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Management of Carbon Monoxide Air Quality The Clean Air Act's mandate to "protect and enhance the quality of the Nations air resources so as to promote the public health and welfare" and subsequent scientific findings by the U.S. Environmental Protection Agency (EPA) served as the basis for the National Ambient Air Quality Standards (NAAQS) for carbon monoxide (CO). Chapter 1 discussed the CO standards, trends in ambient CO, and the studies that were influential in developing the health-based standards. Achieving and maintaining the NAAQS requires monitoring ambient CO, developing emissions invento- ries, implementing emissions regulations and related controls, and tracking progress. This chapter discusses the primary air quality management ele- ments needed to achieve those objectives the basic emissions control strategies used to reduce emissions and the monitoring and modeling tools used to characterize and assess the magnitude of the problem. EMISSIONS CONTROL PROGRAMS CO emissions control strategies have focused on controlling light-duty vehicle (LDV) emissions. The decline in concentrations noted in the earli- est stages of CO management in the 1 970s corresponded to the implementa- tion of a major enhancement in control of motor-vehicle emissions. There 100
Management of CO Air Quality 101 are four approaches for reducing vehicle emissions: ( 1 ) new-vehicle certifi- cation programs, (2) fleet-turnover incentives, (3) in-use vehicle control strategies, and (4) transportation control measures (TCMs) (Guensler 1 99S, 2000~. This section discusses vehicle emissions control strategies in more detail. As LDV emissions decrease, nonroad, area, and smaller stationary sources may become critical for controlling CO in some locations. This section concludes with a brief discussion on the regulation of these other sources of CO pollution. Federal New-Vehicle Emissions Standards Lowering emissions certification standards on new vehicles has been the largest source of reductions in CO emissions from LDVs. For example, the Alaska Department of Environmental Conservation in its most recent SIP for Fairbanks attributed over 70°/O of total emissions reductions over the ~ 995-2001 time period to more stringent federal new-vehicle emissions standards (ADEC 20011. Table 3-1 shows emissions standards for passen- ger cars and light-duty trucks.2 CO emissions standards have dropped by over an order of magnitude since their inception emissions from new passenger cars have fallen from 84 grams per mile (g/mi) before emissions controls were instituted to below the current 3 .4 g/mi, which began in ~ 981 . New vehicle technologies offering much better environmental performance Vehicles are certified using the federal test procedure (FTP) and the supple- mental federal test procedure (SFTP), which specify the preconditioning a vehicle is to undergo before testing, the laboratory conditions the test is to occur in, and a specified driving cycle to be used. Testing is done at temperatures between 68°F and 86°F. Manufacturers are allowed to certify compliance to the 50,000- or 100,000-mile (ml) standards (11 years or 120,000 mi for heavier trucks weighing more than 5,750 lb) using low-mileage cars and an agreed-upon deterioration as- sumption. However, vehicles may be recalled if emissions control systems are found to be faulty. Tier 2 emissions standards, which will begin with model year 2004, are 120,000-mi standards. 2Light-duty trucks have been categorized for emissions certification purposes as light light-duty trucks having a gross vehicle weight rating (GVWR) <3,750 lb (LDT1) or from 3,750 to 5,750 lb (LDT2), and heavy light-duty trucks having a GVWR from 5,751 to 8,500 lb (LDT3 and LDT4~. Trucks with a GVWR greater than 8,500 lb are categorized as heavy-duty vehicles.
102 Managing CO in Meteorological and Topographical Problem Areas TABLE 3-1 Federal Passenger-Car and Light-Truck Exhaust Emissions Standards (g/mija Passenger Cars Light Trucksb ModelYear HC CO NOx HCb CO NOx PrecontrolC 10.6 84.0 4.1 1968-1971 4.1 34.0 8.0 102.0 3.6 1972-1974C 3.0 28.0 3.1 8.0 102.0 3.6 1975-1976 1.5 15.0 3.1 2.0 20.0 3.1 1977-1978 1.5 15.0 2.0 2.0 20.0 3.1 1979 1.5 15.0 3.1 1.7 18.0 2.3 1980 0.41 7.0 2.0 1.7 18.0 2.3 _ .. . . Tier O 1981-1983 0.41 3.4 1.0 1.7 18.0 2.3 1984-1986 0.41 3.4 1.0 0.8 10.0 2.3 1987-1993 0.41 3.4 1.0 0.8 10.0 2.3 1988-1993 0.41 3.4 1.0 0.8 10.0 1.2 Tier 1 (1994-) . . 1994 (100~000-mi 0.25 3.4 0.4 0.25 3.4 1.2 standards in (0~31) (4.2) (0.6) parentheses) 1995 (100,000-mi 0.25 3.4 0.4 0.25 3.4 0.4 standards in (0~31) (4.2) (0.6) parentheses) NLEVC (100,000-mi standards) 1999 0.09 4.2 0.3 0.09 4.2 0.3 aAll standards are for 50,000 mi unless otherwise noted. bStandards before 1988 are for all light-duty trucks. Beginning in 1988, light- duty trucks were separated into two weight classes (1988-1993) and then four weight classes (1994-present). The standards after 1988 are for LDT1, which have a 3,750 lb or less gross vehicle weight (GVW). The National Low Emissions Vehicle (NLEV) Program introduces California low-emissions cars and light-duty trucks into the Northeast in 1999 and the rest of the country in 2001. Sources: Davis 1997; Chrysler Corporation 1998. made these achievements possible. This is in contrast to in-use emissions controls, such as vehicle emissions inspection and maintenance (~/M) pro-
Management of CO Air Quality 103 grams and oxygenated fuels programs, which do not force the adoption of improved vehicle emissions control technologies and hence reduce vehicle emissions by a much smaller fraction (NRC 1999, 2001~. Recent New-Vehicle Emissions Standards Federal passenger car CO standards have remained at 3.4 g/mi (50,000- mi standard). However there are myriad regulations that have resulted in reductions in vehicle CO emissions. For example, though Tier 1 standards did not affect passenger-car CO emissions, they reduced CO standards for light-duty bucks. Tailpipe emissions of CO end hydrocarbon (HC)respond similarly to changes in air-fuel ratios, and CO is reduced by many of the same vehicle emissions control technologies as HC. Thus, the more strin- gent HC standards imposed since 1981 have resulted in concomitant reduc- tions in CO. In addition to these reduced HC standards that result in reduced CO emissions, there are a number of other changes that directly affect CO emissions. With the introduction of Tier 1 standards in 1994, the durability requirements increased from 50,000 mi to 100,000 mi. The supplemental federal test procedure (SFTP), which is discussed in a subsequent section, controls CO during non-FTP conditions of high acceleration and high speed. Cold-start standards, also discussed in a subsequent section, will limit CO emissions during cold-temperature starts. The recently finalized Tier 2 regulations will also impact CO emis- signs. Control of tropospheric (ground-level) ozone (03), which is caused principally by the interaction of nitrogen oxides (NOX), certain reactive volatile organic compounds (VOCs), and sunlight on hot summer days, has been a continuing need. On February 10, 2000, EPA promulgated a new series of vehicle emissions regulations, known as Tier 2, intended to par- tially address this problem by regulating passenger-car and light-duty truck NOX emissions. Tier 2 requires each manufacturer to meet a sales-weighted "corporate average NOX standard" of 0.07 g/mi. Lowering fuel sulfur con- tent, which is discussed in the section on in-use emissions controls, is also an integral part of the Tier 2 strategy. Table 3-2 lists new emissions limits for NOX, non-methane organic gases (NMOG), CO, formaldehyde (HCHO), and particulate matter (PM) by "bin." Manufacturers certify their vehicles in these bins, ensuring that these vehicles comply with all emissions levels associated with the bins.
104 Managing CO in Meteorological and Topographical Problem Areas TABLE 3-2 Tier 2 and Interim Non-Tier 2 Full-Useful-Life Exhaust Mass Emissions Standards (g/mi) Bin Number NO NMOG CO HCHO PM x 11ac 0 9 0.280 7.3 0.032 0.12 10a b d 0.6 0.156/0.230 4.2/6.4 0.018/0.027 0.08 ga,b.e 0.3 0.090/0.180 4.2 0.018 0.06 8 f 0.20 0.125/0.156 4.2 0.018 0.02 7 0.15 0.090 4.2 0.018 0.02 6 0.10 0.090 4.2 0.018 0.01 5 0.07 0.090 4.2 0.018 0.01 4 0.04 0.070 2.1 0.011 0.01 3 0.03 0.055 2.1 0.011 0.01 2 0.02 0.010 2.1 0.004 0.01 1 0.00 0.000 0.0 0.000 0.00 aThis bin and its corresponding intermediate life bin are deleted at end of 2006 model year (end of 2008 model year for HLDTs and MDPVs). Higher NMOG, CO, and HCHO values apply for HLDTs and MDPVs only. This bin is only for MDPVs. Optional NMOG standard of 0.280 glm: applies for qualifying LDT4s and qual- if~ing MDPVs only. Optional NMOG standard of 0.130 g/mi applies for qualifying LDT2s only. fHigher NMOG standard deleted at end of 2008 model year. Source: 65 Fed. Reg. 28 (2000), p. 6855. For example, a manufacturer might certify their sport utility vehicle (SW) in bin 7, their passenger car in bin 5, and an economy car in bin 3. All vehicles must meet the full useful life (which has been raised from 100,000 to 120,000 mi) certification limits for their respective bin. NOX emissions standards for the three bins would then be sales-weighted and compared with the average NOX standard of 0.07 g/mi. Tier 2 regulations also allow manufacturers to trade and bank credits. In years that a manufacturer's corporate average falls below 0.07 g/mi it can generate credits which it can bank and use in years when its corporate average exceeds 0.07 g/mi or it can sell these credits to manufacturers whose corporate average is above 0.07 g/mi. The technologies relevant to the Tier 2 standards will also have benefits for CO reduction. Since the mid-1980s, modern computer-controlled en-
Management of CO Air Quality 105 gines have used electronic fuel injectors rather than carburetors to deliver fuel to cylinders in LDVs and most light-duty trucks. The engine computer system reads the signal from an O2 sensor in the exhaust system and contin- uously adjusts the air-fuel ratio. The continuous feedback adjustment of the air-fuel ratio is known as closed-Ioop control. The feedback provides enough air to burn the fuel while maintaining the optimal catalytic-con- verter efficiency (referred to as the stoichiometric ratio) for control of CO, HC, and NOX. Figure 3-1 shows the air-fuel ratio effects on catalyst con- version efficiency. During hard acceleration and high-speed operations, however, engine computers often use fuel-enrichment strategies to enhance engine perfor- mance for short time periods and to protect sensitive engine components from high-temperature damage. Likewise, fuel-enrichment strategies are often used during cold starts. Cold temperature CO standards and the SFTP, which are discussed in the following sections, are intended to further control CO for these conditions. Thus, in modern engines, CO as well as HC emissions are most prominent during enrichment associated with heavy loads, hard accelerations, and cold starts. Enrichment factors are much larger for CO compared with HC (see Figure 3-2) (M. Barth, University of California, Riverside, personal communication, October 30, 2002; Scora et al. 2000~. Conditions that produce a 10- to 1 00-time increase in CO emis- sions produce a 1- to 10-time increase in HC emissions. The primary methods for meeting the Tier 2 standardsensuring stoichiometric engine operation over a broader range of operation and promoting faster catalyst warm-u~will have benefits for CO reductions. As shown in Figure 3-3, the prototype Tier 2 vehicle maintains a stoichi- ometric air-fuel ratio more effectively than a 1996 vehicle certified to Cali- fo~nia's low emissions vehicle (LEV) standard.3 Although the current 3The CAAA90 authorized California, which has the nation's worst air pollution problems, to impose stricter vehicle emissions standards than those for the rest of the nation. California's low emissions vehicles (LEV) regulations require manufac- turers to meet fleet-weighted average emissions lower than those mandated by the federal Tier I regulations beginning with the 1994 model year. The California LEV program includes five progressively more stringent categories: transitional low emissions vehicles (TLEVs), LEV, ultra-low emissions vehicles (ULEVs), super ultra-low emissions vehicles (SULEVs), and zero emissions vehicles (ZEVs).
