u ° , e ° ,; 3 ° ~, ° ~ ° a ° x ~ ~~ ~ c w ~ ,, - ~~ ~ Y c ~

67 t_ t— <> ~ _ r ~ V rat a0 of oo ~ ~ ~ _4 —) ~ ~ ~ ~ _ 00 co ~ Ax cry cr 0 oo _' ~ _, _I ~ ~ ~ on ~ — ~ my. A. ~ ~ O _ _4 ~ ~ ~ 00 00 ~ ~ ~ O O ~ ~ —' O~ Cry _' ~ ~ _ _ Cry. O 00 ~ — Ch - _ cry O cr _ cr. ~ ~ 00 ~ oo ~ ~ 00 00 ~ O O O ON O — ~ O of ~ ~ ~ ~ ~ O O _ ~ ~ ~ ~ ~ ~ O — O ~ ~ ~ O ~ O ~ C~ — CN O ~ O ~ ~ ~ O oC _ ~ _ _ _ oo ~t ~ oo O0 t_ _ _ _ · _ —, O —) ~ =} ~t t_ =\ r~. ~ ~ ~ ~ oo _ ~ ~ _ ~ _ ~ ~ _ ~ ~ O O ~ oo ~) ~ ~ ~ ~ - 1 ~ ~ . . . . . . . . . . _ ~ ~ o0 CN - , ~ ~ ~ ~ _ ~ _ o ,, c, 3 _ ,~ ~ ~ c ~ ~ c c~ t4 ~° ~ o v o :~3 ~ ~ ~ o ~ ~ o ~ ~ \c, ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ O ~ ~ O ~ ~ ~ ~

68 · lo;, 1 EM no ~ V ~ $- Ct C) ·~= Cd on ~ o ~ ~ .o :> sit Z s 'a C) o .o Ct .= == >< .o ~ o .= He's ~ r~ o ~ .= .= ~ no so C) _I ~ o Ct o C) ~ ~ X ;^ °N o Do Go ') rn _ on Go o - ~ _ _ ~ ~ ~ ~ Go o o _ ~ o o ~ ~ . . . . _ o ~ ~ _ _ _ ~ _ o o ~ o ~ ~ . . . . . . ~ _ ~ ~ o o _ ~ ~ _ ~r t_ - oo ~ o ~ ~ N ) - ~ - ~ - - ~ oo oo oo oo ~ ~ - - o ~ oo 0C ON a~ O ~ ~N cr. r~ ~ ~ oo - u~ ~ ~ ~ ~ o ~ ~ - ~ ~ ~ - - oo o (~) M o ~ ~ oo ~ ~ c`4 - O - } o o ~ t ,~ ~ ~ ~ z ~ o z ~ o _ e O e O 0 5 0 c O c ~ 0 2 ~ O ~, - O a 0 0 ° 2 0 ~ ~ 0 == &v ct (-) s~ ¢ c' v: u, ct ct 0 ¢ ·- ~ ~ 0 s: 0 ~ m .c ,~ c~, m V: 0 ~4 X ° ~ C) Z -= O ~m f3 3 ~ 't 0 s: sc 0 s, ~ c~ ~o ° ~ ~ ¢ ~ . ~ 0 .~ ~ =-= ~ ·O ~ ~ ~ ~ ~ · ~ ~ w 2 j° 2 cv: s: ° ~ D rL, ca ¢ — D ~ O - , O ~ .~ O ~ ~ ~'= C) ~ "o _~ ~ ~ ~ V)

