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9
Emissions Inventories

Introduction

According to the air quality management approach in environmental regulation, emission limits are set according to the stringency needed to achieve a desired concentration of an atmospheric pollutant. Such an approach is based on an understanding of the quantitative relationship between atmospheric emissions and ambient air quality. The task of evaluating this relationship is straightforward for primary pollutants, such as sulfur dioxide (SO2) or carbon monoxide (CO), whose ambient concentrations are directly related to emissions because the pollutant of interest in the atmosphere is the pollutant that is emitted. For many large emission sources of SO2, it is possible to measure simultaneously emissions and ambient air quality in the affected region. With CO, which is emitted mostly by mobile sources—cars and trucks—data on the actual emission rates by source (real-time) axe not available, and the source contribution is much more ubiquitous, but real-time ambient measurement is possible.

The air quality management approach for secondary pollutants, such as ozone, introduces issues additional to those raised for primary pollutants. These issues result from the added complexity introduced by the coupled chemical relationship between ozone production and precursor emissions. One class of the primary emitted precursors—the oxides of nitrogen (NOx), which have attributes similar to those described above for CO—is measurable in the ambient air, and subject to limitations in real-time source monitoring. Point source NOx emissions make up approximately 57% of the national



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Page 251 9 Emissions Inventories Introduction According to the air quality management approach in environmental regulation, emission limits are set according to the stringency needed to achieve a desired concentration of an atmospheric pollutant. Such an approach is based on an understanding of the quantitative relationship between atmospheric emissions and ambient air quality. The task of evaluating this relationship is straightforward for primary pollutants, such as sulfur dioxide (SO2) or carbon monoxide (CO), whose ambient concentrations are directly related to emissions because the pollutant of interest in the atmosphere is the pollutant that is emitted. For many large emission sources of SO2, it is possible to measure simultaneously emissions and ambient air quality in the affected region. With CO, which is emitted mostly by mobile sources—cars and trucks—data on the actual emission rates by source (real-time) axe not available, and the source contribution is much more ubiquitous, but real-time ambient measurement is possible. The air quality management approach for secondary pollutants, such as ozone, introduces issues additional to those raised for primary pollutants. These issues result from the added complexity introduced by the coupled chemical relationship between ozone production and precursor emissions. One class of the primary emitted precursors—the oxides of nitrogen (NOx), which have attributes similar to those described above for CO—is measurable in the ambient air, and subject to limitations in real-time source monitoring. Point source NOx emissions make up approximately 57% of the national

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Page 252 (California's Inspection/Maintenance program) inventory, and 82% of the point sources emit 5000 tons or more annually; 43% of the NOx inventory is generated by mobile sources (EPA, 1989a). Volatile organic compounds, in contrast, are less well characterized from both a real-time emissions and ambient monitoring perspective. This chapter provides an overview of the anthropogenic emissions inventory: how it is compiled, what the. major contributing sources are, and where uncertainties lie. There is a similar overview of the inventory of biogenic emissions, and finally a review of efforts to evaluate the accuracy of emissions inventories. Compilation of Emissions Inventories In 1971, the U.S. Environmental Protection Agency (EPA) established the National Emissions Data System (NEDS) on sources of airborne pollutants. This system was to summarize annual cumulative estimates of source emissions by air quality control region, by state, and nationwide for the Clean Air Act's five criteria pollutants: particulate matter, sulfur oxides, nitrogen oxides, VOCs, and carbon monoxide. At that time the developers did not envision the evolving demands on emissions inventories that have become common with the advent of increasingly sophisticated air quality models. The original intent to compile annual national trends in the emissions of VOCs, NOx, SO2, CO, and particulate matter has been expanded and amended by the need for chemical speciation of VOCs, consideration of additional chemical species, more detailed information on spatial and temporal patterns of inventoried species, and techniques to project trends in emissions. An estimate of emissions of a pollutant from a source is based on a technique that uses ''emission factors,'' which are based on source-specific emission measurements as a function of activity level (e.g., amount of annual production at an industrial facility) with regard to each source. For example, suppose one wants to sample a power plant's emissions of SO2 or NOx at the stack. The plant's boiler design and its Btu (British thermal unit) consumption rate are known. The sulfur and nitrogen content of fuel burned can be used to calculate an emissions factor of x kilograms (kg) of SO2 or NOx emitted per y megagrams (Mg, or metric tons) of fuel consumed. EPA has compiled emission factors for a variety of sources and activity levels (such as production or consumption), reporting the results since 1972 in "AP-42 Compilation of Air Pollutant Emission Factors," for which supplements are issued regularly (the most recent was published in 1985) (EPA, 1985). Emission factors currently in use are developed from only a limited

