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Scope Study for Expanding the Great Lakes Toxic Emission Regional Inventory to include Estimated Emissions from Mobile Sources
Chapter 5 Estimating Air Toxics Emissions from Nonroad Sources
Nonroad sources consist of ten nonroad equipment categories as defined by EPA (EPA, 1992) and are shown in Table 5-1. These categories are differentiated by the purposes of equipment use. There are a total of 79 equipment types within the ten categories. Some examples of equipment types within each category are also shown in Table 5-1. A description of these categories, including the 79 equipment types within them, can be found in the EPA document, "Procedures for Emission Inventory Preparation, Volume IV: Mobile Sources (EPA, 1992)". In general, each equipment type consists of three possible engine types: diesel, 4-stroke gasoline, and 2-stroke gasoline. In addition, some equipment may be fueled by propane or compressed natural gas (CNG).
There are two potential approaches for deriving toxic emissions for nonroad sources:
The following sections discuss these emission estimation techniques.
5-2. Use Toxic Emission Factors Based on Activity Level
Air toxic emissions from nonroad sources can be estimated by multiplying activity data by emission factors. Activity data for toxic emission estimation are described in Section 5-3-2-2. Methods for determining these activity data are explained in Section 5.
In the recent version of FIRE (5.1a) (EPA, 1996), there are no toxic emission factors that can be used directly for nonroad sources. Therefore, appropriate emission factors for internal combustion engines with diesel, gasoline, propane, and CNG need to be used as surrogates for emission estimations. Emission factors can also be extrapolated from the information for light duty gasoline vehicles. In addition, the Society of Automotive Engineers has published a number of papers on emissions from small 2-stroke and small 4-stroke engines and emission factors can be derived from this research.
In the case that all the above information is not available, emission factors may be derived from TOG and PM speciation profiles.
Using the emission factor method, the Wisconsin Department of Natural Resources (WDNR) has developed a toxic air emission inventory for nonroad engines that includes diesel engines (construction equipment), gasoline 4-stroke engines (construction equipment, lawn and garden equipment, inboard boat motors, etc.) and gasoline 2-stroke engines (outboard boat motors, lawn and garden equipment, snowmobiles, etc.). A draft of the WDNR inventory can be obtained form Keith Warnke at (608) 267-0804.
5-3. Combine Total Organic Gases and Particulate Matter Emissions with Speciation Profiles
Building a toxic emission inventory for nonroad sources can also be accomplished by first estimating TOG and PM emissions with activity-based emission factors and then speciating the TOG and PM emissions. Diesel, 4-stroke gasoline, 2-stroke gasoline, propane, and CNG engine emissions have different exhaust TOG speciations. In addition, evaporative emission speciation profiles differ from exhaust emissions in that evaporative emissions consist of more volatile fuel components with no combustion products. Therefore, TOG and PM emissions should be estimated for each type of engine within the source category.
There are two approaches to estimate TOG and PM emissions from nonroad sources. The first is to use EPA inventories with adjustments, and the second is to estimate emissions by using activity data at the county level.
5-3-1. Estimating Total Organic Gases and Particulate Matter Emissions by Using EPA Inventories
EPA developed the 1990 VOC and PM emission inventories for 33 areas listed in Appendix A-5-1. These areas are believed to be geographically representative of areas with significant air pollution problems.
To estimate emissions for a particular county of interest, the states and province may choose one of the 33 areas, which is similar in climate condition and economic activity, then VOC and PM emissions can be estimated by applying the ratio of the population of a county to the area. Equation (5-1) illustrates the calculation.
It is important to note that EPA prepared the inventories for the first nine categories of nonroad engines and vehicles for the 33 areas. Inventories for the last category (commercial and marine vessels) are only available for six areas and have less reliable data than the inventories for the first nine categories.
VOC emissions are divided into four sub-categories:
It should also be noted that the definition of VOC in the EPA inventory includes formaldehyde and acetaldehyde but excludes methane and ethane. In order to apply the TOG speciation profiles, the estimated VOC emissions need to be converted to TOG. In the WDNR inventory, it was assumed that 100% of hydrocarbon emissions are TOG. More study is needed to verify this assumption and determine reasonable conversion factors.
5-3-2. Estimating Total Organic Gases and Particulate Matter Emissions by Using Activity Data at the County Level
This section explains a methodology to estimate TOG and PM emissions by using activity data at the county level. The first subsection discusses the general formula for emission estimates while the second subsection provides information on the activity parameters in the formula.
5-3-2-1. General Formula for Emission Estimation
Shown below are two general equations to calculate emissions for a pollutant (including VOC, PM, and air toxics) by using emission factors based on activity level.
Exhaust and Crankcase Emissions:
Emissions for a pollutant are for a specific equipment category type in units of grams per year (g/year).
An emission factor is defined as the average emissions of the pollutant from the specific equipment category type and may be expressed in units of g/hp-hr or g/gal. The emission factors can be derived from test data with appropriate extrapolations to the different equipment types. Appendix I of the EPA nonroad report (EPA, 1991) contains the VOC and PM emission factors for the 33 areas.
An equipment population is the number of pieces of equipment for the specified equipment category type in the county of interest.
Annual hours of use of the equipment category type is presented in hours/year, days/year, or, in some cases, gallons/year, depending on the unit of the emission factor.
Since emission factors are shown as g/hp-hr in some cases, an average rated horsepower and a load factor should be multiplied in those cases. A load factor is a percent of rated horsepower typically used for the specified equipment category type.
