Key: PM-10 = airborne particulates of less than 10 microns; PM-2.5 = airborne particulates of less than 2.5 microns.
Notes: Not all graph scales are comparable. The indices are calculated using data on emissions from highway vehicles only. Particulate matters include PM without condensibles. Data for 2003 through 2009 are revised. In the National Emissions Inventory (NEI) 2008, on-road mobile source estimates are based on a new methodology. Data for 2008 and beyond may not be comparable to previous years.
Source: U.S. Environmental Protection Agency, National Emissions Inventory (NEI) Air Pollutant Emissions Trends Data, available at http://www.epa.gov/ttn/chief/trends/index.html accessed August 2012.