23The 15 cities are the same ones included in the Integrated Science Assessment for Particulate Matter (2009) which
includes a similar map based on 2005-2007 PM2.5 data.
24Frank, N. H., Retained Nitrate, Hydrated Sulfates, and Carbonaceous Mass in Federal Reference Method Fine
Particulate Matter for Six Eastern U.S. Cities, 'J. Air & Waste Manage. Assoc.' 2006, '56', 500-511.
3. Seasonal and Daily Patterns ofPM2.5
8Fine particles often have a seasonal pattern. Both daily values and quarterly average of
9PM2.5 also reveal patterns based on the time of year. Unlike daily ozone levels, which are
10usually elevated in the summer, daily PM2.5 values at some locations can be high at any time of
11the year. As shown in Figure A-4, PM2.5 values in the eastern half of the United States are
12typically higher in the third calendar quarter (July-September) when sulfates are more readily
13formed from sulfur dioxide (SO2) emissions from power plants in that region and when
14secondary 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
16part because fine particle nitrates are more readily formed in cooler weather. PM2.5 values are
17high during the first (January through March) and fourth calendar quarter (October through
18December) in many areas of the West, in part because of fine particle nitrates and also due to
19carbonaceous particles which are directly emitted from wood stove and fireplace use. Average
20concentration from all locations reporting PM2.5 with valid design values is shown.
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