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4
The Future of Carbon Monoxide
Air Quality Management
When regulations on carbon monoxide (CO) automobile emissions began
in the 1960s, large areas within many cities were experiencing high CO
concentrations. Motor-vehicle emissions controls over the past three de-
cades have greatly reduced ambient CO concentrations. As shown in Chap-
ter 1, the number of monitors showing CO violations has fallen to only a
few, and the monitors that still show violations do so much less frequently.
CO control has been one of the greatest success stories in air-pollution
control. As a result, the focus of United States air quality management has
shifted to characterizing and controlling other pollutants, such as tropo-
spheric ozone, fine particulate matter (PM2 5),} and air tonics.
However, as described in Chapter 2, some locations will continue to be
susceptible to violations ofthe CO health standard because of meteorologi-
cal and topographical conditions that produce severe winter inversions. In
addition, CO remains relevant to air quality managers because it acts as an
indicator of a host of other mobile-source-related emissions, and its control
produces substantial collateralbenefits. Understanding the distribution and
effects of CO exposure will be a continuing challenge. The fixed-site com-
1
IPM2 5 is a subset of particulate matter that includes those particles with an
aerodynamic equivalent diameter less than or equal to a nominal 2.5 micrometers
(~m).
149
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150 Managing CO in Meteorological and Topographical Problem Areas
pliance monitoring system, although important for understanding overall
emissions and air quality trends and area-wide compliance with standards,
is incapable of capturing all locations that exceed the standards and fully
characterizing the spatial variability of CO. The recent exceedances in
Birmingham, Alabama, indicate that there may be locations that have fre-
quent, unmonitored exceedances. Personal exposures to CO and related
pollutants are also not represented by these fixed-site monitors. In this
concluding chapter, the committee focuses on the issues related to expo-
sures and CO management that will be important in the future.
EXPOSURES OF CONCERN IN THE FUTURE
Proximity to CO emissions sources determines human exposure pro-
files, bloodlevels of carboxyhemogIobin, andriskofadverse health effects
In short, place makes the poison (Smith 2002~. Personal exposures may
vary by individual depending on occupation and personal habits. Individu-
als who have long commutes or who drive for a living can be exposed to
CO levels well in excess of those measured at fixed-site monitors. Opera-
tion of nonroad sources, such as construction equipment, gasoline-powered
lawn and garden equipment, snow blowers, snow machines, and other
recreational vehicles, may result in significant personal exposures. Malad-
justed home heating units operated in confined spaces and unventilated
homes remain sources for high CO exposures, as does cigarette smoking
These sources expose individuals to high concentrations but have no mea-
surable effect on any fixed-site monitor.
In addition, CO concentrations are not uniform across a region, and hot
spots with higher levels of ambient CO may occur at discrete locations
Hot spots often occur in places with high vehicle traffic or other local
sources, especially when topographical and meteorological conditions are
conducive to CO accumulation. Demographic data (see Table 1-8 in Chap-
ter 1) also indicate that the monitors that have recently shown violations of
the health-based CO NAAQS tend to be in low-income urban areas. Be-
cause CO is a good indicator of exposure to other air tonics generated by
mobile sources, hot spots may identify locations where individuals are at
higher risk for adverse health effects from a number of urban air pollutants
The committee recommends an active program to identify and charac-
terize hot spots and to better define the upper end of the CO exposure dis-
tribution. Current CO monitoring technologies are sufficiently advanced
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The Future of CO Air Quality Management 151
that accurate mobile monitoring can be deployed in major urban areas and
can be combined with temporary fixed-site monitors to produce the data
necessary to simulate actual exposures experienced by the general popula-
tion. Monitor-equipped vehicles can traverse randomly sampled routes and
destinations in the region, sampling CO concentrations as they move
through the road system. A national program employing mobile, temporary
fixed, and possibly personal monitors would provide tremendous benefits
for exposure assessment, health impacts analysis, model evaluation, and
attainment and maintenance planning.