106 Managing CO in Meteorological and Topographical Problem Areas 100 he ~ 80 z z o Cl) o 60 40 20 WINDOW ~ NOX ~ RICH A/F MIXTURE \ \ BEST OPERATING AREA FOR 3-WAY CATALYST LEAN AJF MIXTURE O _ 13:1 14:1 14.7:1 15:1 16:1 AIR-FUEL MIXTURE RATIO \ 1 J FIGURE 3-1 Catalyst conversion efficiency as a fimction of airmail ratio. Source: Adapted from Canale et al. 1978. Reprinted with permission; copyright 1978, Society of Automotive Engineers. 3.4 g/mi federal new-vehicle standard for passenger cars dates to 1981, the use of advanced-technology three-way catalytic converters and continued improvements in stoichiometric ratio controls have had and will continue to have a collateral CO benefit. However, it will take years for Tier 2 regulations to be implemented (2007 for LDVs and 2009 for heavier light-duty trucks), and even longer for fleet turnover to occur and for the full benefits of the new technologies to be realized. An increase in vehicle durability has accompanied techno- logical improvements. According to Davis (2001) the rational average age of in-use passenger cars has increased from a mean of 5.6 years in 1970 to 8.9 years in 1999. The median lifetime of a 1990 model year passenger car is 4.6 years longer (16.1 years) than that of a 1970 model year car. This increase in vehicle durability will slow the penetration of vehicles with newer emissions control technologies into the fleet. In the meantime, the ongoing improvements resulting from HC standards under Tier 1 and NLEV, the cold-start CO standards, the increased durability required under Tier 1, and the introductions of the SFTP will continue to encourage the downward trend in CO emissions from light-duty vehicles in advance of Tier 2.
Management of CO Air Quality 107 Engine Start Hot Stabilized Exhaust Engine On Acceleration Enrichment Grade F nri~hmPnt Space and Time (seconds) Engine Off FIGURE 3-2 Hypothetical carbon monoxide emissions rates for typical vehicle operation. In addition, some emissions control strategies for controlling cold-start emissions are particular to HCs and do not improve CO emissions. Some of the lowest-emitting vehicles in California (called super ultra-Iow emis- sions vehicles tSULEV]) use a carbon canister to store uncombusted HC emissions during cold starts. The HC emissions are then recirculated through the catalyst after light-off. Such a control strategy does not reduce CO emissions. ~ summary, the impact of Tier 2 requirements is complex. The CO limits for the higher-emissions vehicles, bins 5-8 (bins 9-1 1 disappear after 2009), is 4.2 g/mi based on a full useful life of 120,000 mi. On the surface, this limit is essentially the same as the Tier ~ and NLEV limits. However, Tier 2 standards are 120,000-mi standards, which should improve vehicle in-use performance. These bins will apply to passenger cars as well as all categories of light-duty bucks (LDT1-LDT4~. This wiTIrequirethatLDT2- LDT4, which under Tier 1 standards have 100,000-mi CO standards from 5.5 g/mi to 7.3 g/mi, meet the current CO standards for passenger cars (4.2 g/mi). At a national level, the result of these bins will be a reduction in CO. If bins 6-8 are used for any vehicle, then bins 14 must be used to average NOX below bin 5. Using the example of a manufacturer certifying their SUV in bin 7 (CO standard at 4.2 g/mi), their passenger car in bin 5 (CO standard at 4.2 g/mi), and their economy car in bin 3 (CO standard at
108 Managing CO in Meteorological and Topographical Problem Areas 1996 California Low Emissions Vehicle 15.0 14.9 14.8 14.7- o 14.6 ~ 14.5 2 14.4 14.3 14.2 14.1 14.0 15.0 ~ 14.9 ~ 14.8 ~ O 14.7- ._ 14.6- 14.5- 14.4- 14.3 ~ 14.2 ~ 14.1 ~ 14.0 ~ . ·. ~ ~~ . · ~ . ~ . . . · ~ · ~ ~ %. ~ ~ ~1 ; -. . ~ - .~\ e. ~$.~,S^' , _,' - An_ ~ ~ _ ~ ~ a, ~ ~ · ~ AIMS ~ . . A, ;. - · · : · ~ ~ i. 0 100 200 300 400 500 Time (s) 2003 Tier 2 · ~ ·; 0 100 200 300 400 500 Time (s) FIGURE 3-3 Example of the improved control of air-fuel ratio resulting from new Tier 2 vehicle technologies. Source: Dana 2002. Reprinted with permis- sion from the author.
Management of CO Air Quality 109 2.1 g/mi), the resulting fleet sales-weighted CO will be Tower than 4.2 g/mi. However, this is a national fleet average. Local vehicle fleets may differ from the national average. Cold-Temperature CO Standards As described in Chapter 1, CO is predominantly a winter problem that occurs in regions known for extreme winter conditions (e.g., Fairbanks, Alaska). During cold starts the engine computer signals the fuel injectors to add excess fuel to the intake air to ensure that enough fuel evaporates to yield a flammable mixture in the engine cylinders. A typical engine-com- puter strategy injects several times the stoichiometric amount offue] during the first few engine revolutions, using a fixed fueling schedule to reach idling conditions. Excess fuel continues to be injected until the engine and O2 sensor are warmed up and the exhaust-catalyst inlet temperature reaches about 250-300°C, sufficient for the catalyst to oxidize CO to CO2. This open-Ioop operation, before catalyst light-off (the time it takes the catalyst to reach peak efficiency after start), can continue for several minutes at Tow ambient temperatures. Cold-start enrichment is responsible for a signifi- cant fraction of CO, air tonics, and unburned HCs from properly operating vehicles. Once the engine and emissions control systems are warmed up, combustion becomes stoichiometric, and CO is converted to CO2 in the catalyst, keeping CO emissions very low under typical operating condi- tions. Warm up times under mild ambient conditions, at around 70-80°F, can be around 1 min for modern catalysts and even as short as a few sec- onds for modern close-coupled catalysts (catalysts close to the engine). When ambient temperatures are -20°F or Tower, however, catalyst and engine warm-up times can exceed 5 min (Sierra Research 1999~. In the case of Fairbanks, Alaska, this means that idling and cold-start emissions from LDVs are particularly high and make up a significant proportion of overall CO emissions. ADEC (2001) and NRC (2002) provide more dis- cussion of the role that cold-start emissions play in Fairbanks. Since 1994 new cars and the lightest category of light-duty trucks (LDT1) have been required to meet a CO limit of 10 g/mi on the federal test procedure (FTP) cycle conducted at 20°F. For heavier light-duty trucks (trucks between 3,751 and 8,500 Ib gross vehicle weight tGVW]), the stan- dard is 12.5 g/mi. The cold-temperature CO emissions standard has been unchanged since it was promulgated in ~ 994, though certification data from
110 Managing CO in Meteorological and Topographical Problem Areas EPA's certification database show that there have been continued improve- ment in cold-start emissions (Figure 3-4~. Reducing the 10 g/mi limit for the 20°F cold-start test or reducing the test temperature might provide addi- tional CO emissions reductions for cold northern regions, such as Fair- banks. Indeed, CAAA90 mandated that if six or more areas were desig- nated as nonattainment as of July 1, ~ 997, EPA must require cars to meet a Phase I] cold-start emissions limit of 3.4 g/mi. In their presentations to the committee, representatives of the State of Alaska and the Fairbanks North Star Borough discussed how the adoption of the Phase II cold-start standard would aid in Fairbanks's effort toward long-term attainment ofthe CO NAAQS (Hargesheimer 2001; King 2001; Verrelli 2001~. However, EPA has yet to formally dete~ine the number of CO nonattainment areas that existed as of the deadline. Supplemental Federal Test Procedure An additional source of CO reductions is the SFTP. The technical community has long known about the absence of high speeds and accelera- tions from the FTP. The SFTP introduces speeds as high as 80 MPH and maximum accelerations of 8.4 MPH/s into the certification test (61 Fed. Reg. 54852 t199611. The FTP tests at a maximum speed of 57 MPH and a maximum acceleration of 3.3 MPH/s. Certification to this new cycle will be phased in during the 2000 and 2004 model years. This test procedure should ensure that vehicle emissions control systems will provide improved emissions control over a wider range of vehicle speeds and loads. Much of the improved emissions control will come from reduced use of fuel-rich mixtures at higher Toads. EPA estimates a CO emissions reduction of 1 1% from the LDV fleet in 2020 as a result ofthe SFTP (EPA 1996~. However, it should be noted that for some locations with severe winter Hiving condi- tions, such as Fairbanks, the high speed/high acceleration driving condi- lions within the SFTP are not considered representative. Thus, the benefits from certifying vehicles to the SFTP may be smaller there. Mobile-Source Compliance Programs Mobile-source compliance programs are intended to ensure that vehi- cles meet emissions standards throughout their useful life. There are three
Management of CO Air Quality 111 14 12 50,000-Mile Cold CO Standard 10 ~ ~ 8t _ / 6 2 5.0 O 3.6 PC/LDT1 -- T 1 |~Tier 1~ LEV ~ ULEV LDT2 FIGURE 3-4 Average cold CO standards versus certification data (20°F) for model years 2000-2002 at 50,000 mi. There are 279 vehicles represented in PC/LDT1 data (Tier 1 = 91; LEV = 166; ULEV = 22) and 109 vehicles repre- sented in LDT2 data (Tier 1 = 23; LEV = 71; ULEV = 16~. Source: Dana 2002. Reprinted with permission from the author. major components of the program: preproduction evaluation, production evaluation, and in-use evaluation. Before a motor vehicle can be sold in the United States, EPA requires that it be emissions certified. This involves testing a preproduction prototype to prove that it meets applicable model year emissions standards and durability-deterioration requirements. The certificate requires the manufacturer build every vehicle (or engine) the same as the prototype in all material respects. Once production begins, the manufacturer and EPA conduct end-of-line production audits of randomly selected vehicles. These audits frequently require emissions tests to deter- mine conformity with applicable standards. If a statistically significant number of vehicles fait, the manufacturer takes steps to repair them. Fail- ures may involve a faulty or out-of-specification part or adjustment or improper assembly. Repair measures can include stopping production,
112 Managing CO in Meteorological and Topographical Problem Areas redesigning, repairing vehicles already produced, and recalling those that have been sold. In addition, EPA uses data from state vehicle emissions inspections and maintenance (EM) programs, technical service bulletins, and voluntary emissions recall reports to target testing of in-use vehicles so that in-use compliance can be verified. Recently, manufacturers have been required to develop a general program for in-use testing of customer-owned vehicles. The committee is unaware of any in-use cold-temperature compli- ance testing and is concerned about the lack of such programs. Motor-Vehicle Tn-Use Controls While tougher emissions certification standards are the primary means by which CO emissions have been reduced over the last three decades, in- use vehicle controls have provided additional reductions and ensured vehi- cles are properly maintained. Clean fuels programs and I/M programs are the mainstays of nationwide in-use vehicle controls. In some regions, where winter temperatures are frequently below 20°F and cold-starts con- tribute substantially to the regional emissions inventory, engine preheating also provides valuable emissions reductions. Vehicle Emissions Inspection and Maintenance Programs Vehicle emissions I/M programs are designed to identify vehicles that have higher than allowable emissions and ensure that they are repaired or removed from the fleet. They are the most common measure implemented by state and local governments to reduce CO emissions. I/M programs attempt to control emissions throughout a vehicle's lifetime by ensuring that the vehicle' s emissions control system is maintained and repaired when necessary. These programs are implemented in nonattainment areas and in other areas seeking to improve air quality. The inspection traditionally involves regularly scheduled exhaust tests administered at a certified testing facility. The test measures CO, HC, and sometimes NOX emissions. I/M tests also include a visual inspection of the components controlling evapo- rative and exhaust emissions and may include a functional gas-cap test and a pressure test of the evaporative emissions control system. New testing technologies, such as those using onboard diagnostic (OBD) systems and remote sensing, are also being used.