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Page 253 sampling of the emissions source population for any given category, and the values reported are an average of those limited samples and might not be statistically representative of the population. As illustrated in Figure 9-1 (Placet et al., 1990), 30 source tests of coal-fueled, tangentially fared boilers led to calculations of emission factors that range approximately from 5 to 11 kg NOx per Mg of coal burned. The sample population was averaged and the emission factor for this source type was reported as 7.5 kg NOx per Mg coal. The uncertainties associated with emission factor determinations can be considerable. They are discussed later in this chapter. Figure 9-1 Results of 30 NOx-emissions tests on tangentially-fired boilers that use coal.  An average of 7.5 kg NOx/Mg coal was obtained. Source: Placet et al., 1990. The formulation of emission factors for mobile sources, the major sources of VOCs and NOx, is based on rather complex emission estimation models used in conjunction with data from laboratory testing of representative groups of motor vehicles. Vehicle testing is performed with a chassis dynamometer, which determines the exhaust emission of a vehicle as a function of a specified ambient temperature and humidity, speed, and load cycle. The specified

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Page 254 testing cycle is called the Federal Test Procedure (FIT) (EPA, 1989b). Based on results from this set of vehicle emissions data, a computer model has been developed to simulate for specified speeds, temperatures, and trip profiles, for example, the emission factors to be applied for the national fleet average for all vehicles or any specified distribution of vehicle age and type. These data are then incorporated with activity data on vehicle miles traveled as a function of spatial and temporal allocation factors to estimate emissions. The models used to estimate mobile source emissions have been developed primarily by EPA; California has developed its own model. Recent versions of the EPA and California mobile source emission factor models are MOBILE4 (EPA, 1989b) and EMFAC7E (CARB, 1986; Lovelace, 1990), respectively. The basic approach in estimating emissions therefore is derived from a simple calculation that requires an estimate of an activity level, an emissions factor, and, if the source has a pollution control device, a control factor: Emission =      activity level × emission factor × control factor Although obtaining an estimate of the activity level can be simple and as direct as monitoring fuel use or power plant load for a specified period, it also can be quite complex and indirect, requiring spatial aggregation or disaggregation of estimated activity measures, which may depend on the source type or category and its emission rate. Essential data elements compiled as part of the National Acid Precipitation Assessment Program (NAPAP) point source emissions file are presented in Table 9-1; data elements related to area source compilations are presented in Table 9-2. Anthropogenic Emissions Inventories The 1985 NAPAP emissions inventory prepared by EPA for NAPAP (EPA, 1989a), includes emissions from the U.S. and Canada for 1985. It was developed to provide information for assessment and modeling objectives of the national program. The inventory listed emissions of CO, SO2, NOx, VOCs, total suspended particulate matter, ammonia (NH3), primary sulfate (SO4-2), hydrogen chloride (HCl), and hydrogen fluoride (HF). Of specific interest to this report are the NOx, VOC, and CO emissions, which are summarized by source category in Figure 9-2 and by state in Figure 9-3. The specifics of the compilation are discussed by EPA (1989a) and are only briefly reviewed in this report. The U.S. emissions data were derived primarily using existing methodologies previously developed by EPA (Zimmerman et al., 1988a; Demmy et al.,

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Page 255 TABLE 9-1 Types of Point Source Emissions Data for NAPAP Plant Data State, county, air quality control region, and UTMa zone codes Point Data Point identification number Standard industrial classification (SIC) code UTM coordinates Stack, plume data (height, diameter, temperature, flow rate) Points with common stack Boiler design capacity Control equipment (devices and control efficiencies by pollutant) Operating schedule (season, hr/day, days/week, weeks/year) Emissions estimates for criteria pollutants (method) Process Data Source classification code (SCC) Operating rates (annual, maximum hourly design) Fuel content (sulfur, ash, heat, nitrogen) aUTM, Universal Trans Mercator, a type of map projection. Source: Placet et al., 1990. 1988) and use the NEDS point and area source inventory as a stating point for modification and refinement in the development of the 1985 inventory. Anthropogenic Vocs Forty percent of anthropogenic VOC emissions result from transportation, according to the 1985 NEDS and NAPAP emissions inventories (see Figure 9-2); light-duty cars and trucks make up the largest contributing fraction. Solvent emissions, which are distributed across a broad group of sources,

