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Mobile Source Air Toxic Emissions

EPA obtained mobile source air toxics emissions, with the exception of diesel PM, from the 1996 National Toxics Inventory (NTI). EPA developed onroad pollutant emission estimates using two general approaches. In the first approach, EPA used an emission factor model, MOBTOX5b, to develop emission factor estimates which were then applied to county level vehicle miles traveled (VMT) (see more information about the impacts of control programs on motor vehicle toxics emissions and exposures (pdf)(1317K)). EPA used this approach for Benzene, 1,3-Butadiene, Formaldehyde, and Acetaldehyde. MOBTOX5b is based on the MOBILE5b emission factor model, and incorporates elements of MOBILE6. It applies toxic fractions to toxic organic gases (TOG) estimates in the model. It accounts for the differences in toxic fractions of TOG between different technology types, driving cycles, and normal versus high emitters. It also accounts for the impacts of specific fuel parameters. The VMT estimates used were obtained from the National Emission Trends (NET) database, and in some cases States provided alternative estimates.

For other gaseous air toxics and polycyclic aromatic hydrocarbons (PAHs), EPA used a speciation approach. First, EPA disaggregated county-level volatile organic compound (VOC) estimates into fuel types based on their market share of the fuel in 1996. EPA further disaggregated these estimates into exhaust and evaporative emissions components based on data used to develop the NET (if there was an evaporative emissions component for a particular toxic air pollutant). Finally, EPA applied appropriate speciation profiles to the VOC estimates. The PAH estimates are based only on particle phase PAHs; there are plans to include the gas-phase PAHs in future assessments. EPA estimated emissions of Chromium, Mangesium, Mercury, and Nickel from on-highway light-duty gasoline mobile sources by applying VMT to emission factors from a 1997 Ford Motor Company study. For other vehicle classes and other metals, EPA applied particulate matter (PM10) from the NET to metal/particulate matter fractions.

EPA developed all toxic emission inventory estimates for nonroad engines (e.g. construction equipment, lawn and garden equipment, recreational equipment, marine vessels, locomotives and aircraft) using very limited speciation data. Emissions for nonroad equipment in the NONROAD model were estimated using a speciation approach similar to that used for onroad sources. County-level VOC and particulate matter estimates used to develop the inventory estimates were obtained from the April, 1999 version of the draft EPA NONROAD emissions model (see more information on nonroad vehicle and engine emission modeling.

EPA developed commercial marine vessel nationwide inventory estimates by applying speciation data to nationwide VOC and PM emissions for vessels running on distillate and residual fuel oil. EPA allocated emissions to the county level using port activity data. Similarly, for locomotives, EPA developed nationwide inventory estimates by applying speciation data to nationwide VOC and PM emissions inventory development for commercial marine vessels and locomotives . EPA then allocated air toxics emissions to the State level using State fuel oil sales data and to the county level using population data. 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.

For diesel PM estimates in the national-scale assessment, EPA used the PM inventory developed for the regulation promulgating 2007 heavy-duty vehicle standards (see more on procedures for developing inventories for the heavy-duty engine and vehicle standards and highway diesel fuel rulemaking (pdf)(253K).)

EPA obtained air toxics precursor emission data for 1996 from two separate sources: (1) VOCs that are not also air toxics came from Version 3 of 1996 National Emissions Trends (NET) inventory, speciated for specific organic compounds; (2) data for air toxics that are precursors to other air toxics came from the 1996 NTI. The 1996 NTI is no longer available.

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