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Mobile Source Air Toxic Emissions

Information provided for informational purposes onlyNote: EPA no longer updates this information, but it may be useful as a reference or resource.

EPA obtained mobile source air toxic emissions estimates from the 1999 National Emissions Inventory (NEI). Inventory estimates for all pollutants except diesel PM were obtained from the final version 3 inventory for hazardous air pollutants, whereas diesel PM estimates were obtained from the final version 3 inventory for criteria pollutants. These inventories, along with supporting documentation, are available on EPA's CHIEF Web site

For onroad mobile sources, EPA developed inventory estimates for all States except California, which provided its own inventory. EPA developed the onroad mobile source estimates using its motor vehicle emission factor model, MOBILE6.2 (https://www.epa.gov/otaq/m6.htm) in conjunction with vehicle miles traveled (VMT). VMT estimates were obtained from the Federal Highway Administration and from 13 state and local environmental agencies that submitted VMT data to EPA to use in the NEI.

Within the MOBILE6.2 model, six air toxics (benzene, formaldehyde, acetaldehyde, 1,3 butadiene, acrolein, and methyl tertiary butyl ether [MTBE]) are calculated directly. To estimate emission rates for these pollutants, detailed fuel parameter data are required as model inputs. EPA obtained these fuel data from several sources which survey fuel properties at service stations across the U.S. These data sources, and the properties used in each U. S. county are documented on the 1999 NEI Web site.

Emission factors for an additional 26 air toxics were calculated by MOBILE6 through the use of external data files specifying emission factors for these pollutants in one of three ways: as fractions of volatile organic compounds (VOC), fractions of particulate matter (PM) , or by supplying the basic emission factors. Exhaust and evaporative emissions of gaseous hydrocarbons were estimated as fractions of VOC. These fractions varied by fuel formulation for gasoline vehicles (i.e. non-reformulated, reformulated with MTBE, reformulated with ethanol, winter oxygenated with MTBE, winter oxygenated with ethanol). Polycyclic aromatic hydrocarbons (PAHs) were calculated as fractions of PM, although the data used to calculate mass ratios included both gas and particle phase PAH emissions. Metals were estimated using basic emission factors.

EPA developed toxic emission inventory estimates for nonroad engines in the EPA NONROAD model (e.g. construction equipment, industrial equipment, lawn and garden equipment, recreational equipment) for all States except California and Texas, which provided their own data. Emissions were estimated using a speciation approach. County-level VOC and particulate matter estimates used to develop the inventory estimates were obtained from the 1999 NEI, final version 3. Nonroad HAPs were estimated using one of three approaches:

  1. Gaseous air toxics - Applied toxic to VOC ratios to NONROAD VOC estimates.
  2. PAHs - Applied toxic to PM ratios to NONROAD PM estimates
  3. Metals - Multiplied toxic air pollutant gram per gallon emission factors by county level fuel consumption estimates.
For many sources, data were very limited, and in some cases nonexistent. Where data were nonexistent, speciation profiles for similar sources were used.

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 and shipping lane activity data. Similarly, for locomotives, EPA developed nationwide inventory estimates by applying speciation data to nationwide VOC and PM emissions, or multiplying emission factors by activity, as described above for nonroad equipment in the NONROAD model. EPA then allocated air toxics emissions to the county level using county specific railroad traffic 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.

EPA obtained air toxics precursor emission data for 1999 from two separate sources: (1) VOCs that are not also hazardous air pollutants came from the 1999 NEI for criteria pollutants, version 2, speciated for specific organic compounds; (2) data for air toxics that are precursors to other air toxics came from version 3 of the 1999 NEI for hazardous air pollutants.

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