FUTURE CO MANAGEMENT ISSUES
Future management of CO will contend with both the changing nature
of the CO problem and the changing nature of the air quality management
system. This section discusses the roles of new-vehicle emissions stan-
dards, oxygenated fuels, and transportation-control measures in managing
CO; the appropriate spatial scale for CO management; and the possibility
that increasing VMT and other factors may counter the decline in vehicle
CO emissions per mile. The chapter concludes with a section discussing
the integration of CO into the overall management of air quality in the
United States.
Improvements in Vehicle CO Emissions
Emissions from light-duty trucks and passenger cars will continue to
be the focus of CO management in the future. Current CO controls of
importance include Tier I, NLEV, cold-temperature standards, the SFTP,
and I/M and/or OBDII. Of greatest importance for future CO emissions
control are the effectiveness of cold-temperature, Tier 2 vehicle emissions
standards and the use of low-sulfur gasoline under meteorological condi-
tions conducive to CO buildup.
The cold-temperature CO standards have provided significant reduc-
tions in emissions during the first few minutes of engine operation at low
temperatures. For northern cities such as Fairbanks, Alaska, a more strin-
gent CO cold-start regulation (or a lower cold-start test temperature) would
be beneficial for further reducing emissions under conditions that favor
exceedances. The committee discussed this option in its interim report on
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152 Managing CO in Meteorological and Topographical Problem Areas
Fairbanks and concluded that toughening standards should be considered
in other geographical areas that might also benefit. In Chapter 3 of this
report, the committee reviewed testing information on cold starts and on
new emissions control technologies.
In the absence of compelling evidence, the committee cannot recom-
mend making the cold-start CO standards more strict. In the future, addi-
tional fleet-average CO emissions reductions will come from the increased
number of vehicles certified to cold-temperature standards and from the
introduction of emissions control technologies and low-sulfur fuels that
will be adopted to meet the Tier 2 standards. However, supplemental test-
ing should be done to assess emissions performance below 20°F. Testing
should also be done to determine whether existing onboard diagnostic
(OBD) systems operate properly at 20°F. In addition, CO emissions reduc-
tions from Tier 2 vehicles must be confirmed, especially during cold starts
below 20°F.
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The Future of CO Air Quality Management 153
Oxygenated Fuels Program
The use of oxygenated fuels (or oxyfuels) is required in all areas of the
United States that exceed the NAAQS for CO. Oxygenated fuels programs
have declined in effectiveness and are expected to continue to decline as
more modern vehicles enter the fleet.2 Therefore, the question arises:
Should a mandatory oxygenated fuels program continue?
An oxygenated fuels program aimed at reducing winter CO emissions
appears to be of decreasing value. However, malfunctioning vehicle emis-
sions systems, which might benefit from use of oxygenated fuels, could
dominate vehicle emissions in the future. The committee concludes that
EPA should undertake a review ofthe science end policy behind the current
oxygenated fuels programs to determine the conditions under which these
programs are cost-effective. Low-temperature testing, especially below
20°F, is recommended. The review should also determine when these
programs will no longer be cost-effective because of changes in fleet tech-
nology.
Transportation Control Measures for Reducing CO Emissions
Further reductions in emissions may be aided in the future by transpor-
tation control measures (TCMs). TCMs seek to reduce tailpipe emissions
2An oxygenated fuel is a gasoline containing an oxygenate, typically methyl
tertiary-butyl ether (MTBE) or ethanol, that is intended to reduce production of CO.
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154 Managing CO in Meteorological and Topographical Problem Areas
per mile through improvements to traffic flow and to reduce vehicle-miles
traveled (VMT) through the management of transportation demand. How-
ever, TCMs have accounted for only a small share ofthe overall reductions
in emissions. Studies show that traffic-signal coordination and control
strategies can reduce fuel use from 8°/O to 15% in specific corridors. These
strategies would presumably reduce CO hot spots as well. However, total
regional impacts of these control strategies might only be a 1% to 4°/O re-
duction in fuel use (Cambridge Systematics, Inc. 20011. Although the
empirical evidence is limited, the most successful efforts to manage trans-
portation demand have probably resulted in reductions in VMT of consider-
ably less than 1% (Cambridge Systematics, Inc. 2001~. Many TCMs, par-
ticularly pricing strategies, have proved to be unpopular and politically
infeasible, and those that have been easy to implement, such as voluntary
trip-reduction programs, have shown limited effectiveness. On the bright
side, transportation agencies are increasingly experimenting with new
TCMs, some of which are listed in Table 3-5. The challenge for these
agencies in the future will be to adapt and combine TCMs effectively to
meet the specific needs of their region.
