7 4. Regional and Local Sources of PM2.5


9 Both local and regional sources contribute to particle pollution. Figure A-6 shows how

10 much of the PM2.5 mass can be attributed to local versus regional sources for 13 selected urban

11 areas. In each of these urban areas, monitoring sites were paired with nearby rural sites. When

12 the average rural concentration is subtracted from the measured urban concentration, the

13 estimated local and regional contributions become apparent. Urban and nearby rural PM2.5

14 concentrations suggest substantial regional contributions to fine particles in the East. The

15 measured PM2.5 concentration is not necessarily the maximum for each urban area. Regional

6 Note: derived as the interpolated difference between urban CSN concentrations (squares) compared with nearby

7 IMPROVE site concentrations within 150 km (circles). Annual mean concentrations from 2005-2008 are used. CSN

8 sites not used in the analyses are shown as triangles.26



11 As shown in Figure A-6, we observe a large urban excess across the United State for

12 most PM2.5 species but especially for elemental carbon (EC) and organic mass (OM). Large

13 excess for OM is observed in California, throughout the Northwest, and in the Southeast. The

14 prevalence of urban excess in EC is seen more widely. Large urban excess of nitrates is seen in

15 California. These results indicate that local sources of these pollutants are indeed contributing to

16 the PM2.5 air quality problem in these areas. As expected for a predominately regional pollutant,

17 only a modest urban excess is observed for sulfates.


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