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Page 256 TABLE 9-2 Types of Area Source Emissions Data for NAPAP Source Category Data Stationary (residential, commercial, institutional, and industrial fuel emissions less than 25 tons per year) Mobile (highway and off-highway vehicles, locomotives, aircraft, marine) Solid waste (on-site incineration and open burning) Miscellaneous (gasoline marketing and evaporation of solvents used by consumers, unpaved roads and airstrips, construction, wind erosion, forest fares, agricultural and managed burning, structural fares, orchard heaters) Other (publicly owned treatment works; hazardous-waste treatment, storage and disposal facilities; fugitive emissions from petrochemical operations; synthetic organic chemical manufacturing and bulk terminal storage facilities; process emissions from bakeries, pharmaceutical, and synthetic fiber manufacturing; oil and gas production fields; and cutback asphalt-paving operations) Activity Level Data Fuel use (by gross vehicle weight and type, by state and county) Vehicle miles of travel (VMT, by road type and speed, by state and county) Surrogate geographic and economic data (population, dwelling units, vehicle registration, manufacturing employment, commercial employment, solvent user category employment) aCutback asphalt refers to asphalt that is thinned with volatile petroleum distillates, such as kerosene. Adapted from Placet et al., 1990. contribute 32% of total VOC emissions; the remaining 28% result from other sources such as industrial manufacturing activities and fuel combustion. An independent analysis by the Congressional Office of Technology Assess-

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Page 257 ment (OTA, 1989), reported that 94 cities exceeding the ozone National Ambient Air Quality Standard (NAAQS) during 1986 to 1988 generated 44% of VOC emissions nationwide. In these cities 48% of VOC emissions were from mobile sources, and an additional 25% came from the evaporations of organic solvents and the application of surface coatings. Because these three categories alone account for about 75% of the total estimated VOC emissions in cities where the NAAQS was not met, significant attention to the quality and accuracy of the estimates seems warranted. VOCs emitted from motor vehicles are mainly hydrocarbons that result from the incomplete combustion of fuel or from its vaporization. These contributions are generally categorized and reported as exhaust and evaporative emissions. Within the exhaust emissions category are included the unburned and partially burned fuel and lubricating oil in the exhaust and gases that leak from the engine. The evaporative emissions category includes fuel vapor emitted from the engine and fuel system that can be attributed to several sources: vaporization of fuel as a result of the heating of the fuel tank, vaporization of fuel from the heat of the engine after it has been turned off (hot-soak emissions), vaporization of fuel from the fuel system while the vehicle is operating (running losses), fuel losses due to leaks and diffusion through containment materials (resting losses), and fuel vapor displacement as a result of filling fuel tanks (refueling losses) (EPA, 1990f). Only recently has it been recognized that running losses are not treated adequately in EPA's emissions estimating (MOBILE) models (Black, 1989; EPA, 1989b). The magnitude of running-loss emissions will depend on ambient temperature, gasoline volatility, operating cycle, and engine and emission control system design. An OTA study (1989) reported that, using preliminary emission factors provided by EPA for fleet average running losses, for ambient temperatures of 79ºF (26ºC) and gasoline volatility of 11.5 pounds per square inch (psi), MOBILE4 model estimates of VOC emissions were 1.5 grams per mile (g/mi). In assuming ambient temperatures of 87 ºF and gasoline volatility of 11.7 psi, however, the resulting estimate of VOC emissions increased to 2.9 g/mi, a 93% change. Because of the difference in these estimates it has been suggested that past emissions inventory compilations underestimated mobile source VOC contributions by as much as 30% on hot summer days (OTA, 1989). The use of organic solvents in the dry cleaning industry, in metal degreasing, in cutback asphalt paving, and in a variety of consumer and commercial product manufacturing contributed about 15% of the total VOC emissions in the 1985 national inventory. These sources are difficult to inventory, because almost half of their emissions are estimated to come from facilities that emit less than 50 tons (45 Mg) annually. The emissions of VOCs from surface-coating-related industries contributed