Spatial Scale for CO Management
One critical issue is determining the most appropriate spatial scale for
CO management national, regional, or local. Although national-scale
controls, especially vehicle emissions certification standards, have played
and will play a crucial role in eliminating most exceedances of the CO
standards, future CO attainment will continue to depend on regional and
local control strategies. However, the design of appropriate strategies
depends on a more thorough understanding ofthe sources of CO emissions
in specific areas and the meteorological, topographical, and human factors
that contribute to the formation of hot spots.
The current process of using regional CO emissions inventories for the
analysis of localized exceedances demonstrates this issue. When regional
inventories are used, some sources that do not contribute to local
exceedances might be included in the control plan. In the case of Fair-
banks, the committee noted that the state implementation plan (SIP) emis-
sions inventory contains some sources elevated well above the atmospheric
inversion height (which may be as Tow as 10-20 feet) as well as some dis-
tant sources (over 10 miles away). In Fairbanks, the same meteorological
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The Future of CO Air Quality Management 155
conditions that trap automotive exhaust near the ground keep emissions
from elevated sources from reaching the ground. Determining what sources
actually contribute to an exceedance at a site would require wind pattern
and/or tracer studies. Given the nature ofthe meteorology in Fairbanks, the
committee feels that it is unlikely that all of the sources used in the SIP
actually contribute to CO exceedances. Such difficulties illustrate the need
to improve the spatial and temporal resolution of emissions and meteoro-
logical variables through improved monitoring and modeling in locations
that still exceed the CO standard.
A better understanding of the factors that contribute to localized CO
problems can provide a basis for the development of more effective control
strategies. For example, CO exceedances at the Lynwood, California,
monitor are attributed to a combination of high vehicle emissions and local
topographical and meteorological conditions. Little is known, however,
about the major sources of traffic moving through the area. Strategies to
address emissions from through-traffic might include regional controls as
well as local improvements to traff~c-signal controls. Strategies to address
emissions from locally owned vehicles might include vehicle buy-back or
repair assistance programs targeted to eliminate high emitters.
The committee advises against focusing solely on local controls that
only affect sources in a small geographical area. Given the inability ofthe
fixed monitoring stations to represent the full spectrum of exposures and
the mobility ofthe largest source of CO (the automobile), such an approach
would not ensure that the wider region is adequately protected against CO
and related pollutants. The elimination of hot spots will undoubtedly re-
quire region-wide as well as location-specific efforts.
Impact of Increasing VMT and Longer Fleet Turnover
Advances in motor-vehicle emissions control technology have reduced
CO emissions faster than VMT has increased. Vehicle emissions of CO are
an order of magnitude less now than they were in 1970 for the same VMT.
However, as VMT increases, emissions reductions resulting from control
equipment might not be enough to compensate, and overall CO emissions
may begin to increase at some point in the future. The increased durability
of vehicles may also play a role; owners are keeping their vehicles longer.
The result is that a smaller percentage of lower-emitting vehicles are enter-
ing the fleet each year, and the average fleet age is increasing. Present-day
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156 Managing CO ir' Meteorological and Topographical Problem Areas
vehicles show a deterioration in the effectiveness of emissions controls
over time, which is also likely in future vehicles.
