4.2	EMISSION INVENTORIES

Emission inventories are used in several ways as tools in air quality management. The inventory on its own provides useful information on the magnitude and distribution of emissions across all anthropogenic and natural sources of emissions. The inventory can be tracked over time to present useful information on the growth or reduction of emissions from the various anthropogenic and natural sources. The inventory is also a major input to atmospheric computer models that are used to predict ground level pollutant concentrations and to analyze various air pollution control strategy options.

Two basic types of emission inventories are used in air pollution management studies. The top down inventory is the simpler, and therefore less expensive of the two. Top down inventories are prepared from existing data sources and use estimation techniques that are comprehensive but less specific than the other method. The geographical extent is usually large, often being national in scope. Top down inventories have the advantages of being consistent in the methodology used, can cover very large geographical areas, are relatively inexpensive and can be produced and modified quickly. The U.S. Environmental Protection Agency (EPA) uses top down inventories for regional scale modeling and the tracking of emission trends over time.

Bottom up inventories are prepared using specific locally derived data and may cover an area as small as a single point source or as large as a state. Bottom up inventories are characterized by developing specific emissions data from as many specific sources as possible. This entails direct contact with emission sources, particularly large point sources. Bottom up inventories are usually produced by State and local environmental agencies. They are regarded as having greater accuracy than top down inventories. When combined to form national or regional inventories, inventory method consistency is a concern especially when comparing emissions from one inventory area to another. These inventories are costly and time consuming to produce.

4.2.1. Ozone Inventories

Ozone is a pollutant that is formed in the atmosphere from precursors that are emitted from various sources. The precursors that are important in ozone formation are volatile organic compounds (VOC), oxides of nitrogen (NOx) and carbon monoxide (CO). These precursors have been inventoried for a number of years and inventoried intensively since 1990. A national top down inventory for these precursors has been developed by the EPA and is publicly available via anonymous FTP at earth1. epa.gov under the directory pub/gopher/ EmisInventory . The file names are: US90ARV2, US90MBV2 and US90PTV2. In addition to the ozone precursors, this inventory also contains sulfur dioxide (SO2), ammonia (NH3), particulate matter less than 10 m (PM-10), particulate matter less than 2.5 m (PM2.5) and secondary organic aerosols (SOA). These

atmospheric constituents will be discussed in the next section on particulate matter and regional haze. This top down inventory is comprehensive, with emission estimates presented for all source categories for each county in the United States.

The methodology used to produce this inventory can be summarized by its major components. Point sources are large emission sources that are inventoried individually. Point source emissions were estimated using two methods. For fossil fuel steam electric utility units, emissions were estimated by applying an emission factor (an emission factor relates the quantity of a pollutant emitted to some process activity level - in this case pounds of pollutant emitted per ton of fuel consumed) to the fuel used for each utility unit in the United States based on fuel data submitted by the utilities to the Department of Energy. Other point sources were estimated from the 1985 National Acid Precipitation Assessment Program (NAPAP) emission inventory. The 1985 NAPAP inventory is a comprehensive national inventory that utilized bottom up methodologies to estimate emissions. These point source emissions were scaled from 1985 to 1990 using the Bureau of Economic Analysis (BEA) growth factors for each industrial segment. Area sources are emission sources that are too small or too numerous to be individually inventoried. As with point sources, some of these emissions were scaled from the 1985 NAPAP inventory using BEA growth factors for each industrial segment. Two segments of area source emissions were estimated using different methods. The category of solvent emissions which applies only to VOC emissions, were estimated by obtaining the total quantity of solvents consumed in the United States, assuming that all solvents were eventually emitted to the atmosphere, and apportioning these emissions to each county based on county employment for specific industry groups from the publication County Business Patterns. Area source emissions from residential wood combustion were estimated by using emission factors and a wood burning model developed by the EPA. The wood burning model estimates consumption for all counties in the United States based on survey data from the northeast and several western states. Mobile source emissions were estimated using two methods. Emissions from on-road vehicles were estimated by applying emissions factors to an activity level. The activity level in the case of on-road vehicles is the total mileage driven or vehicle miles traveled (VMT) (one VMT = one mile driven by one vehicle). The annual VMT data for each State were obtained from the Federal Highway Administration's Highway Performance Monitoring System data base. The State VMT totals were apportioned to the counties based on population. The mobile source emission factors for CO, VOC and NOx were calculated for each county using the MOBILE5a model which has been developed by the EPA's Office of Mobile Sources (OMS). This model includes both evaporative and exhaust emissions. Non-road mobile sources includes commercial marine vessels, lawn and garden equipment and construction equipment. Emissions from these non-road mobile sources were estimated from a detailed 1990 inventory prepared for 27 nonattainment areas by the OMS. These inventories were combined and used to create national county-level emissions. These emissions were classified by equipment and engine type and were distributed to the appropriate non-road source categories and were used to estimate non-road emissions for all counties in the United States. Emissions from aircraft and railroads, which are also nonroad mobile sources, were scaled up from the NAPAP inventory as previously described. Natural sources also contribute significant amounts of VOC and NOx to the national emission inventory. These emissions are produced by various plant species and from the soil. The method used to estimate these emissions is the EPA developed Biogenic Emissions Inventory System - Version 2 (BEIS2). Input to this model are land use data and meteorological data. As with the other major sources of emissions, these biogenic emissions were estimated at the county level. This inventory is being used by the EPA in national computer modeling simulations to analyze various options for a revised ozone National Ambient Air Quality Standard (NAAQS).