Management of CO Air Quality 113 The three basic types of EM programs currently in operation are cen- tralized, decentralized, and hybrid. A centralized program consists of a relatively small number (relative to a decentralized network) of stations that perform only emissions tests. Vehicles that fad] the inspection must be repaired elsewhere. This program typically is operated by a government entity or by a contractor with government administration. A decentralized testing program consists of a larger number of low-volume stations that do both emissions testing and vehicle repairs. This type of program links testing to the repair process and is operated by private sector stations. Finally, a hybrid program is one that incorporates elements of both decen- traTized and centralized programs (NRC 2001~. Traditional Exhaust Emissions Tests I/M programs began with an idle test for HC and CO. This test uses a tailpipe probe to measure the steady-state concentrations of CO, HC, and carbon dioxide (CO2) emitted from idling vehicles. A high-idle test in which engine speed is manually increased to approximately 2,500 revolu- tions per minute (rpm) is sometimes performed in addition to the natural or "Iow-idle" test; in traditional idle testing, there is no load applied to the engine.4 NOX concentrations are not measured during idle tests because NOX emissions are low during no-Ioad idle conditions. Because the idle test cannot assess the performance of NOX control technologies, it has been superseded by the IM240 and ASM tests. The IM240 is a Toaded-mode transient dynamometer test that measures the mass of CO, HC, and NOX emissions collected over a simulated 240-second (s), 2-ml driving cycle. The ASM (acceleration simulation mode) test involves the acceleration of a vehicle on the dynamometer to a steady-state speed while measuring exhaust concentrations of CO, HC, and NOX. More than 30 states now operate AM programs with a wide assortment of test designs. An NRC committee that recently looked at I/M programs concluded that they generally have not achieved the emissions reductions originally projected (NRC 2001~. For example, in-program and remote- 4A loaded-mode test, such as the IM240 or the FTP, involves testing vehicle emissions while the vehicle is on a dynamometer that simulates the load a vehicle is under during on-road operation.
114 Managing CO in Meteorological and Topographical Problem Areas sensing data estimated that the Colorado I/M program achieved reductions of 4-l I% compared with a modeled estimated benefit of 17-34% (Stedman et al. ~ 997, 1998; ENVIRON ~ 998; Air Improvement Resources ~ 999~. In addition, there is a need to assess the emissions reduction benefits of I/M for vehicles operating in cold temperatures. However, that committee identified a great need for continuing programs that repair or eliminate high-emitting vehicles from the fleet given the major influence ofthis small fraction of the fleet on total emissions and air quality. The NBC (2001) report estimates that the dirtiest 10% of LDVs produce 50-60% of on-road LDV exhaust emissions. The report also discusses studies that combine data for vehicle ownership, high-emitter frequency, and income levels These studies suggest a strong link between low household income and the likelihood of owning a high-emitting vehicle. Because of this link, the NRC (2001) report recommended that the cost-effectiveness and equity impacts of such policies be explored to provide financial or other incen- tives, such as repair assistance programs, for motorists of high-emitting vehicles to seek repairs or vehicle replacement. Onboard Diagnostic Systems New technologies providing faster, more convenient, end more accurate emissions testing continue to be developed. Many make use ofthe onboard diagnostics (OBD) systems in vehicles. The enhanced onboard diagnostics system installed in model year 1996 and newer vehicles, known as OBDII, can help to detect problems that increase CO emissions. OBDII uses sen- sors to monitor and modify the performance of the engine and emissions control components. The onboard computer monitors signals from the sensors and actuators to identify sensor and control-system failures, illumi- eating the malfunction indicator light (MIL) on the vehicle dashboard, and storing the fault codes (known as diagnostic trouble codes) for later analy- sis. In a garage setting, mechanics can download the OBDU fault codes from the onboard computer with a diagnostic analyzer or "scan tool." The codes identify emissions control systems and components that are malfunc- tioning. It should be noted that OBDII is only a diagnostic system and does not, in the absence of an OBD I/M program, require a vehicle owner to repair an emissions-related problem detected by the system. If an OBD I/M program is operating properly, OBDII inspections should fail vehicles if the vehicle's emissions control components are or
Management of CO Air Quality 115 have been malfunctioning or if the sensors monitoring emissions control components are malfunctioning. In contrast, traditional I/M emissions- testing programs inspect actual vehicle emissions for violations of stan- dards set by individual states. It should be noted that vehicles equipped with OBDII have the ability to maintain low emissions (relative to older technology vehicles) even after a system component has failed. EPA (65 Fed. Reg. 56844 t20013) recently finalized an OBD rule that requires states to implement OBD testing in I/M programs for 1996 and newer OBD-equipped vehicles. However, the proposed replacement of traditional emissions-testing programs with OBDII programs has been controversial. The major issues, identified in the NRC (2001) study end by EPA's Mobile Source Technical Review Subcommittee (EPA 2002e), include the following: (1) the lack of overlap in some studies of vehicles that fad] both the OBD EM tests and traditional tailpipe emissions tests; and (2) the significant fraction of vehicles that failed OBD I/M tests with actual emissions below the vehicle's certification standards. In addition, some components of OBDlI systems (such as exhaust-gas recirculation and O2 sensors) are often disabled by the engine computer during conditions under which the manufacturer cannot guarantee the com- ponents' performance If. Cabaniss, Association of International Automo- bile Manufacturers, personal communication, July 10, 2001~. That tends to be the case for vehicles operating at temperatures below 20°F. There is a need to understand the behavior and performance of OBDU at low tem- peratures, especially if many northern locations begin adopting OBD I/M systems. When a significant number of sensors become inoperative, the OBDlI system's ability to alert vehicle owners of potential emissions-sys- tem failures is diminished. Remote Sensing Remote sensing has also been used as a new I/M testing method. Re- mote sensing is a technique that measures emissions from individual vehi- cles as they drive by a roadside sensor. It offers the possibility of testing vehicle exhaust emissions without requiring the vehicle's presence at a testing facility, though the test is only for a relatively short time (approxi- mately ~0.5 seconds) during which the remote-sensing beam passes through the exhaust. Remote sensing is most accurate for measuring CO. It is currently being used in Colorado and Missouri to identify clean vehicles
116 Managing CO in Meteorological and Topographical Problem Areas that may opt to avoid visiting an emissions testing station for scheduled testing. For example, in the St. Louis area, if a vehicle has two or more successive low-emissions readings measured by remote sensing, the vehicle owner can opt to be excused from scheduled emissions testing. The implementation of remote sensing for identifying high emitters in Arizona, however, was terminated after 5 years by state legislators because of problems including high costs, false failures, and difficulties finding appropriate remote-sensing sites. From mid-May 1998 through early June 1999, over 2 million valid remote-sensing test records were collected, but only 2,987 vehicles were identified as high emitters (Wrona 1999~. Owners of vehicles identified as high emitters were sent letters ordering them to submit their vehicles for lM240 testing within 30 days. About half (55°/O) of vehicle owners responded within that time period; ~ 5-20% of vehicle owners complied later, after their vehicle registration was suspended. Of vehicles that reported for testing, 42% passed the initial IM240 test. A survey indicated that one-third of those vehicles underwent repairs prior to the test (Wrona 1999~. Other vehicles may have been repaired, but owners may not have reported that on the survey. Besides application as a testing device, remote-sensing measurements can be useful for characterizing vehicle emissions, including average emis- sions by mode! year and the fraction of high emitters in the vehicle fleet. Remote sensing can also help assess AM program effectiveness and esti- mate the extent of certain types of program noncompliance.
Management of CO Air Quality I 17 Fuels Vehicle emissions control has also occurred through fuels reformula- tion. This includes increasing the oxygen content of fuels to promote more complete fuel combustion, reducing sulfur content to improve catalyst efficiency, and switching to fuels that inherently produce less CO during vehicle operations. Oxygenated Fuels In 1988, the use of oxygenated fuels (or oxyfuels) was instituted in Colorado to reduce winter CO levels and was subsequently extended to other areas of the United States that were exceeding the NAAQS for CO (typically during winter). EPA mandated that oxyfuels contain an oxygen- ate (normally either methyl tertiary-butyl ether fMTBE1 or ethanol) with oxygen content of 2.7% or more by weight. Adding an oxygenate to the gasoline increases the oxygen-fuel ratio in the combustion process, changing the combustion chemistry and decreasing the emissions of CO formed during incomplete combustion. A 1997 study of the winter oxyfuels program initiated by the White House Office of Science and Technology Policy concluded that at temperatures above 50°F, CO emissions from most vehicles were reduced by about 3-6% per weight percent oxygen (NSTC ~ 997~. CO emissions reductions of 3-7% are pre- dicted by EPA's MOBILES model for the 2010-2015 fleet, mainly because of reduced emissions from pre-1994 vehicles, cold starts, and malfunction- ing vehicles.5 Emissions reductions are generally lower in newer-technol- ogy vehicles (those with closed-Ioop fuel control and three-way catalysts) and higher in high-emitting, older-technology vehicles (those with perma- nent open-loop fuel control and two-way catalysts). O2 sensors and on- board computers in later models control the air-fuel ratio to prevent fuel- rich operations. Earlier versions of the MOBILE model predicted much larger benefits from oxyfuels. In a review of the winter oxyfuel program, the Office of Science and Technology Policy found that the observable reduction in ambient CO levels that could be attributed to the use of fuel oxygenates was lower than the amount pre- dicted by the MOBILE5a model by a factor of 2 or 3 (NSTC 1997).