Ambient CO Pollution' in the United States 69 the regional average, is an anomaly and reflects a recent influx of affluent residents to downtown Denver residents choosing to live in a high-den- sity, high-traffic area. In addition, the number of employed residents who do not drive to work is higher than for the region as a whole in all but three of the monitor areas and is four or more times higher in monitor areas in Spokane, Washington, Denver, Colorado, and Provo, Utah. Residents who walk, bike, or ride transit are likely to spend more time within the monitor area during their commutes and may experience greater exposure to high CO levels. To a limited degree, these demographics may explain the high levels of CO recorded in many monitor areas. Although the residents of these areas are less likely to drive to work, they are more likely to own older vehicles, which in turn are more likely to be high-emitters (Rajan 1993; Granell 2002~. For example, Singer and Harley (1996, 2000) observed a much higher fraction of older vehicles near the Lynwood monitor. The vehicles observed in their study included vehicles passing through the area as well as vehicles owned by local residents. The high emissions rates for older vehicles may offset lower total amounts of driving. In addition, the rela- tively high population densities in all but one monitor area (Table 1-7) suggest higher concentrations of traffic in these areas and thus higher con- centrations of pollutants. However, the traffic generated locally is likely to represent a small fraction of the total traffic in and around most of these monitor areas. These preceding demographics suggest the need for continued attention to the CO problem from the standpoint of environmental justice. In ~ 994, President Clinton signed Executive Order 12898, Federal Actions to Ad- dress Environmental Justice in Minority Populations and Low-Income Populations. The order was related to Title VI of the Civil Rights Act of 1964 and required federal agencies "to achieve environmental justice by identifying and addressing disproportionately high and adverse human health and environmental effects, including the interrelated social and economic effects of their programs, policies, and activities on minority populations and low-income populations in the United States." The order also stipulated that in reviewing other agencies' proposed actions under Section 309 of the CAA, "EPA must ensure that the agencies have fully analyzed environmental effects on minority communities and low-income communities, including human health, social, and economic effects " (EPA 1 998a). EPA and the FHWA have issued their own interpretations of the envi- ronmental justice requirement. EPA defines environmental justice as "the

70 Managing CO in Meteorological and Topographical Problem Areas fair treatment of people of all races, cultures, and incomes with respect to the development, implementation, and enforcement of environmental laws and policies, and their meaningful involvement in the decision making processes of government" (emphasis in original). According to EPA, fair treatment requires that EPA conduct its "programs, policies, and activities that substantially affect human health and the environment in a manner that ensures the fair treatment of all people, including minority populations andlor low-income populations" and that EPA ensure "equal enforcement of protective environmental laws for all people, including minoritypopula- tions and/or low-income populations" (EPA 20016~. As interpreted by the FHWA, environmental justice includes notjust minimizing adverse effects but also the preventing of "the denial of, reduction in, or significant delay in the receipt of benefits by minority and low-income populations" (FHWA ~ 998~. This requirement should apply to benefits from federal policy such as improvements in air quality. Both ofthese interpretations suggest that the remaining locations expe- riencing high CO concentrations represent a potential environmental justice concern. According to EPA's final guidance for incorporating environmen- tal justice concerns into National Environmental Protection Act (NEPA) compliance analyses, a minority population is present "if the minority population percentage of the affected area is 'meaningfully greater' than the minority population percentage in the general population or other ' appro- priate unit of geographic analysis"' (EPA 1998a). This is clearly the case for areas surrounding most of the monitors registering CO exceedances since 1995. In addition, EPA notes the following: Minority communities and Tow-income communities are likely to be dependent upon their surrounding environment (e.g., subsis- tence living), more susceptible to pollution and environmental degradation (e.g., reduced access to health care), and are often less mobile or transient than other populations (e.g., unable to relocate to avoid potential impacts). Each of these factors can contribute to minority and/or low-income communities bearing disproportion- ately high and adverse effects (EPA 1998a, p. 571. The extent of the areas with CO concentrations that exceed the NAAQS is unclear because the number of monitors in each area is limited. Therefore, measures of the exposures to CO experienced by low-income and minority populations are imperfect. While the declining number of exceedances of the CO standard and the design of the standard to ensure a

Ambient CO Pollution in the United States 71 reasonable margin of safety are encouraging, the correlation between CO and other pollutants creates uncertainty in the degree to which "adverse human health and environmental effects" might be occurring in these Tow- income and minority communities. Nevertheless, the demographic patterns suggest that the impacts that occur are disproportionately high in these areas. In addition to providing an important impetus for the continuation of efforts to eliminate CO exceedances, these results suggest a need for monitoring and personal exposure research programs designed to more fully characterize the distribution of CO and other mobile-source-related pollutants.

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The regulation of carbon monoxide has been one of the great success stories in air pollution control. While more than 90 percent of the locations with carbon monoxide monitors were in violation in 1971, today the number of monitors showing violations has fallen to only a few, on a small number of days and mainly in areas with unique meteorological and topographical conditions.

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