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Page 258 Figure 9-2a NAPAP 1985 national emissions inventory for NOx  and VOCs by source category. Source: Placet et al., 1990. about 9% of the total in the 1985 national inventory. The sources include automotive, furniture and appliance manufacturing, printing, and metal and plastic fabrication industries. As with stationary-source solvent evaporation, this source category is not well quantified, and the combined contributions of the two categories are expected to have significant uncertainties. Voc Speciation by Source Category The evolving knowledge of the chemical reaction mechanisms that provide quantitative information on the VOC-NOx relationship to ozone production has highlighted the need for compound-specific information on the chemical composition of the compounds in the VOC emissions inventory. The methods developed to generate compound-specific information are similar to those

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Page 259 Figure 9-2b NAPAP 1985 national emissions inventory for NOx  and VOCs by source category. Source: EPA, 1989c. used for emission factors. Each point or area VOC emissions source, identified by its source classification code (SCC), has an associated VOC speciation profile, which provides a weight-percent breakdown of the individual compounds that contribute to total VOC mass emissions from the source. Speciation profiles are derived typically from compilations of detailed gas chromatographic analyses of the VOC emissions from sources in representative categories. The data sets used to derive the speciation profiles are quite limited and within a given source category can be highly variable. The current profile data base (Shareef et al., 1988), although the most comprehensive to date, suffers from major uncertainties in estimating compound-specific emissions. Shareef et al. (1988) used a rating procedure considered in EPA's AP-42 emission factor analysis technique (mentioned above) to assign subjective data quality rankings to the speciation profiles used in the NAPAP 1985 inventory. They used a scale of

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Page 260 Figure 9-3 NAPAP 1985 national emissions inventory for NOx and VOCs by state. NOx  emissions are reported as thousand tons NO2/ year. Based on data from EPA, 1989c. A through E, for the highest to the lowest quality, respectively, and their subjective analysis indicates that about 50% of the national VOC emissions of the most reactive chemical species would fall into the class B quality rating. The scale does not have quantitative error estimates associated with the letter ratings. Uncertainty issues associated with the chemical speciation will be discussed further in a later section.

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Page 261 Anthropogenic Nox Nitric oxide (NO) is formed by high-temperature chemical processes during combustion of fossil fuels, from both the nitrogen present in fuel and from oxidation of atmospheric nitrogen. Detailed inventories are available for Canada, the United States, and western Europe that describe the spatial patterns of NOx emissions from combustion of fossil fuels and from industrial processes (Lubkert and Zierock, 1989; Placet et al., 1990). Table 9-3 lists several estimates (cf. Placer et al., 1990) of NOx emissions associated with fossil fuel combustion in the United States. Between 40 and 45% of all NOx emissions in the United States are estimated to come from transportation, 30-35% from power plants, and about 20% from industrial sources. About half the NOx emissions associated with transportation come from light-duty gasoline trucks and cars and approximately one-quarter are from heavy-duty gasoline and diesel vehicles. TABLE 9-3 Estimated Annual U.S. NOx Emissions from Anthropogenic Sources Obtained from Recent Inventories Source category Emissions (teragrams of nitrogen/year)   NAPAP Inventorya EPA Trendsb MSCETc EPRId Electric utilities 1.8 2.1 1.9 2.2 Nonutility combustion 1.1 1.0 1.1 1.3 Transportation 2.4 2.7 2.3 2.4 Other sources 0.3 0.2 0.2 0.4 Total 5.6 6.0 5.5 6.3 aEPA 1989a. bEPA 1990a. cMSCET Month and State Current Emissions Trends; Kohout et al. 1990. dEPRI Electric Power Research Institute Heisler et al. 1988. Amounts are for 1982. The other amounts presented in the table are for 1985. Source: Placet et al. 1990.