118 Managing CO in Meteorological and Topographical Problem Areas There is a lack of information on the effectiveness of oxyfuels at tem- peratures below 50°F. EPA (Mulawa et al. 1997) tested three vehicles at 20°F, 0°F, and -20°F at its cold-weather facility using unleaded gasoline containing 10% ethanol (3.5°/O oxygen by weight). Two ofthe cars showed substantial improvement in CO emissions; the third showed none. The Colorado Department of Public Health and Environment found an average 11% decrease in CO emissions by switching to 10°/O ethanol blended fuels in 24 vehicles that it tested at 35°F (Ragazzi and Nelson ~ 999~. One of the problems with these oxyfue] studies is that the vehicles used might not be operating the most current control systems and might be even less represen- tative of Tier 2 vehicle technology. Theory suggests that oxygenated fuels would provide emissions benefits under extreme cold-start conditions be- cause cars run under open-Ioop conditions for longer periods. However, available data are not sufficient to support or refuse that argument. Because CO has become less of a problem in many places, the number of new stud- ies looking at the effectiveness of oxyfuels at low temperatures has de- creased considerably. Although oxyfuels provide some air quality benefits, concerns have been raised about the widespread use of MTBE as an oxygenate. The offensive odor and taste of MTBE, and the potential adverse effects of MTBE leaking into drinking water supplies have raised questions about whether the benefits gained from using MTBE (reducing high ambient CO concentrations ~ or 2 days per year) are greater than the possible negative consequences. Reformulated Gasolines Reformulated gasoline (RFG) is mandated by EPA for use in areas exceeding the NAAQS for ozone. RFGs must meet a number of require- ments both in fuel composition (benzene < 1.0% and oxygen > 2.0% by weight) and in reduction of exhaust emissions of VOCs and air tonics, including benzene. In practice, this translates into an RFG aromatic content of <25%, and fuel sulfur concentrations of about 30 ppm (compared to >350 ppm for non-RFGs). Kirchstetter et al. (1999) concluded that the use of reformulated gasolines in California led to reduced CO emissions from hot-stabilized vehicles, consistent with the body of data from the Auto/OiT Air Quality Improvement Research Program (NRC 1999~. The decrease in CO emissions associated with RFGs is in part due to their lower sulfur content compared with regular gasolines (NRC ~ 999~.
Management of COAir Quality 119 Low-Sulfur Fuel A key finding of the Auto/Oil project was that reducing fuel sulfur decreases exhaust emissions (Benson et al. 1991~. Sulfur in gasoline ad- versely affects the efficiency of vehicle emissions control systems by poi- soning the catalyst. This decreases pollutant conversion efficiency and potentially lengthens the time needed after ignition for the catalyst to be- come effective. The 1991 Auto/OiT study concluded that reducing sulfur concentrations from 450 ppm to 50 ppm would result in a 13% decrease in CO exhaust emissions in 1990 Tier 0 technology vehicles. In addition, low-sulfur fuel is expected to lengthen lubricant and engine life as well as reduce emissions of HC, NOX, hydrogen sulfide, sulfur dioxide, sulfuric acid aerosols, and other air tonics. Reversing the effects of sulfur on cata- Tytic performance requires fuel-rich conditions and aggressive accelerations that achieve high catalyst temperatures (about 1,200°F). However, sul- fur's effects are not easily reversed in the newer-model Tower-emissions vehicles (Truex 1999~. To guard against the poisoning effects of sulfur, it is best to operate these newer-model vehicles on low-sulfur fuel only. To address concerns about the increased sensitivity of the newer-tech- nology vehicles to sulfur poisoning, EPA included new fuel standards requiring refiners to meet an average sulfur concentration of 30 ppmbegin- ning January I, 2006, in its Tier 2 proposals (65 Fed. Reg. 6697~200011. However, additional studies on the effect of high-sulfur gasoline on catalyst efficiency and light-off time in cold climates are necessary. Hybrid Gasoline-Electric Vehicles Hybrid vehicles combine a conventional internal combustion engine with an electric motor. The internal combustion engine can be run on various alternative fuels; however, this discussion relates to those vehicles powered by a normal gasoline-fueled engine alternating or in concert with an electric motor, with the battery system being charged by the gasoline- powered engine. Compared with a regular gasoline-fueled vehicle, this arrangement allows greater gasoline mileage to be achieved while maintain- ing Tow exhaust emissions. Hence, hybrid vehicle emissions are compara- ble to emissions from gasoline-fueled vehicles with the same emissions rating (see DOE/EPA k2002] for emission ratings of available hybrid vehi- cles). The committee was unable to find emissions data for hybrid vehicles
120 Managing CO in Meteorological and Topographical Problem Areas under cold-weather temperature conditions like those encountered in Fair- banks. However, the extreme cold temperatures may make use of the elec- tric motor/batteries infeasible. Compressed Natural Gas and Liquified Petroleum Gas Vehicles fueled by compressed natural gas (CNG) and liquefied petro- leum gas (LPG) are inherently cleaner, at least in terms of reactive HC emissions and ozone formation, than gasoline-fueled vehicles. As shown in Table 3-3, CO emissions, estimated using the FTP from five CNG-fueled vehicles (one 1999 passenger car and four 1994-1995 pick-up/light-duty trucks) and three LPG-fueled trucks, were significantly lower than the federal CO emissions standard. However, it is unknown how these vehi- cle's emissions rates will deteriorate with increasing mileage. Collateral Emissions Reductions from Emissions Standards and In-Use Controls As noted in Chapter 1, CO controls can reduce emissions of other pollutants generated during fuel-rich or cold-start engine operation, such as PM, benzene, 1 ,3-butadiene, polyaromatic hydrocarbons (PAHs), and alde- hydes. A recent assessment by EPA found that one ofthe major sources of air tonics exposure nationally is automobile emissions (EPA 2000c). The agency recently finalized regulations on the control of air-toxics emissions from mobile sources (66 Fed. Reg. 17230 t200 11~. The EPA analyses show that programs already in place, such as the reformulated gasoline program, the national low-emissions vehicle program, and the Tier 2 emissions stan- dards and fuel sulfur controls, will yield significant reductions of mobile- source air tonics. Table 34 displays emissions reductions estimated to occur due to existing federal programs for four selected mobile-source air tonics. Quantifying air toxic s emissions reductions from fuel reformulation is rather uncertain at present. The primary toot used for that assessment is the COMPLEX mode! from EPA. It is based on a rather limited range of auto- mobile control technologies and temperatures (NRC 2000) and should be updated to provide better estimates for emissions responses for air tonics, including CO's response across a range of relevant conditions. Model im-
Management of CO Air Quality 121 TABLE 3-3 Summary of FTP Emissions Results for the Test Fleet in the AFV Study ModelYear Vehicle Fuel CO (g/mi) 1999 Honda Civic GX CNG 0.026 1995 GMC Sonoma PU CNG 0.977 1994 Dodge Caravan M~nivan CNG 0.200 1994 Dodge Ram 350 van CNG 0.913 1994 Dodge Ram 350 van CNG 0.217 2000 Ford F-150 XL LPG 0.145 1999 Ford F-250 XLT LPG 0.420 1992 Chevrolet S10 PU LPG 0.492 Abbreviations: AFV, alternative Mel vehicles; CNG, condensed natural gas; FTP, federal test procedure; GMC, General Motors Corporation; LPG, liquified propane gas. Source: J. Norbeck, University of California, Riverside, unpublished matenal, 2002. provements wit! improve the assessment of the collateral impact of CO emissions reductions on air tonics. It should be noted that some CO control technologies might lead to increased emissions of other pollutants. For example, oxygenates can increase NOx emissions, which in turn can increase concentrations of both ozone and particulate nitrate (NRC ~ 999~. In addition, the use of oxygen- ated fuels for control of CO is expected to decrease emissions levels of PAHs and benzene but increase the levels of certain emitted aldehydes. Thus, the picture is complex and strongly argues for an integrated approach to air quality management that does not isolate pollutants. Transportation Control Measures Transportation control measures (TCMs) are actions designed to change travel demand or vehicle operation characteristics to reduce motor- vehicle emissions, energy consumption, and traffic congestion. Transporta- tion agencies are increasingly experimenting with new TCMs, some of which are listed in Table 3-5 . TCMs include transportation-supply improve- ment (TSI) strategies and transportation-demand management (TDM)
122 Managing CO in Meteorological and Topographical Problem Areas TABLE 3-4 Estimated Percent Reduction for Selected Toxics (for Nationwide On-Highway Vehicles Only) Cumulative Percent Reduction from 1990 Compound 1996 2002 2007 Benzene 33% 65% 73% Fonmaldehyde 33% 69% 76% Acetaldehyde 23% 58% 67% 1,3-Butadiene 35% 67% 72% Source: 65 Fed. Reg. 6697 [20003. strategies. TS1 strategies attempt to reduce emissions by changing the physical infrastructure of the road system to improve traffic flow and to reduce stop and go movements. In contrast, TDM strategies attempt to reduce the frequency and length of automobile trips by changing driver behavior using regulatory mandates, economic incentives, voluntary pro- grams, and education campaigns. TSI strategies can be grouped into three categories: traffic signaTiza- tion, traffic operations, and enforcement and management. Traffic signalization strategies include programs to optimize the timing of individ- ual traffic signals and to coordinate traffic signals over a designated area Traffic operations strategies include converting two-way streets to one-way streets, restricting left turns, "channelizing" roadways and intersections, and selectively widening roadways and intersections to reduce bottlenecks. Enforcement and management strategies include incident management programs to reduce delays from accidents and other roadway incidents, ramp metering to improve the flow of traffic on freeways, and general enforcement oftraffic andparkingregulations. These techniques have been used for decades and are considered cost-effective strategies for reducing congestion, but their effects on vehicle emissions are difficult to measure and predict (EPA ~ 99Sb). The Federal Highway Administration (FHWA) has described methods for estimating the emission reductions of many TCMs for the Washington, D.C., area (FHWA 1995~. Demand-management measures include, but are not limited to, no drive days, employer-based trip-reduction programs, parking management, park and ride programs, work-schedule changes, transit-fare subsidies, and public-awareness programs. These measures fall into four categories: trip- reduction mandates, market incentives, voluntary programs, and education
Management of CO Air Quality 123 TABLE 3-5 Transportation Control Measures (TCMs) IMPROVED PUBLIC TRANSIT Incentives for single occupancy vehicle commuters to use convenient and rea- sonably priced mass transit alternatives. The three major ways of increasing ridership on public transit are (1) system/service expansion, (2) system/service operational improvements, and (3) inducements to increase ridership. TRAFFIC FLOW IMPROVEMENTS Strategies that enhance the efficiency of a roadway system, without adding ca- pacity, including traffic signalization, traffic operations, and enforcement and management. HIGH OCCUPANCY VEHICLE (HOV) LANES Roadways dedicated for HOV use. INTELLIGENT TRANSPORTATION SYSTEMS Traffic detection and monitoring, communications, and control systems. Exam- ples include traffic signal control, freeway and transit management, and elec- tronic toll collection systems. BICYCLE AND PEDESTRIAN PROGRAMS Includes sidewalks, bicycle lanes, and bicycle racks. COMMUTE ALTERNATIVE INCENTIVES Incentives, usually employer based, to encourage commuters to carpool or use transit services. TELECOMMUTING Working at home using electronic communication instead of physically travel- ing to a distant work site. GUARANTEED RIDE HOME PROGRAMS Ensures transportation (e.g., taxi or transit passes) for carpooling employees in the case of an unforeseen circumstance. WORK SCHEDULE CHANGES Adjusting hours worked to control peak emissions. Examples include stag- gered hours, flextime, and a compressed workweek. TRIP REDUCTION ORDINANCES (REGULATORY MANDATES) Regulations that attempt to adjust personal travel decisions through employer- based incentive/disincentive programs. CONGESTION PRICING Financial disincentives to driving on highly used roadways, or priced alterna- tives to a congested roadway. (Continued)
