Why Carbon MONOXIDE Still Matters K. JOHN HOLMES NATIONAL RESEARCH COUNCIL ARMISTEAD G. RUSSELL GEORGIA INSTITUTE OF TECHNOLOGY
CO regulation, one of the great successes in air pollution control in the United States, must be maintained.
egends and myths of tragic events stemming from carbon monoxide (CO) poisoning can be traced back thousands of years. CO generally occurs when insufficient oxygen is available during combustion of carbonbased fuels. Incidents of CO poisoning increased with the advent of coal and industrialization, thereby linking the toxic gas to the development of civilization itself (1, 2). By the early part of the 20th century, CO was recognized as a significant occupational health hazard and, by the 1950s, was seen as an indicator of poor air quality in urban areas, resulting from motor vehicle emissions. When CO emissions were first regulated in the United States in the 1960s, elevated concentrations were widespread and long-lasting. Cities such as Denver, Colo.; Lynwood, Calif.; and Fairbanks, Alaska, have experienced hundreds of days with high CO per year. In 1965, Chicago had an average CO concentration of 17 ppm, almost twice the current 8-h standard. Following requirements for “criteria pollutants” established under the 1970 Amendments to the Clean Air Act (CAA), the U.S. EPA in 1971 designated healthbased National Ambient Air Quality Standards (NAAQS) for ambient CO concentrations at 9 ppm for an 8-h average and 35 ppm for a 1-h average. According to a 1977 National Research Council (NRC) study, CO was “probably the most publicized and best known criteria pollutant,” in part because of its acute effects (2). Substantial controls on automobile CO emissions were introduced, and reducing CO pollution became central to air quality management efforts in many locations. Because ambient standards for CO have been largely attained and the estimates of its health impacts are much smaller than those from the other pollutants, contemporary air quality management in the United States now focuses on attaining new NAAQS for tropospheric ozone and fine particulate matter (PM2.5) and on managing air toxics (3, 4). Figure 1, for example, displays the dramatic reduction in the number of days that exceeded standards in Lynwood since the early 1970s. However, as of 2001, some locations with problematic meteorological and topographical conditions continue to occasionally violate the 8-h NAAQS for CO. In its fiscal 2001 appropriations for EPA, Congress asked NRC to study the issue. The NRC Committee on Carbon Monoxide Episodes in Meteorological and Topographical Problem Areas met in many affected locations to hear from experts on local conditions and national and local controls. The committee’s findings and recommendations are described in Managing Carbon Monoxide in Meteorological and Topographical Problem Areas (5), which we summarize in this article.
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© 2004 American Chemical Society
Meteorological and topographical conditions The committee recognized that all areas with air quality problems—whether from CO or other pollutants— have a component of physical geography that makes exceedances more likely. This study focused on areas subject to severe winter inversions and low wind speeds that, in combination with confined topography and significant localized emissions, are extremely effective in trapping the products of incomplete combustion, including CO. These conditions can occur not only within major metropolitan areas, such as Lynwood, but also in smaller cities, such as Fairbanks and Kalispell, Mont. Except for Birmingham, Ala., all the locations shown in Table 1 are west of the Mississippi and experience the most severe inversion conditions in the winter. The CO problem in Birmingham is different. It is not seasonally dependent, but rather caused by a single point source that produces violations when wind conditions direct emissions toward a nearby monitor. Table 1 focuses on areas with wintertime problems. It does not include exceedances in Weirton, W.Va., where, like Birmingham, CO problems are linked with operational malfunctions at a single facility. Meteorological and topographical conditions influence CO concentrations through effects on vertical mixing, wind speeds, temperature, humidity, and emissions. Atmospheric inversions occur when the temperature of the atmosphere increases with altitude and greatly reduces vertical mixing in the atmosphere. Combined with low wind speeds, inversions prevent air circulation because colder air is trapped near the ground by the warmer air above. Low humidity plays a role by allowing more infrared radiation from the earth’s surface to pass into space, rapidly reducing ground-level temperature and producing inversions close to the surface after sundown. These meteorological and topographical conditions contribute to the buildup of a host of pollutants, such as CO, PM2.5, hydrocarbons (HC), and other atmospheric pollutants. AUGUST 1, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY ■ 289A
FIGURE 1
Exceeding the 8-h CO standard in Lynwood, Calif. The number of days over the limit has decreased steadily in the past 30 years.