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Page 292 Figure 9-9 VOC/NOx ratios measured during summer 1985. (VOCs do not include methane.)  These measurements were made during the morning rush hour, 6:00 a.m. to 9:00  a.m. The triangles are the average of the measurements made at each site and the  bars show the standard deviation. The squares are the median. Adapted from Baugues, 1986. contention that anthropogenic emissions are the predominant sources for the ambient VOCs concentrations recorded in urban areas. In assessing the implications of this finding, several factors must be considered. First, as discussed in Chapter 5, the natural VOCs are, in general, more reactive than are the anthropogenic VOCs. As a consequence, any chemical processing of the VOCs that occurs before early-morning samples are taken will tend to increase the concentrations of the anthropogenic VOCs relative to those of the more reactive natural VOCs. Second, because of their greater reactivity, the natural compounds tend to be discriminated against in the sampling method (container sampling) used to collect the data. This discrimination tends to reduce the measured concentrations of natural VOCs relative

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Page 293 TABLE 9-10. Comparison of mobile-source contribution deduced from emissions inventory data with estimates deduced from ambient measure meats. The VOC inventory data for highway sources were taken from the State Implementation Plans. City Percentage of 1980 emissions inventory Estimated from measurements Year Boston, Massachusetts 46 82 1985 Philadelphia, Pennsylvania 32 50 1984     69 1985 Washington, D.C. 66 87 1984     96 1985 Cincinnati, Ohio 41 50 1984 Cleveland, Ohio 45 70 1985 Houston, Texas 26 39 1985 St. Louis, Missouri 27 63 1985 Source: Baugues 1986 to the anthropogenics. Third, because of the temperature and light dependence, the emissions of the natural VOCs will tend to increase during the daytime and peak during the periods of greatest photochemical activity. Finally, the concentration of samplers near street-level in downtown areas tends to discriminate against natural VOCs, whose sources tend to be concentrated in areas outside the core city. These measurements, therefore, might not be a reliable gauge of the importance of the natural compounds in the portion of the atmosphere responsible for the ozone sampled at these urban locations.

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Page 294 Figure 9-10 Biogenic VOC concentrations (ppb Carbon) measured during the summers of  1984 and 1985. The medians of the concentrations of isoprene and a-pinene  are listed for 10 cities studied during both summers. The 1984 measurements  are the triangles; the 1985 measurements are the diamonds. Bars represent the  range of concentration. Adapted from Baugues, 1986. Rural Measurements As air masses leave the urban areas, they mix with air masses from other urban centers that can contain a somewhat different mix of ozone precursors. To this mixture are added the emissions from more isolated industrial sources or power plants, with their characteristic emission patterns. In addition, there is a relatively large input of natural compounds; in particular, the emissions of VOCs from forests. Photochemical processing and deposition serve to reduce the concentrations of the more reactive and polar compounds relative to the less reactive and nonpolar compounds. Finally, as the concentrations of the compounds are reduced through dilution, chemical destruction, and

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Page 295 Figure 9-11 Percentage of biogenic VOCs compared with total VOC measured during the summers  of 1984 and 1985. (The VOCs do not include methane.) The medians of the percentage  contribution of isoprene and  a-pinene to the total VOCs for 10 cities studied during  both summers are listed. The 1984 measurements are the triangles; the 1985 measurements  are the squares. Adapted from Baugues, 1986. deposition, the difficulties associated with measurement increase the chance of sizable errors in measurement. This makes the extraction of useful information for emissions inventories extremely difficult. However, large data sets measured with high-quality instruments afford some interesting insights. As is the case in the urban studies, atmospheric measurements made in rural areas are not very reliable for establishing the relative importance of emissions sources. Most of the measurements are not designed to determine source strength, distribution, or allocation, but rather to establish air concentrations of the compounds for model simulations of ozone production or to indicate the photochemical processes that control ozone production. In addi-