124 Managing CO in Meteorological and Topographical Problem Areas TABLE 3-5 Continued PARKING PRICING Programs that encourage single-occupant vehicle users to switch to over means of Ravel by imposing fees for parking or that encourage shifting times for vehicle starts away from peak CO penods. PARKING MANAGEMENT Allocation of parking spaces intended to encourage single-occupant vehicle users to use other means of travel. Source: EPA 2001g. and exhortation campaigns (Guensler 1998~. In general, trip-reduction mandates, such as trip-reduction ordinances that require employers to estab- lish demand-management programs, have not proved effective in the United States (GuensTer 1998~. However, these mandates may be effective for very large employers, such as universities and major government centers. Market incentives, including transit-fare subsidies, may offer greaterpoten- tial. Various strategies for increasing the direct cost of drivingmarket disincentives have also been proposed but are rarely implemented. The success of voluntary control strategies depends on consumer behavior and the availability of alternatives, so public education and exhortation pro- grams figure prominently in all of these strategies. In a recent review of the Congestion Mitigation and Air Quality Tm- provement Program (CMAQj, which funds TCMs in ozone and CO non- attainment areas, the Transportation Research Board (TRB 2002) found broad support for the program among transportation planners, air quality officials, and interest groups. Although the limited evidence available suggested that TCMs were less effective in terms of costs per emissions reduced compared with other emissions reduction strategies, the program offers the opportunity for nonattainment areas to experiment with nontradi- tional transportation approaches to pollution control. In addition, CMAQ also funds some promising TCMs that receive limited if any support from traditional transportation funding sources. Most TCMs have been developed for large metropolitan areas and for areas in nonattainment for ozone; TCMs for Los Angeles were described by Bae (1993~. Smaller regions and regions facing CO problems must adapt these TCMs or develop ones specific to their needs. In Fairbanks, Alaska, for example, CO emissions in winter are substantially increased by
Management of CO Air Quality 125 cold starts. The Fairbanks North Star Borough has adopted a "plug-in" program as one of its primary transportation control measures. Vehicles do not start readily at temperatures below 0°F, so residents install electric engine block heaters to keep their engines warm when parked for extended periods oftime. The borough's plug-in programinvolves two components. The first is a public education campaign to encourage residents to plug in their vehicles at temperatures from 0°F to 20°F, when CO emissions are high, even though vehicles can start without being plugged in. The second component is an ordinance requiring large employers to install electric outlets in their parking lots (NRC 2002~. Public Education Programs Public education programs are designed to increase public awareness and understanding of air quality problems and may lead to changes in be- havior that result in emissions reductions. Available evidence suggests that public awareness and understanding levels of air quality problems are low. A study conducted by the U.S. Department of Transportation and EPA as a part of the Transportation and Air Quality Public Information Initiative concluded that citizens do not understand the link between transportation choices and air quality, are largely unaware of the range of alternatives to solo driving available in their communities, and do not place a high priority on air quality and transportation issues (DOT/EPA 20021. For example, although Fairbanks has a fairly active public-information campaign con- cerning the connection between vehicle plug-ins at temperatures above 0°F and improved air quality, most individuals responding to a survey said they plugged in for ease in starting their vehicles (ADEC 2001~. Public education programs have been implemented in numerous metro- politan areas throughout the United States by local, regional, and state governments as well as nonprofit organizations such as the American Lung Association, with help from the federal government. In May 1999, the FHWA, Federal Transit Administration (FTA), and EPA's Of flee of Trans- portation and Air Quality developed the "It all adds up to cleaner air" pro- gram (DOT/EPA2003~. This program provided federal support in the form of market research, advertising, a "Comprehensive Resource Toolkit," an orientation workshop, andlimited funding for 14 demonstration communi- ties and provided materials related to public education to many others. However, the cost-effectiveness of public education programs has not been documented.
126 Managing CO in Meteorological and Topographical Problem Areas Episodic control programs aim to change travel and other kinds of behavior on days when exceedances of air quality standards are possible. These programs, also called "action day" programs, largely depend on public service announcements and other forms of public education but may also involve incentives to change behavior (e.g., free transit fares). As of 1996, at least 35 regions in the United States had implemented or were developing episodic control programs, in maintenance areas as well as nonattainment areas (EPA 1 997b), and by 2002, the number had grown to well over 50 (EPA 2002f). Most of these programs target ozone, a more pervasive problem than CO, but the Air Pollution Control Division (APCD) ofthe Colorado Depart Eminent of Public Health and Environment issues advi- sories for CO and PM in winter months that activate mandatory wood- burning restrictions and call for voluntary driving reductions (Regional Air Quality Council 2002~. Rigorous evaluation of the effectiveness of these programs is not available, but EPA has said that episodic controls "have the potential of being more effective in reducing short-term air quality viola- tions" than long-term emissions-reduction measures (EPA 1 997c). In addi- tion, these programs may be more acceptable to the public than long-term restrictions on driving or gasoline use (EPA 1997b). In order to implement an episodic control program, a regional air qual- ity agency must be able to predict when conditions will be conducive to an exceedance. In most programs, alerts are issued the day before and depend primarily on weather conditions, including winds, temperature, and cloud cover. An ability to predict exceedances with perfect accuracy is not essen- tial, but calling too many alerts is likely to reduce the effectiveness of the program. As discussed in the committee's interim report (NRC 2002), the Fairbanks North Star Borough has called 16 alerts over the past four win- ters; four were correct (an exceedance actually occurred), and 12 were not.6 During the same period, three exceedances occurred that were not forecast. In 1997, EPA established a policy for incorporating voluntary mea- sures, such as public education programs and episodic control programs, into state implementation plans (SIPs) and giving SIP credits for them. Called the Voluntary Mobile Source Emission Reduction Program, its aim is to make it easier for state and local governments to achieve air quality 6While it is possible that emissions were reduced in response to broadcast alerts, a preliminary analysis of vehicle traffic does not indicate significantly less driving on alert days.
Management ofCOAir Quality 127 goals by providing greater flexibility in determining the best measures for their communities. The policy allows as much as 3% ofthe total reductions needed for attainment to be from voluntary mobile-source programs. To claim credit, states must provide areaTistic estimate ofthe emissions impact and commit to monitoring the success of the program and remedying any shortcomings (EPA 19976~. Land-Use, Urban Population Growth, and Sprawl The sprawling patterns of land development typical of metropolitan areas in the United States contribute to high levels of automobile travel and thus to air quality problems. The defining characteristics of "sprawl" in- clude Tow-density development, unlimited outward expansion, and "leap- frog" development (Burchell et al. 20021. Most metropolitan areas in the United States are growing faster in land area than in population. Between ~ 982 and 1997, urbanized land increased by 47%, while population grew by only 17% (Fulton et al. 2001~. This Tow-density pattern of growth has two important effects on travel: longer trip distances and greater reliance on the car. Land-use policies are increasingly recognized as a consideration in formulating an overall strategy to combat congestion and are also now recognized by EPA as a tool for improving air quality. "Smart growth" programs designed to counter sprang] are popular throughout the United States. These programs use both regulations (such as zoning) and financial incentives to encourage development within existing urbanized areas, which is conducive to public transit, biking, and walking. Smart growth strategies have the potential to reduce vehicle travel by reducing trip dis- tances and reliance on the car (EPA 2001i). However, the full impact of
128 Managing CO in Meteorological and Topographical Problem Areas such programs is uncertain. In recognition of their potential to reduce emissions, EPA now allows state and local communities to account for the air quality benefits of smart growth strategies in SIPs as a part of the Vol- untary Mobile Source Emission Reduction Program (EPA 2001g). The benefits of smart growth strategies may be more likely to accrue at the regional level than they are at the local level. Smart growth policies may lead to higher densities of development in certain areas within the metropolitan region and thus to potentially higher levels of vehicle traffic in those areas. The increase in traffic could, in turn, lead to higher local- ized concentrations of CO and other motor-vehicle pollutants. It is thus possible that smart growth strategies will prove effective in reducing re- gional levels of ozone but at the time result in the creation of new areas of high CO and related pollutants. However, given the continuing reduction in CO emissions through improvements in vehicle controls, the possibility that such areas would produce CO exceedances seems remote for most locations. Control of Stationary and Area Sources Virtually any process that burns fossil fuels or biomass will produce CO, though in varying quantities. The more efficient the combustion pro- cess, the lower CO emissions will be. Thus, because of high combustion temperatures, power plants and most other industrial processes have very low CO emissions relative to the amount of fuel burned.7 As was discussed in Chapter I, the situation in Birmingham, Alabama, is an exception. An industrial source (a mineral-woo! production facility) has been the cause of numerous violations of the CO NAAQS. Many industrial facilities have controlled CO emissions, either directly or indirectly, in efforts to reduce VOC emissions. Residential sources, including wood burning fireplaces, coal, oil and gas-fired space heaters, and lawn and garden engines, also produce CO emissions. Lawn and garden engines have the greatest share, approx- 7It should be noted that, since the dissociation of CO2 to CO and O2 is endo- thermic, at sufficiently high temperatures the CO to CO2 ratio can be appreciable. High temperatures during combustion also account for the production of NO and NO2 from N2 and O2 in other endothermic reactions.