25 20 15 10
30
5
25
Sources and control 20 15 10
ar Ye
73–74 75–76 77–78 79–80 81–82 83–84 85–86 87–88 89–90 91–92 93–94 95–96 97–98 99–00
Number of exceedances
Number of exceedances
30
residents and is located in a three-sided topographical bowl. Its high latitude and low winter temperatures lead to greatly reduced solar heating at midday. Its atmospheric conditions are typically dry, and its topography keeps wind speeds at a minimum in the winter. These conditions combine to produce extreme atmospheric inversions close to the surface. A temperature increase of several degrees Celsius per 100 m is considered a strong inversion; in Fairbanks, inversions can be as much as 30 ºC/100 m (7 ). In the past five years, the month of February had the most episodes of high CO. In February, a ground inversion is rapidly established or strengthened near 5 p.m., which traps pollutants emitted when sunset and maximal emissions from vehicles occur around the same time.
5 0 June May April Mar Feb Jan Dec Nov Oct Sept Aug July Month
Fairbanks is an example of how meteorology, topography, and human activities can combine to produce high CO episodes. Because of concerns about Fairbanks’ ability to attain and maintain compliance with the standard, Congress designated the city as a case study for the NRC committee. (Findings from this case study are summarized in an interim report [6].) The city of Fairbanks has about 30,000
CO is emitted from virtually all combustion processes that involve fossil fuels. Anthropogenic sources include power plants, industrial processes, on-road vehicles, and non-road sources. Its largest urban source is gasoline-powered passenger cars and lightduty trucks. In 2001, these vehicles contributed >58% of national CO emissions (8). In urban areas, the fraction of CO emissions from motor vehicles is even higher. For example, emissions inventories for five of the locations listed in Table 1 estimate that on-road vehicles made up 62–84% of emissions. EPA suggests that on-road vehicles contribute >95% of CO emissions in cities that it classifies as having serious air pollution (9). In particular, CO emissions are highest under cold starts and increased load (e.g., climbing a hill or rapid accelerations) when a “richer” fuel mixture, defined as a higher proportion of fuel to air, is used to improve the vehicle’s performance. High emissions can also result from engine malfunctions. The common element among cold starts, increased loads, and engine malfunctions is that vehicles are operating under fuelrich conditions where insufficient oxygen is available to completely oxidize fuel carbon to carbon dioxide. Given the dominant role of gasoline-powered vehicles
TA B L E 1
Locations with significant days with exceedances of the 8-h CO standard during 1995–2001 City, State
Birmingham, Ala. Calexico, Calif. Lynwood, Calif. Fairbanks, Alaska Phoenix, Ariz. Las Vegas, Nev. Spokane, Wash. Anchorage, Alaska El Paso, Texas Denver, Colo.
1995
1996
N/A 15 12 8 3 1 4 0 0 2
2 9 17 1 2 3 2 3 2 0
Number of exceedance days per year 1997 1998 1999 2000
2 10 10 2 1 1 0 0 1 0
6 6 9 2 0 2 0 0 0 0
17 11 8 2 1 0 0 1 0 0
9 6 2 1 0 0 0 0 0 0
2001
Total
33 2 0 0 0 0 0 1 0 0
69 59 58 16 7 7 6 5 3 2
Source: Laurence Elmore and Jake Summers (U.S. EPA) provided the data. Marcella Nystrom (California Air Resources Board) provided the data for Calexico in 2001.
290A ■ ENVIRONMENTAL SCIENCE & TECHNOLOGY / AUGUST 1, 2004
in emissions, CO management has focused largely on improving combustion efficiencies and post-combustion catalyst controls for these vehicles. New motor vehicle emissions standards and controls. Lowering new-vehicle emissions certification standards has resulted in the most significant CO emissions reductions. Passenger cars since model year 1981 must not emit more than 3.4 g CO/mile for certification tests run at standard temperatures (68– 86 °F). Before controls were introduced, average newvehicle emissions were 84 g/mile! A separate newvehicle emissions standard limits passenger cars built since 1994 to 10 g CO/mile at 20 °F. Both the normal and cold temperature standards were met through a combination of combustion and post-combustion controls. Combustion controls for CO primarily involve reducing the time the engine operates in fuelrich conditions through sophisticated on-board computers, oxygen sensors, and fuel-injection systems. Post-combustion controls include catalytic converters that oxidize CO to carbon dioxide before the exhaust exits the tailpipe. Reducing cold-temperature CO emissions required further refinements in catalyst design. Both combustion and post-combustion controls have a significant impact on other motor vehicle emissions, including HC.
