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. Ammonia data for 2003 through 2005 are revised. Data for 2006 through 2008 are revised.
Source: U.S. Environmental Protection Agency, Clearinghouse for Inventories and Emissions Factors (CHIEF), Current Emission Trends Summaries, available at http://www.epa.gov/ttn/chief/trends/index.html as of August 2011.