Management of CO Air Quality 129 imately ~ ~ million tons per year (about half of the nonautomotive fraction). Those emissions, as well as emissions from forest fires, typically occur in summer when CO does not approach nonattainment levels. However, other sources such as chain saws and generators may be used year-round. Oxy- genated fuels might reduce CO emissions from these sources. Although space heating, particularly from wood burning, comprises a small part ofthe inventory (about 3%), timing and spatial scale in high-CO areas can make that contribution more significant, particularly in places like Fairbanks, Alaska; Missoula, Montana; Denver, Colorado; and other areas where wood burning for both recreational and functional purposes is com- mon. Substituting cleaner-burning fireplaces and stoves or switching to natural gas can reduce CO and PM emissions. These controls can have a greater role in reducing human exposure in cases where the emissions are trapped in a confined space (e.g., indoors). Missoula, Montana, for exam- ple, has had a persistent problem with PM, and their emissions inventory for CO in 1990 attributed 28% of CO to residential wood burning (Therriault 2002~. As a result, the city banned installing new wood stoves and discouraged the use of those already in operation. This reduced the contribution of residential wood burning to 1 8°/O of the CO emissions in- ventory by 1996. MONITORING, MODELS, AND INVENTORIES There are three important tools for air quality management of CO and other pollutants: monitors, models, and inventories. Monitors provide mea- surements of ambient pollutant concentrations. These measurements- made either on a short-term special-project basis or at permanent sta- tionscan provide assurance that ambient concentrations do not pose a health risk to vulnerable members of society and can show where levels are high enough to potentially put an area in nonattainment of the NAAQS. Mathematical models are used for a variety of purposes locally and regionally from estimating current or future emissions for inventories to computing pollutant concentrations that can be expected for a given (time- gPM~o is a subset of PM that includes particles with an aerodynamic equivalent diameter less than or equal to a nominal 10 micrometers.
130 Managing CO in Meteorological and Topographical Problem Areas dependent) emissions rate and set of meteorological conditions. Forecasts can be made to enable officials to announce air quality alerts or to demon- strate that proposed mitigation measures will reduce ambient concentra- tions sufficiently to bring an area into attainment or to maintain attainment. Emissions inventories are assessments that identify the sources of an air pollutant in a given area and their annual contributions to total emis- sions. Inventories can be very helpful to policy-makers by showing which sources produce the most pollution and therefore which mitigation mea- sures are likely to be most effective. Emissions inventories are also a use- fu] tool in conformity determinations when compared with the emissions budget (Howitt and Moore 1999~. CO emissions rates for inventories or other purposes are seldom measured directly; they are estimated using models and/or emissions factors.9 Models are verified and improved by comparing their results with measurements of actual concentrations. Thus the three major tools of air quality managementmonitoring, models, and inventories are linked. Monitoring At the heart of air quality management strategies are ambient measure- ments. Air pollution monitoring is done at different temporal and spatial scales, depending upon the planned use of the data collected. Short-term monitoring is often used for specific projects, and long-term monitoring at permanent stations is used to determine air quality trends. in many cities, monitoring has been in place for decades, although some smaller cities were identified as CO problem areas in the l 990s and have shorter histori- cal records. CO concentrations in most cities are falling far below exceed- 9Emission rates can be determined directly by measuring airflow rates and compositions (e.g., from a power plant smokestack or vehicle tailpipe). However, the development of emissions inventories relies on emissions factors, which express CO as the mass of an emission per unit of activity, for example, grams of CO per mile driven, or tons of CO per million kilowatt-hours (kWh) of electricity gener- ated. The mass of CO could be calculated as the emissions factor times the annual number of miles driven or the number of kWh generated.
Management of CO Air Quality 131 ance levels, so reduced support for CO monitoring is being considered (EPA 2002g). Short-Term Monitoring Short-term microscaTe monitoring is sometimes done to compile project-specific information. This type of sampling was much more prevalent in the late 1970s and the 1980s when corridor analysis for highway projects was being carried out. Now emphasis is placed on using models to predict specific, microscaTe pollutant concentrations. Large, controversial roadway projects may still include monitoring to assess local air quality, and research for microscaTe modeling still depends on measure- ments for mode! verification and validation. The primary purpose of short-term microscaTe monitoring is to deter- mine the existing concentrations of pollutants near a planned activity, such as roadway construction. Often, this type of monitoring is conducted near hot spots in the vicinity of the project. Sampling is typically done upwind and downwind near the existing source, often at multiple locations, to determine its emissions contribution. The upwind sites represent the back- ground level (the concentration before the contribution of the source). Sampling times are related to source activity periods and are usually on the order of hours or days. Sampling may be reported at intervals as small as 1 min. Usually a very limited number of pollutants is measured, CO being the most common. Measured 1 -hour and 8-hour concentration averages can be directly compared to the NAAQS. Short-term saturation studies are especially important for understanding how meteorology, topography, and emissions activities affect the distribu- tion of CO emissions and their effects in a location. Saturation studies attempt to develop a detailed understanding of the horizontal and vertical distribution of CO and to explore the role that physical and human factors play in elevated concentrations. Because they can also be used quaTita- tively in assessing the human exposure to CO, they have already been dis- cussed in Chapter 1. Permanent Monitoring Stations Fong-term, permanent stations are used for monitoring in most large urban areas in the United States as part of EPA's ambient air monitoring
132 Managing CO in Meteorological and Topographical Problem Areas program.~° They are often referred to as state and local air monitoring stations (SLAMS) and are quite extensive, as shown in Figure 3-5. The primary purpose of these stations is to determine attainment status and monitor air quality trends in the area, but the data serve other purposes. They are applicable in the following: · Activating emergency control procedures that prevent or alleviate air pollution episodes. · Observing pollution trends throughout the region, including non- urban areas. . Providing a database forresearch evaluation ofthe effects of urban, land-use, and transportation planning; development and evaluation of abate- ment strategies; and development and validation of diffusion models. . Determining highest concentrations expected to occur in the area covered by the network. . Determining representative concentrations in areas of high popula- tion density. · Determining the impact of significant sources or source categories on ambient pollution levels. · Determining general background concentration levels. Once placed, monitoring stations are only moved under special circum- stances. At many of the stations multiple criteria pollutants are measured along with several meteorological variables. The data from most sites are transmitted to databases maintained by EPA. Some of the information can be accessed through the aerometric information retrieval system (AIRS) (EPA 2002h). AIRS allows long-term trends to be examined; local area average concentrations to be established; effects of abasement measures and impacts of local sources to be evaluated; and unusual occurrences to be recorded. Whereas the results from short-term monitoring tend to be used for planning and research, data from long-tenn monitoring stations can have a direct impact on federal policy. Because these stations are used to deter- mine air quality trends and to establish attainment status, the measure- ments made there play a large role in federal air quality management. For '°See EPA 2002g for a description of EPA's ambient air monitoring program, its objectives and proposed changes.
Management of CO Air Quality 133 I /1~ ~ /e ~ ~ ~ ~ - o ~ Y' ems ~ £~ I. ~ ~ ~ ~-\ \~ ~ i K-. ~1 A~':4 -'a'' ':L!-~ ~ .. - ~ sits :,~: I-~ Am.' `~ .':; Odor ·~¢ ', (1) FIGURE 3-5 Locations of state and local air monitoring stations (SLAMS) and national air monitoring stations (NAMS). Source: EPA 2001a. example, trends exhibited at permanent monitoring stations were part ofthe basis for the changes made to the national ozone standard. Models In recent years, much more emphasis has been placed on modeling than on monitoring to determine local concentrations. Accordingly, model development and appropriate use are crucial to the overall air quality man- agement process. Models for mobile sources, stationary sources, and re- gional impacts are prescribed by EPA in Appendix W of the Code of Fed- eral Regulations, Title 40, Part 51. States are allowed to use these pre- ferred models to estimate local area concentrations and to compare them with the NAAQS. Efforts to evaluate the effectiveness of CO air quality management are inherently interdisciplinary. As shown in Figure 3-6, estimating CO emis-
134 Managing CO ir' Meteorological and Topographical Problem Areas CO AIR QUALITY ESTIMATES Travel-Demand Estimation Model MOBILE On-Road Mobile Model Emissions Estimates Vehicle-Miles of X Emissions Rates = Carbon Monoxide Travel and Average by Speed and from On-Road Speeds ~ L: ~ Vehicles | Nonroad Mobile l_ ~ Area Source 1_ | Stationary Source l_ _ Meteorology Input Data _ Rollback, Dispersion, or Air Quality Model Carbon Monoxide Concentrations FIGURE 3-6 Use of models in He estimation of ambient CO concentrations. signs and assessing their impacts on air quality require the interaction of three different models and the related areas of expertise: travel-demand models and other methods of estimating activity levels, emissions models, and air quality models. Travel-Demand Forecasting Determining emissions estimates requires data on vehicle activity, usually vehicle-miles traveled (VMT). Although direct traffic counts can be used to estimate existing emissions, travel-demand techniques are neces- sary to predict future traffic volumes. For existing facilities, past trends can be analyzed and extrapolated to future conditions. In the case of new facili- ties or future traffic volumes, demographic data is used to provide the loca- tions of households and employment in small traffic-survey zones within the urban region and to forecast regional economic growth, land-use pat- terns, and future demographic trends. The change in traffic is estimated using the four-step travel-demand modeling process that encompasses. · Trip generation. The estimation of the number of trips by zone per time of day and type (both trips originating in a zone, called trip produc- tion, and trips terminating in a zone, called trip attraction).
