5 The composition of PM2.5 also varies by season and helps explain why mass varies by

6 season. Figure A-5 shows the average composition by season (spring, summer, fall and winter)

7 for PM2.5 data collected during 2008-10. In the eastern United States, sulfate are high in the

8 spring (March-May) and summer (July-September). Nitrates are most evident in the midwest and

9 western cities where its percentage is moderately high in the spring and fall (October-and highest

10 during the winter.) Organic mass (OM) is high throughout the year.

5 The composition of the highest daily PM2.5 values may be different than that for the

6 annual average. Table A-1 provides 2008-10 data on daily PM2.5 values and their composition on

7 high mass days for various sites within large metropolitan areas (in the east: Birmingham, AL;

8 Atlanta, GA; New York City, NY; Cleveland, OH; Chicago, IL; and St. Louis, MO; in the west:

9 Salt Lake City, UT; and Fresno, CA). Mass is proportioned into five components: sulfates,

10 nitrates, OM, elemental carbon (EC) and crustal material. For each site, the table shows the

11 2008-2010 annual average speciation profile, the breakdown for the top 10 percent of days per

12 year and corresponding FRM mass. The table shows some notable differences in the percentage

13 contribution of each of the species to total mass when looking at the high end of the distribution

14 versus the annual average. Except for the southeast (where there is little nitrate in PM2.5),

15 nitrates are slightly higher in the top 10 percent of the PM2.5 days. For the 2008-2010

16 measurements, the percent of sulfates is currently similar or slightly less on the top 10 percent of

17 the days as compared to the annual averages. The portion of OM appears to be similar on the

18 high days compared to the annual averages.



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