Lowering new-vehicle emissions certification standards has resulted in the most significant CO emissions reductions. In-use emissions controls. During public sessions, the committee heard from state and local officials about the programs implemented locally to further control in-use motor vehicle emissions. Motor vehicle emissions inspection and maintenance (I/M) programs and oxygenated fuel programs are the most common methods used locally to augment federal emissions standards. The CAA mandates both I/M and fuel programs for the worst CO areas. Although emissions decreases that arise from I/M and oxygenated fuel programs probably have fallen short of the benefits that were originally projected (10–15), they are pursued as some of the largest potential sources of emissions reductions beyond new-vehicle emissions standards. The committee also heard about other locationspecific motor vehicle controls, which include implementing programs to improve traffic flow, encouraging engine preheater use, and using alert day or voluntary trip reduction strategies. For example, catalytic converters on vehicles in Fairbanks can take 10 min or more to warm up and become fully effective during winter (16). Cold-start emissions in Fairbanks are estimated to contribute 45% of all motor vehicle emissions (17). The local government, the Fairbanks North Star Borough, is making substantial efforts to control these emissions by making available and encourag-
History of criteria pollutants under NAAQS As defined in the Clean Air Act Amendments of 1970, criteria pollutants are air pollutants emitted from numerous or diverse stationary or mobile sources. The amendments direct the U.S. EPA to set National Ambient Air Quality Standards (NAAQS) to protect human health and public welfare. The original criteria pollutants were carbon monoxide, total suspended particulate matter, sulfur dioxide, photochemical oxidants, hydrocarbons, and nitrogen oxides. Lead was added to the list in 1976, ozone replaced photochemical oxidants in 1979, and hydrocarbons were dropped in 1983. Total suspended particulate matter was revised in 1987 to include only particles with an equivalent aerodynamic particle diameter of ≤10 micrometers (PM10). A separate standard for particles with an equivalent aerodynamic particle diameter of ≤2.5 micrometers (PM2.5) was adopted in 1997. ing the use of electric outlets for engine-preheat devices known as “plug-ins”. These devices improve vehicle starting and greatly reduce the time needed for the catalyst to become fully operational. Control of other sources. There is much less focus on controls of CO emissions from other industrial and nonroad sources. However, these other sources increasingly contribute a larger fraction of emissions. Since CO problems can be caused by fairly localized emissions sources, there also are instances when a single large stationary source is the cause of the problem and the sole focus of controls. For example, in Birmingham and Weirton, the CO-control strategy focuses on modifying the operations of a single industrial production facility (18). Changes in emissions and air quality. According to EPA, nationwide CO emissions were reduced by 32% between 1982 and 2001 (8). Light-duty gasolinevehicle emissions decreased by 43%, and all other sources combined decreased by 8% during the same time period (8). The improvement in CO emissions occurred despite an approximately 43% increase in vehicle miles traveled (VMT) in the United States over the same period (19). Trends in national average ambient CO concentrations do not mirror trends in nationwide emissions. Using the average of the second-highest 8-h annual concentration from >200 monitoring sites, EPA estimates that the average ambient CO concentration has declined by 65% during the 1983–2002 period (9). This amount is substantially more than the estimated reduction in national CO emissions. As shown by various studies of vehicle emissions and air quality, some of the discrepancy may be due to uncertainties and inaccuracies in models and data used to estimate national and local emissions inventories (5, 20–24). Because most CO monitors are located in urban areas, a larger issue may be that changes in air quality tend to track changes in urban air emissions rather than in total emissions. Because light-duty vehicles dominate urban emissions, the improvements in ambient CO concentrations disproportionately reflect reductions in emissions from this source. AUGUST 1, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY ■ 291A
