14.1.3 Annual PM2.5

Similar to episode selection for the daily standards, the most important consideration for

the annual PM2.5 NAAQS is to model a variety of meteorological conditions throughout the year.

The easiest way to accomplish this is to model a full year. Since all days count toward the

annual average, each season should be represented in the modeling. If less than a full year is

modeled then a minimum number of days should be modeled from each quarter.

One study (Environ, 2004) examined how well a subset of modeled days in each quarter

represented modeled quarterly and annual averages. The study calculated the uncertainty

associated with absolute model predictions and RRFs that were based on 7, 14, or 28 days from

each quarter (the periods were contiguous). They found that the relative uncertainty of the

annual average concentrations was in the range of 40-70% for SO4, 30-50% for anthropogenic

organics, and 60-90% for nitrates (the low percentage is for 28 days and the high percentage is

for 7 days). The uncertainty was lowered by about half when considering the uncertainty in

RRFs for the 7-28 day periods vs. the annual average. This shows the advantage of using the

models in a relative sense. But it also shows that modeling less than a full year introduces

99A careful analysis is needed for the days that exceed the 24-hour NAAQS. It may be

necessary to divide days by species as well as by quarter. For example, exceedence days with

high nitrates may respond differently than exceedence days with high organics (in the same

quarter). Averaging the response from many dis-similar days may give an unrealistic mean



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