In addition, bottom up inventories were required by the Clean Air Act (CAA) and were prepared by the States for all ozone nonattainment areas for the year 1990. These State Implementation Plan (SIP) inventories have two applications. The CAA requires that VOC emissions be reduced by specified amounts. The SIP inventories and follow-up periodic inventories are used to determine if the required VOC reductions were achieved. For some of the nonattainment areas, the SIP inventories were also used as an input to photochemical grid models to predict future year ground level ozone concentrations and to evaluate various control strategies needed to attain the ozone NAAQS.

Since these SIP emission inventories were prepared by 33 States plus the District of Columbia, there is some variation in the emission estimation methodologies used. General statements can be made to show some of the major areas of difference between the SIP inventories and the top down methodology discussed above. The most significant difference occurs in the point source portion of the inventory. The EPA guidance required all VOC sources emitting 10 tons/year and NOx and CO sources emitting 100 tons/year to be inventoried individually as point sources. This included all sources in the nonattainment area plus sources that were located in a 25 mile wide buffer zone surrounding the nonattainment area. The methods used to estimate emissions ranged from actual measured emissions data from plant continuous emission monitors to the application of emission factors and activity levels determined from survey responses. The States were required to use the EPA MOBILE model to estimate on-road mobile source emissions. However, significant variation in the application of the MOBILE model occurred. The MOBILE model has a number of input options that allow the user to accept built in default values or to provide location specific values. Some States conducted extensive studies to develop specific inputs while others relied primarily on the default values. Non-road emissions and biogenic emissions were primarily estimated using the same EPA methodologies discussed under the top down methodology described above. Area sources were estimated individually by area source category for the nonattainment area. Estimation methodologies varied, but often were based on a per capita or per employee emission factor for a particular area source category. The activity level, e.g., population or number of employees, could be locally determined or obtained from national census data. The SIP inventories were submitted by the States to the EPA Regional Offices (RO). The RO's reviewed each SIP inventory using a detailed check list. This quality assurance function insured that all SIP inventories satisfied a set of minimum requirements. The States have submitted a total of 97 ozone nonattainment inventories which includes 357 counties of the total 3141 United States counties.

In the eastern United States, transport has been recognized as a significant problem. The strategy of treating each nonattainment area independently has not been successful because emissions from one nonattainment area can impact other areas. The Ozone Transport Assessment Group (OTAG) consists of 38 eastern States and has been formed to address this problem. The approach being followed is to construct an emission inventory for the entire region and use it in a photochemical grid model to develop a region wide attainment strategy. The emission inventory will be a blend of the EPA's national top down inventory and the State's SIP inventories for VOC, NOx and CO and will be based on 1990 emissions. All of the States have reviewed and submitted their SIP inventories to the EPA. The States supplied SIP data are presumed to be more accurate than EPA' top down results and are therefore being used. For those portions of the 38 state area not included in the SIP nonattainment area inventories, the EPA will use its top down inventory results.

National maps of NOx and VOC emissions are displayed below in Figure D-2.

