Mobile Source Air Toxic Emissions

Note: EPA no longer updates this information, but it may be useful as a reference or resource.

EPA obtained mobile source air toxics emissions from the 1999 National Emissions Inventory (NEI), draft Version 3 for Hazardous Air Pollutants.

EPA developed highway vehicle pollutant emission estimates using the air toxics module for MOBILE6.2 (see https://www.epa.gov/otaq/m6.htm to obtain the model and technical guidance on its use). MOBILE6.2 estimates milligram per mile emission rates by vehicle class for benzene, 1,3-butadiene, formaldehyde, acetaldehyde, and acrolein directly, based on fuel parameter descriptions and emissions control technology. Information on fuel parameters and the mix of emission control technologies is used to estimate air toxic to HC (hydrocarbon) ratios which are then applied to HC emission rates to estimate the toxic emission rates. Emission rates for other hazardous air pollutants are calculated based on information supplied in external data files, as described in the documentation for the highway vehicle portion of the NEI (ftp://ftp.epa.gov/EmisInventory/draftnei99ver3/haps/documentation/orcrit_hap.pdf). For gaseous pollutants except for polycyclic aromatic hydrocarbons (PAHs), toxic to HC ratios provided in the external data files were applied to HC emission rates. These ratios varied by vehicle class and by fuel type for some pollutants. For gaseous and particle phase PAHs, PAH to exhaust particulate matter (PM) ratios provided in external datafiles were applied to total exhaust PM emission rates. Metal compounds were estimated using milligram per mile basic emission rates provided in the external data files. The vehicle miles traveled (VMT) estimates used were obtained from the National Emission Inventory (NEI) database. In most cases, EPA estimated VMT based on data compiled by the Federal Highway Administration, but in some cases, States provided their own VMT estimates. For several pollutants, California provided its own highway mobile source inventory estimates.

EPA developed toxic emission inventory estimates for most nonroad engines (those included in its draft NONROAD model, such as construction equipment, lawn and garden equipment, recreational equipment) using limited speciation data which were then applied to criteria pollutant (VOC or PM) emission estimates at the county level. For gaseous pollutants except for polycyclic aromatic hydrocarbons (PAHs), toxic to VOC ratios were applied to county level VOC estimates. For gaseous and particle phase PAHs, PAH to PM ratios were applied to county level PM estimates. Metal compounds were estimated by multiplying milligram per mile emission rates by national activity estimates, then allocating to the county level using the ratio of county level PM10 to nationwide PM10. EPA developed nationwide HAP inventory estimates for engines not in the draft NONROAD model, i.e., from commercial marine vessels, locomotives and aircraft, using approaches similar to those described above, and then allocating emissions to the county level. For commercial marine vessels, EPA allocated emissions to the county level using port activity data. Similarly, for locomotives, EPA allocated emissions to the county level using GIS based rail activity data provided by the U.S. Department of Transportation. EPA used the same general approach to develop the aircraft inventory, with nationwide air toxics emission estimates allocated to the county level using national air carrier activity data (see more information on inventory development for nonroad equipment at ftp://ftp.epa.gov/EmisInventory/draftnei99ver3/haps/documentation/). It should be noted that for some pollutants and some nonroad engine sources, States provided their own estimates. This is detailed in the documentation for the inventory.

Mobile Source Air Toxic Emissions

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