6 3. Seasonal and Daily Patterns of PM2.5


8 Fine particles often have a seasonal pattern. Both daily values and quarterly average of

9 PM2.5 also reveal patterns based on the time of year. Unlike daily ozone levels, which are

10 usually elevated in the summer, daily PM2.5 values at some locations can be high at any time of

11 the year. As shown in Figure A-4, PM2.5 values in the eastern half of the United States are

12 typically higher in the third calendar quarter (July-September) when sulfates are more readily

13 formed from sulfur dioxide (SO2) emissions from power plants in that region and when

14 secondary organic aerosol is more readily formed in the atmosphere. Fine particle concen15

trations tend to be higher in the first calendar quarter (January through March) in the Midwest in

16 part because fine particle nitrates are more readily formed in cooler weather. PM2.5 values are

17 high during the first (January through March) and fourth calendar quarter (October through

18 December) in many areas of the West, in part because of fine particle nitrates and also due to

19 carbonaceous particles which are directly emitted from wood stove and fireplace use. Average

20 concentration from all locations reporting PM2.5 with valid design values is shown.


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