write:particularly where measurements of PM2.5 have had a longer history. Annual average PM2.5 concentrations decreased by 50% on the southern coast of California and 30% in the San Fran

IN 34122 Honolulu
HI 2458 Los Angeles
CA 47101 Louisville
KY 3743 New Haven
CT 52178 New York
NY 4786 Philadelphia
PA 34531 Phoenix
AZ 4492 Provo
UT 40209 Santa Fe
NM 1535 San Francisco
CA 2972 St. Louis
MO 44101 Steubenville
OH 40177 Source: From USEPA
EPA/600/AP-95/001c
EPA
Washington
D.C.
1995. particularly where measurements of PM2.5 have had a longer history. Annual average PM2.5 concentrations decreased by 50% on the southern coast of California and 30% in the San Francisco Bay area during the period 1990–1995. The availability of PM10 and PM2.5 concentration data has made other assessments possible. By subtracting concentrations of PM2.5 from PM10
an estimate of coarse particle concentrations can be made. The nationwide median annual concentration for coarse particles (PM10–2.5) is ~10 µg/m3
with values as high as 54 µg/m3. Highest coarse fraction concentrations are reported for western states
especially California. PM10:PM2.5 mean ratios of 0.70 have been reported for the Northeast
Southeast
and the industrial Midwest; lower ratios have been reported for the Northwest (0.50)
Southwest (0.38)
and southern California (0.47). Ambient PM10 and PM2.5 vary seasonally
with highest concentrations typically reported during summer months during high-pressure
hazy days. Diurnal variations in PM2.5 concentrations have also been reported
with maximum concentrations occurring during the morning and evening. 2.4.8 Long-Range Transport from Sources Outside the U.S. It has been known for decades that both precursor gas-phase molecules and
to a lesser extent
particulate substances (e.g.
acid sulfates) can be transported hundreds of kilometers or more. Because of the availability of data from intensive monitoring studies of particulate aerosols and analysis of their chemical constituents
as well as satellite imaging
there is substantial evidence that particles are transported into the U.S. from very distant sources (thousands of kilometers). This includes transport from biomass Atmospheric pollution and pollutants 71