4.2.2	Particulate Matter and Regional Haze Inventories

From the emission inventory perspective, the inventory requirements are the same for particulate matter (PM) and regional haze. Therefore, the following discussion applies to both regulated pollutants. The atmospheric constituents that are important to the formation of both regulated pollutants are: VOC, Nox, SO2, NH3, PM-10 and PM- 2.5. The top down inventory methodology for VOC and NOx was discussed in Section 4.2.1. Much of the methodology also applies to SO2, PM-10 and PM-2.5. The main differences are for on-road mobile sources, fugitive dust sources and ammonia sources. Emissions of PM-10, PM-2.5 and SO2 from on-road mobile sources were estimated using the PART5 model developed by the EPA's OMS. This model was used to calculate emission factors from vehicle exhaust, brake wear, tire wear and re-entrained road dust from paved and unpaved roads. The emission factors from this model were then used with the VMT discussed in Section 4.2.1 to estimate on-road mobile source emissions. There are some source categories that are unique PM sources that are included in this inventory. These are significant sources of particulate emissions and include wind erosion, agricultural tilling and construction activities. Wind erosion was estimated from agricultural land by obtaining crop land areas from the Department of Agriculture, meteorological data and a wind erosion model to estimate emissions. Agricultural tillage emissions were estimated based on the area planted, crop type, soil parameters and the number of tillings per year. Construction emissions were estimated based on an emission factor, the area of land under construction determined from the cost of construction and the average duration of construction activity. Ammonia emissions are dominated by emissions from animal husbandry activities. Emission factors were applied to animal populations obtained from the census of agriculture produced by the Department of Agriculture.

Bottom up inventories have also been prepared for PM-10 nonattainment areas and for regional haze in the Grand Canyon area. The PM-10 SIP emission inventories had similar CAA requirements to the ozone SIP inventories. There are two factors that may impact the PM-10 inventories that are different from the ozone inventories. First, the PM-10 nonattainment areas are generally much smaller than the ozone areas. Furthermore, some of these areas are dominated by a single source category, e.g., residential wood combustion or fugitive dust. Therefore, the extent and scope of these inventories are more limited than the ozone inventories. Secondly, the PM-10 inventories were generally not used as inputs to computer modeling simulations. As a result, the additional quality assurance that occurs through use of inventory data in models did not occur. The EPA regards the level of rigor for the PM-10 SIP inventories to be less than that of the ozone SIP inventories. The States have submitted a total of 70 PM-10 nonattainment inventories.

The Grand Canyon Visibility Transport Commission has developed an emission inventory for the 11 Western State Region to evaluate regional haze. This emission inventory combined both top down and bottom up methodologies in developing their emission inventory. The starting point for the inventory was the EPA developed top down ozone inventory discussed under Section 4.2.1. The particulate matter and visibility pollutants were added to this inventory using top down methodology. Certain source categories received special emphasis, e.g., prescribed burning, in the preparation of the inventory. Each of the 11 States were participants in the inventory and had to agree with the inventory values for their State. Two of the States, California and Oregon, supplied complete data sets so that none of the original top down derived values were used. Most of the other States supplied some data, primarily point source data.

4.2.3 Emission Factors

As stated above, emission factors are a primary tool in the development of emission inventories. Table 4.2-1 is a presentation, by pollutant, of the emission factors currently contained in the EPA's Factor Information Retrieval Data Base (FIRE).

TABLE 4.2-1. FIRE EMISSION FACTORS

POLLUTANT

NUMBER OF EMISSION FACTORS

PERCENT OF EMISSION FACTORS RATED A OR B
CO	912	24
NOX	1232	17
VOC	2487	10
SO2	1358	27
PM	2008	27
PM-10	1212	23
PM-2.5	0	-
NH3	0	-