FIGURE 2
Relationship of CO to other pollutants (a) CO (blue line) and benzene (green line) vary diurnally. Adapted with permission from Ref 41. (b) Variation of CO, relative mass, total particle number, and black carbon concentrations versus downwind distance from a freeway. Adapted with permission from Ref 42. (a) 1.0
CO (ppm), Benzene/3 (ppb)
0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Hour of the day
(b) 1.0
Relative concentration
0.8
Relative mass Carbon monoxide Particle number Black carbon
0.6
0.4
0.2
0.0 Upwind
–300
–200
–100
0
100
200
300 Downwind
Distance to the 405 Freeway (m)
Air quality management In the near term, the committee found that conditions for CO attainment in those few areas that continue to experience violations and those that have recently come into attainment should continue to improve. The biggest factor is the continuing decline in motor vehicle CO emissions rates. Yet, this decline occurs without the tightening of passenger car emissions standards for CO at normal temperatures (68–86 °F) since 1981 and with VMT continuing to increase. Improvements are due to fleet turnover, with an increasing fraction of vehicles in the fleet being certified to tighter CO standards, including cold temperature (20 °F) standards. Improvements are also a collateral benefit of tightening emissions standards for HC and nitrogen oxide (NOx) emissions. Some of the technologies adopted to meet these standards— especially for HC control—reduce CO emissions. Future regulations affecting CO emissions rates from 292A ■ ENVIRONMENTAL SCIENCE & TECHNOLOGY / AUGUST 1, 2004
motor vehicles include the new Tier 2 standards that tighten NOx and HC emissions standards and reduce fuel sulfur content (25, 26). How long will advances in motor vehicle emissions control technology reduce CO emissions rates faster than VMT increases? Some information presented to the committee suggested that regional CO emissions will continue to decrease well into the future (27, 28), whereas other sources showed that emissions would start to increase again as early as 2005 (29, 30). The continued decline in CO emissions will depend not only on the rate of increase in VMT but also on the extent to which technologies adopted to reduce NOx and HC emissions rates will reduce CO emissions rates. However, for locations outside of those listed in Table 1, the increase in CO emissions, if it occurs at all, may occur when ambient concentrations are well below the NAAQS. At least some locations listed in Table 1 will remain vulnerable to exceedances of the 8-h NAAQS for CO because of meteorological and topographical conditions that produce severe winter inversions. Future large increases in population might compound these adverse natural conditions, especially in small cities such as Fairbanks, whose populations can surge with the initiation of a single large project such as the construction of military facilities or a trans-national natural gas pipeline. The role of meteorological and topographical conditions in causing CO exceedances prompted some local officials to suggest that these locations should be granted some form of exemption from the CAA. However, the committee concluded that a similar argument could be made for other regions with regard to various air quality problems. Thus, the committee recommended that local air quality management agencies continue to enhance programs to identify and repair or remove high-emitting vehicles and to improve programs tailored to local conditions, such as the Fairbanks plug-in program. Although new-vehicle emissions standards for HC and NOx emissions control should provide a collateral benefit for CO, testing should confirm the extent to which emissions reductions for CO actually occur, especially at temperatures 11,000 deaths from accidental CO poisoning have been avoided over the 1968–1998 period because of the more stringent vehicle emissions standards (34). This “collateral” benefit is not accounted for in EPA’s recent reports on the benefits and costs of the CAA (3, 4), yet it dwarfs the estimated direct benefits ascribed to CO control.