An aging fleet and increased VMT could result in an exacerbation of
the CO problem in existing nonattainment areas and an increased risk of a
return to nonattainment status in maintenance areas. As described in Chap-
ter 3, Davis (2001 ) documents the increase in the national average age and
median lifetime of in-use passenger cars. The Energy Information Agency
(EIA 2003) forecasts an increase in VMT by LDVs by 2.4% per year
through 2020, which is larger than the 2.2% growth projected just 1 year
earlier (EIA 2001~. Some agencies have forecast that on-road CO emis-
sions will begin to rise again after about 2005 due to increasing VMT (New
York State Department of Environmental Conservation 1999; Colorado
Department of Public Health 2000; City of Fort Collins 2001~. However,
those estimates are based on earlier versions of the MOBILE model. The
same result will not hold when emissions projections are updated using
MOBILES, because MOBILES overestimated the increase in CO as vehi-
cles age (the deterioration rate). Albu (2002), in contrast, forecasted fleet
turnover would continue to decrease mobile-source CO emissions in CaTi-
fornia through 2020. CO problem areas with a greater percentage of older
vehicles or where VMT is rapidly increasing because of population growth
or other behavioral changes (localized increases in traffic) are expected to
be the most susceptible to exceedances.
INTEGRATING CO CONTROL INTO THE
OVERALL AIR QUALITY MANAGEMENT SYSTEM
Historically, all criteria pollutants have been considered independently.
As such, CO tends to be managed in isolation even though other pollutants
have similar emissions sources and CO can play a substantial role in the
formation of ozone (03) (NRC 1999~. Because hydrocarbon emissions
have dropped over the past few decades, CO is causing a more significant
fraction ofthe tropospheric O3 in urban areas; yet CO emissions reductions
are typically not pursued for the control of urban O3. Further, CO control
can reduce emissions and resulting exposures from other mobile-source
pollutants.
The ability to use similar assessment tools and controls for copollutants
can provide savings for areas that have related problems. For example,
areas facing both CO and PM2 s problems will use regional PM2 5 modeling
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The Future of CO Air Quality Management 157
to account for the impacts of controls and regional growth on PM2 5 levels.
Including CO in such modeling requires few additional resources and could
help identify hot spots for other pollutants. In addition, the primary source
of CO fuel-rich operations of light-duty vehicles is a major source of
other pollutants of concern, including PM2 5 and air tonics. There are many
control programs that provide benefits for all of these pollutants, including
those aimed at reducing cold-start emissions, removing or repairing high-
emitting vehicles, and improving the effectiveness of the vehicle catalyst
(e.g., with low-sulfur gasoline).
Thus, the committee recommends that CO be better integrated into the
management of other related pollutants. The committee recognizes that the
focus of air quality management in the near future will be on attaining the
new PM2 5 and 8-hour O3 standards as well as reducing air tonics. How-
ever, winter inversion conditions that characterize the remaining high-CO
areas not only affect the build up of CO but they are also related to higher
concentrations of PM2 5 and some air tonics. The committee recommends
that EPA assess the relationship of CO to these other pollutants of concern
when the CO criteria are updated.
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158 Managing CO in Meteorological and Topographical Problem Areas
Finally, the issue of integrated air quality management is important in
considering the reallocation of CO monitoring resources. Very few areas
in the country continue to violate CO standards. Because CO nonattain-
ment is very unlikely in many regions, state and regional air quality plan-
ning agencies have expressed interest in reducing or eliminating CO moni-
toring in their areas. From a scientific standpoint, current CO monitors
provide valuable information for long-term air quality management plan-
ning. The marginal savings of eliminating the remaining CO monitors,
especially those co-Iocated with monitors for other pollutants, should be
weighed against the continuing benefits of collecting these CO data for use
in planning, analysis, and assessment. These data can be very useful in
illustrating long-term trends, evaluating the regional effectiveness of emis-
sions controls, and conducting historical health assessments. Support from
federal and other sources may be necessary to continue monitoring opera-
tion.
The research community and regulatory agencies should address the
development and deployment of the most useful CO monitoring network
for the purposes of planning and modeling. Monitors that will provide
benefits in terms of assessing transportation impacts should be retained. In
addition, to accurately assess local air pollution, permanent monitors need
to reflect changes in growth and development patterns. Furthermore, as
national interest in toxic air contaminant concentrations and environmental
justice issues continues to increase, regulatory and research communities
should evaluate placing additional monitoring stations to address those
Issues.
OCR for page 159
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Representative terms from entire chapter:
oxygenated fuels