The emission factors for evaporation are expressed in units of grams per hour (g/hr) or grams per day (g/day). The evaporative emission season for diurnal VOC emissions is assumed to be 229 days/year which includes all summer days, no winter days, and most other days.
It should be noted that emission factors are engine-type specific and most of the emission factors are based on tests of new engines. Limited information shows that emissions from in-use (older) engines could be as much as two times higher than from new engines (EPA, 1991). In the nonroad study, EPA also provided a set of emission factors for VOC that includes a gross adjustment for in-use deterioration. However, uncertainties are associated with the in-use adjustment, and better adjustment factors need to be developed. Attention should be paid to these uncertainties before use of the in-use adjustment.
In addition, since emission factors are available for VOC but not TOG, appropriate conversion should be made to get TOG emissions.
5-3-2-2. Determining Activity Parameters within Each Category Type at the County Level
The following activity parameters need to be determined at the county level:
These activity parameters can be determined through surveys. However, in most cases, the survey data are not available at the county level. The method, presented below, that was described in the EPA nonroad study (EPA, 1991) may be used.
Since annual hours of use, average horsepower, and load factors remain essentially constant throughout an area and, in some cases, on a state or national level, determining equipment populations in each category type at the county level is the most challenging. Determining equipment populations can be accomplished by using activity indicators. EPA contracted with Energy and Environmental Analysis, Inc. to conduct a Nonroad Engine and Vehicle Emission Study (EPA, 1991). In that report, equipment populations were estimated by using regression analysis of state level populations and activity indicator statistics. The indicators were derived from economic data presented in various census publications. A linear relationship between specific activity indicators and an equipment categorys state population was then determined by using regression analysis. The activity indicator which has the strongest relationship with the equipment category of interest was used to determine the equipment categorys population at the county level. Table 5-2 shows the activity indicators for each equipment category.
The general formula to estimate county equipment populations is shown in Equation (5-4).
The states and province can use other indicators if these indicators show stronger relationships with equipment populations under their local conditions.
Data for national and state equipment populations, annual hours of use, load factors, average horsepower, 2-stroke /4-stroke distribution, and propane/CNG penetrations can be found from a market research database commercially available through Power System Research. This database is updated frequently through surveys of equipment manufacturers and users.
5-3-3. Speciation Profiles
Speciation profiles for nonroad sources can be obtained from the same information sources as identified in Section 4-4-2-3. Tables 5-3 and 5-4 show the existing EPA and the California Air Resources Board (CARB) TOG and PM speciation profiles for nonroad sources (EPA, 1993; CARB, 1991).
In addition to the SPECIATE profiles, EPA indicated the percentages by weight of hydrocarbons for benzene and 1,3-butadiene (EPA, 1991). These values are shown in Table 5-5.
The formaldehyde emissions are assumed as 60% of total nonroad aldehyde emissions in the EPA nonroad source study. The emission factors are available for aldehyde as well as VOC and PM in Appendix I of the EPA nonroad report (EPA, 1991).
5-4. Seasonal Adjustments
Emissions estimated by either using toxic emission factors based on activity level or combining TOG and PM emissions with speciation profiles are annually based. These annual emissions can be apportioned to a typical summer day or a typical winter day by using seasonal adjustment factors (SAF) developed by EPA (EPA, 1991). The adjustment procedures are shown in the following equations.
Typical Summer Day Emissions = Annual Emissions * SAFSummer
Typical Winter Day Emissions = Annual Emissions * SAFWinter
Values for the percent activity during summer and winter are based on eight regions. These regions are also classified into three seasonal designations: cold, medium, and warm, based on average January temperatures.
Table 5-6 shows the regions and their seasonal designations.
In the Great Lakes region, the St. Lawrence Seaway and the Soo Locks close during the winter season halting commercial ocean-going marine vessel traffic. The states and province should obtain the actual operation days when making the seasonal adjustments.
Since airport service vehicles, logging equipment, or light commercial equipment are typically used year-round, no seasonal adjustments in emission estimates need to be made for these categories. Summer and winter percentages of yearly activity for the remaining categories can be found in Appendix A-5-2.
CARB, Speciation Manual, Volume 1: Identification of Volatile Organic Compound Species Profiles, Volume 2: Identification of Particulate Matter Species Profiles, 2nd Edition, State of California Air Resource Board, August 1991.
EPA, Factor Information Retrieval System (FIRE) Version 5.1a, U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards, Research Triangle Park, NC, May 1996.
EPA, Nonroad Engine and Vehicle Study, Report and Appendices, U.S. Environmental Protection Agency, Office of Office of Air and Radiation, Washington, D.C., EPA -21A-2001, November 1991.
EPA, Procedures for Emission Inventory Preparation: Volume IV: Mobile Sources, U.S. Environmental Protection Agency, Office of Mobile Sources, Ann Arbor, MI and Office of Air Quality Planning and Standards, Research Triangle Park., NC, EPA-450/4-81-026d (Revised), 1992.
EPA, Volatile Organic Compounds (VOC)/Particulate Matter (PM) Speciation Data System (SPECIATE), Version 1.5, Emission Inventory Branch (MD-14), Office of Air Quality Planning and Standards, U.S. Environmental Protection Agency, Research Triangle Park, NC. 1993a
Table 5-1. Nonroad mobile source equipment categories and examples
Table 5-2. Activity indicator for determining equipment population at the county level