KEY: CO = carbon monoxide; NO2 = nitrogen dioxide; PM-10 = particulate matter 10 microns in diameter or smaller.
NOTES: Numbers of monitoring sites that provide raw data vary by pollutant and year. For 1975-77: CO (141 sites), lead (43 sites), NO2 (40 sites), ozone (149 sites). For 1978-87: CO (208 sites), lead (160 sites), NO2 (93 sites); ozone (320 sites). For 1988-97: CO (368 sites); lead (195 sites); NO2 (224 sites), ozone (660 sites).
SOURCE: U.S. Environmental Protection Agency, Office of Air and Radiation, Air Quality and Planning and Standards, National Air Quality and Emissions Trends Report, 1997, 454/R-98-016, December 1998, available at www.epa.gov/oar/aqtrnd971, table A-9.