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Page 296 tion, there have been no long-term studies of rural areas that reveal the trends in the emissions of these compounds. Dodge (1989) suggested that aged urban air reaching rural locations has a ratio of VOC to NOx of 17:1. To this would be added local emissions dominated by stationary rather than mobile sources with VOC/NOx ratios of 2:1. In those simulations natural VOCs also were added in varying amounts to test sensitivities of the mechanism to the compounds. This scenario would suggest that the VOC to NOx ratio in rural areas depends strongly on the photochemical processing of the compounds and on the effect of urban versus local anthropogenic or natural sources. During the summer of 1986, D.D. Parrish (pets. comm., NOAA, 1990) observed a daytime VOC/NOx ratio of 7.2:1 at a rural site in western Pennsylvania. Of the total non-methane hydrocarbons, 45% were identified as natural, with the dominant natural compound being isoprene. The bulk of the remaining compounds were alkanes (39%) and aromatics (8%), presumably anthropogenic. Neglecting the biogenic hydrocarbons, the VOC/NOx ratio was approximately 4:1. This is much lower than the ratios typically registered in the urban locations. A principal-components analysis was carried out on a similar data set obtained at this site in 1988 (M.P. Buhr, pets. comm., University of Colorado, 1990). This analysis indicated that the atmospheric concentrations of alkanes, aromatics, and CO correlated with each other, suggesting they are dominated by anthropogenic, probably mobile, sources. NOx on the other hand correlated most strongly with SO2, suggesting that NOx was most strongly associated with stationary anthropogenic sources. By contrast, in 1987 similar measurements were carried out at a rural site in the Colorado mountains (D.D. Parrish, pers. comm., NOAA, 1990). In this case VOC/NOx was 15.2:1. Of the total VOCs, 28% were identified as natural, principally terpenes. Neglecting the biogenic compounds, VOC/NOx was approximately 11:1. This is very similar to the ratios typically registered in the urban locations. The bulk of the remaining compounds were alkanes (58%) and aromatics (9%), presumably of anthropogenic origin. A principal-components analysis was carried out on a similar data set obtained at this site in 1989 (M.P. Buhr, pets. comm., University of Colorado, 1990). This analysis indicates that the anthropogenic VOCs and NOx correlated with each other, suggesting they are dominated by an anthropogenic urban source, the Denver metropolitan area. It must be recognized that these rural measurements are strongly biased. The measurements were made at the surface in forest clearings. Hence, they tend to amplify the concentration of the natural VOCs relative to anthropogenic VOCs. A much lower relative concentration of these compounds could

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Page 297 be expected on average throughout the boundary layer. In addition to the VOCs, CO was measured at the rural locations in Pennsylvania and Colorado, as well as a suburban site near Boulder. The lifetime of CO in the atmosphere is long enough that there is a global background concentration, which is known to vary systematically with season because of photochemical processing. For Northern Hemisphere midlatitudes, the CO concentration varies between 127 parts per billion in winter and 84 ppb in summer (Seller et al., 1976). For this reason the amount of CO relative to NOx or VOC measured in the atmosphere will be greater than the relative amounts contained in the primary emissions depending on the degree of photochemical processing that has occurred between the point that the emissions entered the atmosphere and the location where this air mass is sampled. Thus, when the concentrations of NOx and the NMHCs are large near anthropogenic sources, the ratio of NOx or NMHCs to CO will approach the ratio of compounds expected for that source. Likewise, in so far as NOx, the NMHCs, and CO are derived from anthropogenic sources, when NOx or the NMHCs become quite sparse the concentration of CO will approach background levels. Figure 9-12 shows the mixing ratio of CO versus NOy (NOx + PAN + HNO3 + ...) measured near Boulder (Parrish et al., 1991). NO. is the more conserved atmospheric reservoir of the primary NOx emissions because NOy comprises NOx as well as its oxidation products. The curves on the graph show the expected relationship between CO and NOx if the CO concentration is equal to the wintertime CO background mixing ratio, 127 ppb, along with the CO emitted by sources that have a CO/NOx ratio of 5:1, 10:1, or 20:1. During this period, when photochemical processing is reduced, the data indicate that the sources of anthropogenic emission influencing the atmospheric concentrations of these compounds have a CO/NOx ratio of 10:1 to 20:1. This can be compared with a CO/NOx ratio for the Denver metropolitan area of 7.3:1 estimated by the 1985 NAPAP inventory or 8.2:1 estimated by the Colorado Department of Health (R. Graves, pers. comm., 1990). Atmospheric Measurements Versus Emissions Inventories Fujita et al. (1990) compared CO/NOx and VOC/NOx derived from ambient measurements taken between 7:00 a.m. and 8:00 a.m. at eight sites in the South Coast Air Basin of California during the summer phase of the 1987 South Coast Air Quality Study (SCAQS) (Lawson, 1990) with corresponding ratios derived from the day-specific, gridded emissions inventory for the same