CO is unique among criteria pollutants because of its significance for both ambient air quality management and public safety. CO as an indicator. The buildup of CO in the ambient environment demonstrates the role that meteorology and topography have on ground-level emissions (especially those from motor vehicles) during winter under conditions of low vertical mixing. As such, CO can indicate other directly emitted mobilesource pollutants, such as toxic HCs and components of PM. Figure 2 shows that the relationship to other pollutants can be strong. This figure shows the correlation between benzene and CO concentrations by time of day and indicates how CO and two aspects of
PM (particle number and black carbon) disperse from a highway. The locations of CO hot spots may indicate distributional issues with regards to motor vehicle-related pollutants. Because CO concentrations are unevenly distributed, exposure to CO within the population varies. Individuals living in or near hot spots are exposed not only to higher concentrations of CO, but also to higher concentrations of other mobile-source-related pollutants. Although the network of monitors is too sparse to identify all locations with elevated ambient CO, the characteristics of the residents living near the locations shown in Table 1 can be examined. Data analysis from the 2000 U.S. Census shows that the CO monitors in Table 1 are typically found in areas that have greater percentages of low-income and minority residents and a lower fraction of vehicle ownership than their surrounding regions (Table 2). Because CO is a relatively unreactive pollutant, its temporal and spatial distribution provides an effective diagnosis of atmospheric dispersion patterns. In this regard, CO has potential applications for other air quality management issues and can improve the understanding of the dispersion of chemical, biological, and radiological materials. However, the resolution of models typically used in local CO management is too coarse to capture the variability in pollutant concentrations and to accurately represent unusual meteorological conditions. In addition, field-saturation studies, which typically rely on portable monitors to “saturate” a geographical area with samplers to assess the spatial distribution of pollutants, are fairly expensive. Developing more physically comprehensive
TA B L E 2
Population characteristics during year 2000 Monitor location
% Non-white Monitor areaa Regionb
Anchorage, Alaska (3201 New Seward Hwy.) Birmingham, Ala. (Shuttlesworth and 41st Sts.) Calexico, Calif. (129 Ethel St.) Denver, Colo. (Broadway-Camp) Denver, Colo. (Speer Blvd. and Auraria Pkwy.) El Paso, Texas (North Campbell) Fairbanks, Alaska (Cushman St.) Las Vegas, Nev. (East Charleston Blvd.) Lynwood, Calif. (Long Beach Blvd.) Phoenix, Ariz. (Grand Ave. and Thomas Rd.) Spokane, Wash. (Third Ave.)
Per capita income ($) Monitor area Region
% Non-driving Monitor area Region
33.6
27.8
26,260
25,287
17.4
11.0
96.9
32.7
8085
21,142
28.7
4.8
51.0 48.6
50.6 20.6
10,193 20,300
13,239 26,206
16.0 47.2
10.3 10.1
19.5
20.6
68,944
26,206
55.9
10.1
17.4
26.1
3,907
13,139
19.9
7.9
31.2
22.2
20,921
21,553
32.6
10.4
34.2
26.2
15,935
21,697
13.1
9.8
65.3
51.3
7739
20,683
12.7
14.6
38.5
23.0
13,109
21,907
18.6
10.0
12.7
8.6
19,016
19,233
43.5
11.0
a
Monitor area defined by census tracts immediately surrounding monitor site (except Birmingham monitor area defined by block group). b Region defined by Metropolitan Statistical Area except for Fairbanks North Star Borough and Imperial County (Calexico).
AUGUST 1, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY ■ 293A
models that can accurately simulate the variability of CO and evaluating such models against field data would be useful for assessing atmospheric dispersion and human exposure to various airborne materials. Some believe that CO would be most useful in microscale setting where polluted concentrations vary dramatically over short distances, for example, with distance from a roadway. This setting is important because some studies have found a link between health impacts and proximity to major roadways (35– 37 ), although other studies have found no such correlation (38–40). CO is less reliable in representing regional distributions of other pollutants and, at regional scales, is probably a poor indicator of mobile-source air toxics, such as formaldehyde and acetaldehyde, that react rapidly and have substantial sources in the atmosphere.
(7) (8) (9)
(10)
(11)
(12) (13) (14) (15)
(16)
Where do we go from here? Extremely high ambient CO pollution has disappeared from most urban areas in the United States, greatly reducing human exposure to ambient CO. However, in a few major metropolitan areas and smaller cities, it remains a significant part of air quality management activities. The common element for most of these locations is a susceptibility to wintertime inversions that limit dilution of motor vehicle emissions. Given the reduced number of air quality violations and newly promulgated standards and regulations for PM, ozone, and air toxics, CO will not be the focus of air quality management. Consequently, new controls will be directed at other pollutants. Nevertheless, in addition to health concerns, continued studies of CO remain critical for understanding the impact of meteorological and topographical conditions on air pollution, particularly during the winter; the spatial and temporal distribution of pollution; the relationship between atmospheric concentrations and human exposures to motor vehicle-related pollution; and how increases in VMT and changes in the characteristics of the vehicle fleet alter motor vehicle emission trends. K. John Holmes is a senior staff officer with the National Research Council’s Board on Environmental Studies and Toxicology, where he is involved in studies on automobile emissions and other air quality issues. Armistead G. Russell is a professor at the Georgia Institute of Technology and chaired the NRC Committee on Carbon Monoxide Episodes in Meteorological and Topographical Problem Areas.
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