In addition to the number of emission factors for each pollutant, the table also contains information on the percentage of these factors that have the higher data quality ratings of A or B. It can be seen from this table that there is good coverage for the ozone pollutants although the quality of VOC is lower relative to NOx and CO. The particulate matter and haze pollutants are less well covered. Good coverage exists for NOx, VOC and SO2. The coverage for PM-10 would also appear to be good. However, the great majority of the PM-10 emission factors were derived from the PM emission factors shortly after the EPA promulgated the current PM-10 NAAQS. This effort involved applying existing particle size distribution data to the PM emission factors to derive the new PM-10 factors. The data quality ratings for the new PM-10 emission factors were not lowered as should have been the case. Therefore, the coverage for PM-10 is still good but the confidence in the factors is lower. No emission factors for PM-2.5 are in this data base. The factors that were used in the EPA top down inventory were derived from the PM emission factors using a calculator program that included particle size distribution data for the various source categories. The confidence in this size distribution data is low, especially for the PM-2.5 size interval. The EPA is currently collecting field test data for PM-10 and PM-2.5 emissions from paved and unpaved roads. Both of these fugitive particulate source categories are major emission sources. When these data are available, it will improve the confidence in the fine particle emission factors. Emission factors for NH3 are not in the EPA FIRE data base. The NH3 factors that were used in the EPA top down inventory are a compilation of NH3 factors that are based on recent studies primarily from Europe. Confidence in these factors is also low.

4.3.3	Emission Trends

Summaries of emission trends extracted from the recent EPA Emission Trends report (EPA, 1995) are provided in Figures ES-1, ES-2 aand Table ES-1.

Table ES-1. Summary of National Emissions

(thousand short tons, 1.1 million short tons equals 1 million metric tons)

Year	Carbon Monoxide	Nitrogen Oxides	Volatile Organic Compounds	Sulfur Dioxide	Particulate Matter (PM-10) w/o fugitive dust	Fugitive Dust (PM-10)*	Lead

(short tons)
1900**	NA***	2,611	8,503	9,988	NA	NA	NA
1905**	NA	3,314	8,850	13,959	NA	NA	NA
1910**	NA	4,102	9,117	17,275	NA	NA	NA
1915**	NA	4,672	9,769	20,290	NA	NA	NA
1920**	NA	5,159	10,004	21,144	NA	NA	NA
1925**	NA	7,302	14,257	23,264	NA	NA	NA
1930**	NA	8,018	19,451	21,106	NA	NA	NA
1935**	NA	6,639	17,208	16,978	NA	NA	NA
1940	93,615	7,374	17,161	19,953	15,956	NA	NA
1945****	98,112	9,332	18,140	26,373	16,545	NA	NA
1950	102,609	10,093	20,936	22,358	17,133	NA	NA
1955****	106,177	11,667	23,249	21,453	16,346	NA	NA
1960	109,745	14,140	24,459	22,227	15,558	NA	NA
1965****	118,912	17,424	30,247	26,380	14,198	NA	NA
1970*****	128,079	20,625	30,646	31,161	13,044	NA	219,471
1975	115,110	21,889	25,677	28,011	7,617	NA	158,541
1980	115,625	23,281	25,893	25,905	7,050	NA	74,956
1984	114,262	23,172	25,572	23,470	6,220	NA	42,217
1985******	114,690	22,860	25,798	23,230	4,094	40,889	20,124
1986	109,199	22,348	24,991	22,442	3,890	46,582	7,296
1987	108,012	22,403	24,778	22,204	3,931	38,041	6,857
1988	115,849	23,618	25,719	22,647	4,750	55,851	6,513
1989	103,144	23,222	23,935	22,785	3,927	48,650	6,034
1990*******	100,650	23,038	23,599	22,433	3,882	39,451	5,666
1991*******	97,376	22,672	22,877	22,068	3,594	45,310	5,279
1992*******	94,043	22,847	22,420	21,836	3,485	40,233	4,899
1993*******	94,133	23,276	22,575	21,517	3,409	39,139	4,938
1994*******	98,017	23,615	23,174	21,118	3,705	41,726	4,956
Note(s):	*	Fugitive dust emissions not estimated prior to 1985. They include miscellaneous-agriculture & forestry, miscellaneous-
fugitive dust, and natural sources-wind erosion.

**	NAPAP historical emissions3,4

***	NA denotes not available.

****	Combination of revised transportation values and NAPAP historical emissions.

*****	There is a change in methodology for determining on-road vehicle and non-road sources emissions (see chapter 6).

******	There is a change in methodology in all sources except on-road vehicles and non-road sources and all pollutants except lead, as reflected by the dotted line.

*******	1990 through 1994 estimates are preliminary. The emissions can be converted to metric tons by multiplying the values by 0.9072.