NONROAD MOBILE Emissions Modeling

This paper describes the EPA nonroad mobile source emissions model now under development. The NONROAD model, as it is called, will be released in a beta version in early 1998, and then in final version in late summer 1998. The current working version of the model estimates emissions from nonroad agricultural, construction, industrial, recreational marine, and lawn and garden equipment; emissions from commercial marine, locomotive, and aircraft equipment may be added before the final release of the model if resources are available. Engine types covered by the model include twoand four-stroke gasoline and diesel engines, and CNG/LPG engines. These engines are used in both rural and urban areas, and their operation can range from primarily steady-state (e.g., generators) to extremely transient (e.g., backhoes or other construction equipment). Modeling emissions from the nonroad sector is especially challenging because of difficulties in determining in-use population, activity, load, and emission rates. The in-use population for nonroad engines is difficult to determine because the equipment is not registered or permitted as it is for highway mobile or stationary sources. In this paper, the capabilities, structure, basic calculations, input data required, and output of the model will be described. The paper will also include a discussion of the current sources of in-use population estimates, equipment age distribution, emission factors, activity estimates, growth estimates, and geographic and temporal allocation factors. INTRODUCTION Air pollution in the United States results from the emission of a wide variety of manmade and natural pollutants such as volatile organic compounds (VOC) , nitrogen oxides (NO ), air toxics, and x particulate matter (PM). Under the Clean Air Act, the U.S. Environmental Protection Agency (EPA) is given specific responsibilities to reduce the emissions of pollutants from a range of sources in order to provide clean and healthy air in the United States. Generally speaking, the EPA classifies anthropogenic (manmade) emissions into three broad categories, mobile, stationary (point), and area sources. Mobile source emissions (historically included in area source emissions) are further disaggregated into on-road (e.g., cars, trucks, and motorcycles) and nonroad emission categories. Because of the significant contribution of nonroad emission sources to the total mobile source emission inventory, it has become critical over the past several years for the EPA to provide state and local pollution control agencies with the ability to easily create and project accurate, reproducible inventories of nonroad emissions to satisfy the specific requirements of the Clean Air Act Amendments of 1990. In a report issued by the EPA in 1991, the agency reported that nonroad vehicles and equipment were a significant source of VOC, NO , and PM emissions (1). The report showed that in some areas of the x country, nonroad emissions contributed to as much as a third of the total mobile source NO and VOC x inventory and over two-thirds of the mobile source PM inventory. To assist states and local regulatory agencies in the creation of accurate nonroad emission inventories, the EPA is currently developing a national nonroad emissions model, referred to as NONROAD. The beta version of the nonroad emissions model, which will be released in early 1998, will predict emissions for all nonroad equipment categories and subcategories with the exception of locomotive and aircraft emissions; future model updates will include modules for estimating emissions from those sources if resources are available. The model estimates emissions for current years, and includes growth and scrappage functions to predict emissions for future years as well as to backcast emissions for past years. Structurally, the model is made up of three specific components: a graphical user interface (GUI), which allows the user to easily input model parameters; the core model, which contains all computational algorithms; and a reporting utility for viewing and summarizing modeled emissions estimates. The current schedule calls for the release of the beta version of the model in early 1998 for testing and peer review by air quality planning agencies and other interested parties, finalizing the model in mid-1998, and releasing the model in August 1998. NONROAD MODEL DESCRIPTION Equipment Types The NONROAD model estimates emissions from a diverse collection of vehicles and equipment types in the following categories: • airport ground support, such as terminal tractors; • agricultural equipment, such as tractors, combines, and balers; • construction equipment, such as graders and back hoes; • industrial and commercial equipment, such as fork lifts and sweepers; • recreational vehicles, such as all-terrain vehicles and off-road motorcycles; • residential and commercial lawn and garden equipment, such as leaf and snow blowers; • logging equipment, such as shredders and large chain saws; • recreational marine vessels, such as power boats. C underground mining equipment; and C oil field equipment. The model includes more than 80 basic and 260 specific types of nonroad equipment, and further stratifies equipment types by horsepower rating. Fuel types include gasoline, diesel, compressed nocturnal gas (CNG), and liquified petroleum gas 9LPG). The user has the option of estimating emissions for all source types, or for one or more selected main categories of nonroad equipment. The user can also specify all fuels or one or more specific fuel types. Furthermore, the user can select one or more specific types of nonroad equipment with a specified fuel type(s). While the current working version of the model does not include emissions from aircraft, locomotive, or commercial marine equipment, these sources may be added before the final release if resources are available. Pollutants Reported The NONROAD model estimates emissions for six exhaust pollutants: hydrocarbons (HC), NO , X carbon monoxide (CO), carbon dioxide (CO ), sulfur oxides (SO ), and PM. The user selects among 2 X five different types for reporting HC — as total hydrocarbons (THC), total organic gases (TOG), nonmethane organic gases (NMOG), non-methane hydrocarbons (NMHC), or volatile organic compounds (VOC). Particulate matter can be reported as total PM, PM of 10μ or less (PM ), or PM of 2.5μ or less 10 (PM ). The model also estimates emissions of non-exhaust HC for six modes — hot soak, diurnal, 2.5 refueling, resting loss, running loss, and crankcase emissions. Geographic and Temporal Coverage In each run of the model, the user selects what geographic area(s) are top be included. At the broadest level, the model estimates national total emissions. More commonly users will specify emissions by state, or for one or more counties within a state. At the most detailed level, the user can estimate sub-county emissions; however, this is an advanced feature and the user must supply subcounty input data of the types described below. Thus the model is capable of estimating, in a single run, emissions for nonattainment areas which may consist of multiple counties or sub-counties in one or more states. The NONROAD model can estimate current year emissions for the specified geographic area as well as project future year emissions and backcast past year emissions. In estimating future year projections and in backcasting, the model includes growth and scrappage rates for equipment in addition to a variety of control program options (discussed further below). The model can calculate emissions for a variety of time periods — an entire year, one of four seasons, or any particular month. Emissions for the period selected are estimated either for the total period or for a typical day (weekday or weekend) in that period.