Distributions of PM10 and PM2.5,1999–2005, surveillance-oriented sites from contiguous U.S. region.

aFollowing the promulgation of the National Ambient Air Quality Standard for PM2.5 in 1997, routine collection of PM2.5 was implemented in 1999. No attempt was made to convert PM10 concentrations to PM2.5, which requires a scaling factor based on a presumptive proportion of PM2.5 in the PM10 mass.bSites describe unique sampling points indicated by longitude/latitude. Those providing no geodetic datum information were not included.cWhile PM was typically measured at a frequency of every six days or higher, many sites took daily sampling.