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Page 298 Figure 9-12 Correlation between CO and NO measured at a suburban site in Boulder. NOy  comprises NOx and its oxidation process. The individual 5-min averages of the  measurements are shown. The background CO concentration for this season was  taken to be 130 ppb. The curves indicate the various emission ratios of CO and NOy  being added to this background CO concentration. Source: Parrish et al., 1991. period. Similar ambient CO/NOx ratios at each of the monitoring locations measured at the same times suggested a common emissions source. Both the ambient CO/NOx and VOC/NOx ratios were about 60-80% higher than corresponding ratios derived from emissions inventories (Figures 9-13 and 9-14). The accuracy of the VOC speciation was evaluated by comparing the

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Page 299 Figure 9-13 Ambient versus inventory CO/NOx ratios, South Coast Air Basin, August 1987.  Excludes NOx concentrations <50 ppb. EI refers to CO/NOx ratios derived from  emissions inventory data ().  Ambient ratio refers to ratios obtained from ambient  measurements taken between 7:00 a.m. and 8:00 a.m. Source: adapted from Fajita et al., 1990. composition of emissions at the monitoring sites with the observed ambient composition. Figure 9-15 is a comparison of VOC/NOx derived from ambient measurements and the emissions inventory for seven cities (Morris, 1990); the mismatch is believed to be a nationwide phenomenon. Summary For 2 decades, EPA has compiled inventories of emissions of volatile organic compounds (VOCs), oxides of nitrogen (NOx), and other airborne pollutants. As sophisticated air quality models have been developed, emissions inventories have been expanded to provide detailed information on

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Page 300 Figure 9-14 Ambient versus inventory VOC/NOx, South Coast Air Basin, August 1987.  Excludes NOx concentrations <50 ppb. EI refers to VOC/NOx ratios derived  from emissions inventory data (). Ambient ratio refers to ratios obtained from ambient  measurements taken between 6:00 a.m. and 9:00 a.m. chemical speciation of VOCs, spatial and temporal patterns of inventoried species, and projected trends in emissions. The development of sound ozone control strategies requires knowledge of the precision and accuracy of emissions estimates. However, estimates of the uncertainty in emissions data have, for the most part, been highly subjective. The methods used to estimate emissions have not been adequately checked by intercomparison or field measurements. Ambient monitoring data from many urban and rural areas of the United States, along with data from roadside motor vehicle emissions tests, tunnel studies, and remote sensing studies of on-road vehicle exhaust, show that current inventories underestimate anthropogenic VOC and carbon monoxide (CO) emissions by large margins. The motor vehicle portion of the emissions inventory has been demonstrated conclusively to underestimate VOC and CO emissions. Moreover, roadside tests and remote-sensing data indicate that

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Page 301 Figure 9-15 Comparison of VOC/NOx ratios derived from ambient measurements and  emissions inventories for seven cities. Source: Morris et al., 1990. approximately 10% of the vehicles on the road contribute at least 50% of the CO and VOC emissions. The VOC bias in current emissions inventories is a serious impediment to progress in designing effective ozone reduction strategies. These findings have substantial national implications for strategies to control VOCs. A rigorous program to resolve differences between on-road emissions and those predicted by emission models must be undertaken immediately. Resolution of the on-road versus model-predicted vehicle emission rates will have to account for the mass emission rate differences as well as the

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Page 302 emission rate ratios that imply rich on-road operation. Issues such as super-emitters, speed-dependent emission rates, and off-cycle operation must be considered. Biogenic VOC emissions appear to be of comparable magnitude to anthropogenic VOC emissions in the United States as a whole. Biogenic emissions can also be a significant source of VOCs in urban airsheds. For yearly or seasonal periods, these emissions are not well quantified. Moreover, because of the large variability in emissions that can occur over the growing season, much larger errors can be incurred when annual or seasonal inventories are applied to a given single- or multiple-day episode of high ozone concentrations. Because natural VOC emissions tend to be highly reactive and to increase during the day, past measurements of these emissions may have understated their importance relative to anthropogenic VOC emissions. Much research is needed to improve the methods used to calculate biogenic VOC emissions. If VOC emissions have been underestimated as much as the studies discussed in this chapter suggest, then VOC emission reductions in many areas of the United States will be less effective than was previously believed (see Chapters 6 and 11). Hence a major upward revision in VOC emissions inventories could force a fundamental change in the nation's ozone reduction strategy, which has been based primarily on VOC control.