Management of CO Air Quality 135 . Trip distribution. The pairing of trip productions with trip attrac- tions resulting in a full spatial pattern oftrave] by purpose and time of day. Mode choice. The mode oftrave] used, specifically walk, bicycle, single occupancy vehicle, high occupancy vehicle (HOV), bus, rail, or truck. . . Route assignment or choice. Trips are assigned to paths in the transportation infrastructure by minimizing travel times, or travel times and costs, and incorporating average speed and other impedance feedbacks. Travel-demand models provide the emissions factor mode] (typically the MOBILE model) with information on average VMT and vehicle speeds for each roadway segment that can be aggregated by roadway type or faciT- ity (e.g., freeways, arterials, collectors, and freeway ramps). At a microscale level, traffic simulation models can be used to estimate emissions producing activities. These models assess queuing and traffic flow along specific roadway segments or at specific intersections. The models are also combined with instantaneous emissions models to predict emissions inventories (NRC 2000~. Emissions Factors Models The emissions inventory is an important tool for estimating the relative impacts of emissions controls. Inventories predict the total mass emitted annually from all contributing sources of a particular pollutant, such es CO. Changes in sources can then be quantified in terms of the reduction in the mass released, and the control options that yield the greatest reductions can be determined. Estimated future total-mass emissions can be compared with existing emissions to determine the overall effectiveness of a mitiga- tion program. However, as noted previously in this report, uncertainty and errors in emissions models are likely substantial. More evaluations com- paring modeled emissions with observations as well as more uncertainty analysis of emissions models are needed. On-Road Emissions Moclels Because mobile sources contribute such a large fraction of CO emis- signs and depend on many more factors than typical stationary sources, special computer programs have been developed to estimate the appropriate
136 Managing CO ir' Meteorological and Topographical Problem Areas emissions factors. In 49 states, the MOBILE series of models is used MOBILE6 is the latest version (EPA 2002i).~ California, the one excep- tion, uses EMFAC2002.~2 Both MOBILES and EMFAC2002 forecast emissions factors for many different types of vehicles and fleet mixes by age, fuels, and operating conditions. Outputs of these models can be used to estimate CO emissions (in grams per mile) for starting, idling, and run- n~ng. Uncertainties and errors in MOBILE are well documented (e.g., Hallock-Waters et al. 1999; NRC 2000; Sawyer et al. 2000; Holmes and Russell 2001; Frey and Zheng 2002; Parrish et al. 2002; Pokhare] et al. 2002~. Concerns over these issues prompted a recent NRC committee charged with reviewing MOBILE to recommend that enhanced model evaluation studies, in tandem with uncertainty studies, begin immediately and continue throughout the long-term evolution and development of mobile-source emissions models (NRC 2000; Holmes and Russell 2001~. The committee recommended that such studies be done with oversight and guidance from an outside reviewing body that includes users and technical experts. MOBILE6 contains many improvements on previous versions of the model. However, Dulia and Heirigs (2002) expressed concern that MOBILES understates the benefits of technology improvements on CO emission rates, especially in areas with colder climates such as Alaska where the benefits of off-cycle controls are assumed to be negligible during winter driving. The modest decline in CO emissions rates forecast by MOBILE6 is much lower than what would be expected given the introduc- tion of control-system technologies designed to meet KIEV and Tier 2 requirements. One reason for the modest projection is that forecasts of CO emissions rates are extremely sensitive to gasoline sulfur content levels when mode! runs use the standard assumptions for Fairbanks (including no benefits Tom the SFTP). Because MOBILE6 forecasts for CO emissions in Alaska are largely a function of projected gasoline sulfur levels and have little to do with improvements in emissions control designs, Dulla and Heirigs (2002) are concerned that projected CO emissions rates understate the benefits of NLEV and Tier 2 technologies and may overstate the level of local controls needed. 'iSee EPA 2002j for a detailed description ofthe MOBILE model. i2See CARB 2002 for a description of the EMFAC model.
Management of CO Air Quality 137 Nonroad Sources Airports are important nonroad sources of CO emissions. To help in the preparation of airport emissions inventories, the Federal Aviation Ad- ministration has released the Emissions Dispersion Modeling System (EDMS), Version 4.~3 This computer model includes emissions rates for most aircraft types, ground support equipment, motor vehicles (based on MOBBED, typical stationary sources at airports, and training fires. The model requires operational data for each of these sources and generates an emissions inventory for the airport. EDMS contains a Gaussian dispersion model, which is discussed in the following section, and also requires mete- orological data to predict local CO concentrations. Construction equipment contributes a large amount of nonroad emis- sions. Until recently, these sources were mainly ignored because their emissions were thought to be of short duration. Large multiyear projects, such as the "Big Dig" in Boston (Big Dig 2000), have greatly changed this outlook. Studies have shown that construction equipment can exacerbate local air quality problems, and off-road sources are now considered more often than they were before. The contributions of other nonroad engines and off-road vehicles listed in Table 1-2, such as off-road recreational vehicles (snowmobiles, in particular), lawn and garden equipment, and boats, have also come under review as additional CO reductions are sought. In some locations, their emissions can be substantial. As emissions from on-road vehicles decrease because of tighter emis- sions standard, fuel-sulfur controls, and less deterioration of emissions control devices, the emissions estimation techniques for nonroad mobile sources will continue to increase in importance. The NRC (2000) con- cluded that, primarily because of a lack of data, the current off-road emis- sions model (NONROAD) does not accurately estimate emissions from off- road vehicles and nonroad emissions inventories or the effects of emissions controls on these sources. That report recommended a major effort be un- dertaken by EPA to update NONROAD with better population and activity data and real-worId emissions factors. EPA has been developing enhance- ments to the NONROAD model to improve emissions estimates.~4 i3 See FAA 2002 for a description of the EDMS. '4See EPA 2002k for a description of NONROAD.
138 Managing CO in Meteorological and Topographical Problem Areas Stationary Sources Stationary sources, especially power plants and large industries, may also have a large impact on local CO concentrations. As previously men- tioned, stationary-source emissions factors can be determined from AP42 (EPA 1995~. Stationary-source operations are usually more consistent then mobile-source operations, thus stationary emissions are easier to quantify. Activity, such as fuel usage (often in the form of BTUs generated or amount of fuel consumed per year), is multiplied by an emissions factor to estimate the total mass of CO emitted per year. However, CO exceedances in Birmingham, Alabama, demonstrate that an unregulated point source that experiences process upsets can become a large emissions source sufficient in itselfto create CO exceedances. Utilizing emissions factors from AP42 would underestimate the contribution from sources such as the one in Bir- mingham. In addition, estimating emissions from area sources, such as residential heating, is likely to be highly uncertain. During this study, the committee noted that the emissions inventory for Missoula, Montana, attributed ~ 8°/0 of CO emissions to wood burning, whereas the inventory for Fairbanks, Alaska, attributed only 3°/0 of CO emissions to that source. The disparity existed despite Missoula's fairly substantial effort to control emissions from wood stoves. The committee also questioned whether the increasing popularity of fuel-oi! stoves has resulted in the underestimation of this source in inventories. It is clear that emissions inventories for stationary sources need improvement. Air Quality Models Air quality modeling is an essential element of air quality management. Models can be used to evaluate plans for attainment of an NAAQS (also referred to as an attainment demonstration), to evaluate the effects of new construction projects, and to conduct further research into what causes pollution episodes and how they can be predicted. A number of modeling techniques requiring various levels of scientific expertise, input data, and computing resources are available for these purposes. The simplest mod- els, rollback models, assume a direct correlation between emissions and ambient pollutant concentrations; the most complicatedmodels, grid-based air quality models, resolve temporal and spatial variations in pollutant concentrations and the effects of meteorology, emissions, chemistry, and
Management of CO Air Quality 139 topography. Models are also characterized by the size of the problem they address: microscale models simulate pollution from a point source or intersection; mesoscale models simulate metropolitan or multistate pollu- tion; and large-scare models simulate continental or global pollution. In attainment demonstrations presented in SIPs, states are required by EPA to model how emissions reductions will lead to the desired air quality improvements. Three types of models have been used to demonstrate at- tainment ofthe CO NAAQS: rollback (also knows as statistical rollback), Gaussian dispersion, and numerical predictive models. Rollback Models The simplest of the three models used for attainment demonstrations is the statistical rollback mode] in which the needed reduction in emissions is assumed to be proportional to the required reduction in ambient CO concentrations (ADEC 2001~. CObaSeYear CONAAQS /0 reduct~orl= C°baseyear Background where CObase year = the second highest 8-hour average in the base year; CONA 4QS = the NAAQS of 9 ppm (or sometimes 9.4 ppm); and Cobackground = an average regional background CO in the absence ~ . . 01 emissions. Although easy to implement, rollback models do not explicitly consider the role of meteorology or the spatial heterogeneity of CO emissions and con- centrations. EPA has allowed states to use rollback models rather than the more resource-intensive dispersion and urban-airshed models described below, to demonstrate attainment in smaller cities. An improvement on the simple rollback model is the probabilistic rollback model used in CO mod- eling for the Puget Sound area of the State of Washington (Ioy et al. 1995~. Gaussian Dispersion Models A second type of model that has been used for CO-attainment demon- strations is a Gaussian dispersion model, which is typically used to simulate CO concentrations for microscale analysis in the vicinity of intersections
140 Managing CO in Meteorological and Topographical Problem Areas or along major traffic corridors (EPA 1992~. One of the first effective Gaussian dispersion models for mobile sources was CALrNE3, which is still in use. Inputs for this model include meteorological data, such as wind speed and atmospheric inversion strength in the vicinity of the pollutant source, and temporally resolved emissions. Emission factors developed from other emissions models (MOBILE and EMFAC), along with traffic volumes, roadway geometries, and intersection information, are used to determine the emissions along a roadway. Dispersion modeling then in- cludes transport and mixing to calculate local concentrations. The model is Gaussian in nature, meaning it assumes that a plume of pollutant gas released from a point source can be described by a widening Gaussian function (a bell-shaped curve) as it travels downwind (Wayson 1999~. The model also makes the assumption that roadway segments can be cut into small sections with a point source approximation applied to each and their plume concentration contributions summed at a receptor site. This concept allows roadway curves or winds nearly parallel to the roadway to be mod- eled effectively. The shortcoming of CALINE3 is that it is only useful for vehicles that are moving at a constant rate of speed. At locations of high CO emissions (such as intersections), increased emissions due to vehicle delay and idling must be accounted for. To do that, two models are in use today: CAL3QHC and CALLNE4. Both use the same general approach to estimate dispersion as CALINE3 does. CAL3QHC is used in 49 states, and CALLNE4 is used in California. Gaussian dispersion models are typically used for local area (micro- scaTe) analysis and are used extensively in CO-related evaluations, includ- ing project-level conformity determinations. Modeling is done for the worst hour to compare with the 1-hour average CO NAAQS. Worst-case conditions (a windspeed of 1 MPH and a stable atmosphere) are often used. A persistence factor, whichis a multiplier ofthe peak 1-hour concentration that is based on changes in wind patterns and traffic, is used to estimate an 8-houraverageconcentrationforcomparisonwiththeS-hourNAAQS. The model results often determine whether a project can go forward. The American Meteorological Society (AMS) policy statement on dispersion modeling (Henna 1978) concluded that these models are accu- rate within a factor of 2 for reasonably steady horizontally homogeneous conditions; however, they will be less accurate, for example, when obstacle wakes flows (e.g., from buildings or vehicles) and extremely stable thermo- dynamic lapse rates occur. Dispersion accuracy will also be Tower, as listed
Management of CO Air Quality 141 in the AMS statement, for "dispersion over forests, cities, water and rough terrain." Grid-based Air Quality Models The most complicated models used for attainment demonstrations simulate how a pollutant concentration varies with time and space over an entire urban area. These numerical predictive models, generally intended for regional analysis, can simulate emissions from multiple sources end the dispersion, advection, and photochemical reactions of gaseous pollutants in the atmosphere. These models are integrated separately from meteoro- Togical models. Grid-based models, such as Models-3 and the urban air- shed mode] (UAM), have been used for many years to simulate 03, which is a region-wide or mesoscale pollutant. The UAM has been adapted to simulate CO in Denver (Colorado Department of Public Health and Envi- ronment 2000~. Because ofthe local nature of high-CO episodes, extensive modeling ofthe entire urban airshed may be unnecessary for CO-attainment demonstrations. Airshed modeling is resource-intensive, requiring detailed knowledge of an area's meteorology (usually based on the output of a mesoscaTe weather model constrained by observations), spatially and temporallyresolved emissions inventories, and measurements ofthe pollut- ant at several locations to allow model evaluation. Highly trained person- net are needed to conduct the simulations. More complicated models are not always appropriate for attainment demonstrations, but they can be valuable in improving the understanding of the interactions among atmospheric processes. Even better research tools than the numerical predictive models described above (such as Models-3 and the UAM) are process numerical models, which allow pro- cesses specific to air quality modeling and meteorology to be coupled within a single computational framework. Process numerical models typi- cally are formulated by adding pollutant emissions, chemistry, and trans- port into an existing meteorological model rather than simply using the meteorological data as a mode! input. The relatively nonreactive behavior of CO makes it an ideal chemical species for simulation in a weather model. Predictions of CO, for example, can be straightforward in the Na- tional Weather Service Eta model, Is which has a horizontal grid framework Resee NWS 2002 for information on the NWS Eta model.
142 Managing CO in Meteorological and Topographical Problem Areas of 12 x 12 km over the contiguous United States. However, this resolution is insufficient for most CO problem areas. Initial work to simultaneously simulate atmospheric flow and diffusion of CO at high spatial and temporal resolution is described by Fullerton (20021. Box Models Box models are another tool available for microscale analysis of air pollution. The "box" is some volume of air into which emissions are in- jected. Box models may divide a region into cells of equal volume and use mass balances to treat the transfer of CO among cells. In their simplest application, they can consist of a single box. The cells may also be sepa- rated in the vertical direction. Air within each cell is assumed to be well mixed. Simplifications ofthis concept lead to the common single-cell box model. Though box models are not used in attainment demonstrations, they are particularly useful to understand how various emissions scenarios and meteorological conditions affect pollutant concentrations. For example, a box model for CO in Anchorage, Alaska, has been used to quantify how mechanical turbulence from roadway traffic might increase the mixing height and reduce CO concentrations on severe-stagnation days compared with concentrations observed in residential neighborhoods (Morris 2001~. Appendix C describes a single-cell box model, with and without recirculation. The committee's interim report on Fairbanks describes the application of such a model to Fairbanks, Alaska (NRC 2002~. Summary of Air Quality Models There is no single air quality model that is the best for CO for all Toca- tions. Typically the choice depends on the severity of the problem, the available data, and the resources available for modeling. It its interim report (NRC 2002), the committee recommended that Fairbanks, Alaska, use a simple box-model approach for air quality planning purposes in the near term. A box model simulates the effects of emissions end meteorology in a well-mixed controlled volume. The committee felt that such an ap- proach could provide greater insights into the effects of the timing of CO emissions and of meteorological variables, in this particular situation, given the limited vertical dispersion and available data. Box models could sup-
Management of CO Air Quality 143 plement Fairbanks's current approach of using a simple rollback model, which they used in their attainment demonstration (ADEC 20011. In the long-term, the committee recommended that more work be done to develop, apply, and evaluate more sophisticated, physically comprehen- sive models that would simulate how CO concentrations vary with time and space. Because CO is relatively conservative on time scales of hours, knowledge ofthe temporal and spatial distribution of CO emissions and of the observed CO concentration field provide an effective diagnosis of atmo- spheric dispersion patterns. For chemical species that are eliminated by reactions in the atmosphere, knowledge of the CO dispersion provides an observational constraint on the concentration fields ofthe reactive species. The committee concluded that more physically comprehensive models should be used for planning, forecasting, and assessing human exposures to high CO concentrations. It is important that mode] development and testing be specific to the extreme conditions that occur in CO problem areas such as Fairbanks. However, model development must occur in concert with improved monitoring to enable model evaluation. The committee believes that even in areas such as Fairbanks, which has experienced very few exceedances since 1996, and none since 2001, the development of comprehensive models is still worthwhile. The number of periods of ele- vated CO levels experienced in Fairbanks indicates that the city is still susceptible to exceedances. Furthermore, CO modeling can be used to better understand and characterize CO hot spots as well as other criteria pollutants and air tonics. The development of a better modeling approach today will benefit all problem areas in the future. Despite advances in air quality modeling capabilities over the last 30 years, many improvements are still possible and necessary. One problem is that the vertical and horizontal resolution of models is too coarse to capture the variability in pollutant concentrations, which is necessary to identify local hot spots and is important for determining local concentra- tions downwind of hot spots. In addition, the validity of mode] representa- tion becomes questionable when unusual meteorological conditions occur, and that could lead to errors in the prediction (Pielke 2002~. Models used for regulatory purposes can suffer a loss of realism as a result of such short- comings, leading to costly errors in planning. Models also need more real- istic three-dimensional dynamics (advection, pressure gradient forcing' turbulences and more realistic parameterizations of smaller-scale processes (e.g., turbulence fi om buildings, radiative flux divergence changes in the temperature profile associated with aerosols in the lower levels ofthe atmo-
144 Managing CO in Meteorological and Topographical Problem Areas sphere). The models also need higher spatial and temporal resolution. Ensemble runs of the models should be performed to provide a more realis- tic envelope of simulated dispersion patterns. However, the committee recognizes that this adds cost and time to the evaluation. Not only can these models be used for air quality applications, models with higher reso- lution can also assist in homeland defense because they can help understand the dispersion of accidental or deliberate releases of chemical, biological, and radiological materials. In 2003, a large dispersion research project will be undertaken to help define important dispersion parameters, primarily for homeland security purposes (DOE/DOD 2002~. The project will be a month-Ion" study con- ducted by a combination of federal and state governmental agencies with support from multiple universities. Research will include releases oftracer gases with careful measurements of meteorological parameters to determine dispersion trends for city-wide dispersion, dispersion in street canyons, infiltration to buildings, and effects of topography. Statistically Robust Methods to Assist in Tracking Progress The air quality models described above assess the effectiveness of emissions controls and the prospects for attaining the CO standard by repre- senting critical processes within a physically based model of the system. An alternative to those physical models is to take a statistical approach assessing the relationship among human activities, CO emissions, meteorol- ogy, and ambient air quality, as described below. Probability of an Exceedance Reddy (2000) carried out an analysis of the probability of a future CO exceedance in Denver that might be broadly applicable to other areas. The analysis took into account the historical variability in CO concentrations as a result of meteorology and unusual traffic events. The purpose of his analysis was to determine the risk of a CO exceedance associated with eliminating or altering the oxyfuels program during the first week in Febru- ary for the future years 2002-2013. He used CO monitoring data from the CAMP site (AIRS ID 08-031-002), which is the site in Denver that has historically shown the greatest number of exceedances. He used daily peak
Management of CO Air Quality 145 8-hour average CO concentrations for the first week in February for the 20- year period of 1975-1994. Because these values depended on the emissions during those years in addition to stochastic meteorology and occasional unusual traffic, Reddy corrected past CO concentrations for each year to what they would have been if the emissions for that year had been the same as those projected for 2002. The natural logarithms of the corrected peak 8-hour average CO con- centrations were normally distributed; the 8-hour averages themselves were not. By estimating future emissions inventories for the years 2002-2013, based on projected fleet composition and VMT, and assuming that the lognormal distribution would hold for future years, Reddy was able to calculate the probability of an exceedance on a single day of the first week in February (P~ ~ for the future years. He then used Equation 1 to compute the probability of one or more exceedance days during an entire week (P79 P76 = 1 - (1-P~ 97. (1) Reddy found a better than 5°/O chance that an exceedance might occur if Denver immediately suspended the oxyfuels program for the period 2002- 2013. The study also found that Denver would likely not have an exceed- ance if 1.5% oxygen (which is less than the oxygen content used in the current oxyfuels program) was used in fuels for 2002 and 2003 before suspending the use of oxyfuels for 2004 through 20 ~ 3. Equation ~ assumes that exceedance events are independent over time (thus the probabilities can be multiplied, as in the second term on the right hand side of the equation). The assumption might not hold, for example, exceedance events might be positively associated over time. Given this possibility, Reddy' s method might overestimate P7 a. Alternatively, we can modify Reddy's equation as follows: Expected number of exceedances = N X Pi 4, (2) where N denotes the number of days in the time period being considered, under the assumption that the probability of exceedance is uniform over the time period. For Reddy's application, the time period considered is the first week in February, thus N= 7. Under more general conditions, Equation 2 can be modified as follows:
146 Managing CO in Meteorological and Topographical Problem Areas Expected number of exceedances = I. ~ Pi ~ (3) where Pi denotes the probability of exceedance on the i-th day. Equation 3 does not assume that the exceedance probability is uniform over time. For example, one might use a different exceedance probability for week- days versus weekends. The same procedure that Reddy used, or the modified one discussed above, could be applied to monitoring sites in other cities and for times other than the first week in February (e.g., a whole winter season), provided that there are enough historical data to establish the distribution of CO concentrations and to estimate emissions inventories for past and future years. Meteorological De-trending Ambient CO concentrations across the nation are going down. Un- doubtedly many of these reductions are due to emissions controls. Part of the kend, however, may also be meteorological. A warmer winter with less stagnation can lead to Tower winter CO levels. As noted by Neff (2001), Denver may be experiencing lower CO levels than would be expected from emissions reductions alone because of warmer winters with greater vertical mixing. How can the impact of meteorological trends on the observed concentrations be removed in order to assess the impact of emissions con- trols and to show true progress towards meeting air quality standards in the future, when meteorological conditions may not tee so favorable? One must "de-trend" the observations. Meteorological de-trending is accomplished by identifying how meteo- rological variables impact pollutant concentrations and removing the influ- ence of those variables. One way would be to create a physically realistic model that can simulate many years, developing emissions-to-air quality relationships and showing how they respond to meteorological influences. However, this approach would be cumbersome and would introduce signifi- cant uncertainties. The influence of meteorology is more typically identi- fied using an empirical approach. Many years worth of concentration data are analyzed, along with the corresponding meteorological data, to develop a statistically based model. That model is then used to remove meteorolog- ical impacts (Kuebler et al. 2001; Porter et al. 2001).
Management of CO Air Quality 147 Recent work by Flaum and colleagues used a multistep process to resolve the trends in ozone (03) into four components: a long-term trend, presumably due to emissions controls; a seasonal component; a component driven by meteorological fluctuations; and a noise component (Flaum et al. 19961. Kuebler et al. (2001) used a similar approach, not only for 03, but also for CO, NOX, and VOCs, and compared the meteorologically de- trended concentrations of the primary pollutants with the trends in emis- sions estimates. From that, a direct relationship between the emissions levels and pollutant concentrations could be established. The latter approach appears appropriate here given its prior use for CO, though the explanatory variables may depend on location. For example, in Fairbanks, a nonlinear response to temperature is expected because CO concentrations appear to be highest at about -20°F to 20°F, not at much lower or much higher temperatures. This approach is convenient for local air quality management organizations because it requires relatively little data (e.g., a long-term record of CO concentrations and meteorological variables such as temperature and windspeed would suffice, though more factors are useful) and nominal computational power. The de-trending analyses also can provide extra information for air quality planning. As noted above, de-trending can be used to help develop
148 Managing CO in Meteorological and Topographical Problem Areas probabilities of exceeding the NAAQS for CO at various emissions levels. From that, the necessary level of emissions can be identified in a more statistically robust fashion.
