More limited research has been conducted related to PM2.5 modeling. Most model
applications for the annual PM2.5 NAAQS have modeled a full year (CAIR, RPOs, etc). This is a
logical goal since every day of the year is included in the calculation of the annual NAAQS.
Several tests have also been made using less than a full year of meteorological data. One study
(Environ, 2004) found that modeling 14 days from each quarter provided results with a
reasonable amount of uncertainty compared to modeling the full year. The following sections
contain further recommendations for choosing appropriate time periods to model for attainment
demonstrations and glidepath assessments.
At a minimum, four criteria should be used to select time periods which are appropriate
1) Simulate a variety of meteorological conditions:
a) 8-Hour Ozone- Choose time periods which reflect a variety of meteorological
conditions which frequently correspond with observed 8-hour daily maxima > 84
ppb at multiple monitoring sites.
b) 24-Hour PM2.5- Choose time periods which reflect a variety of meteorological
conditions which frequently correspond with observed 24-hour averages > 65
ug/m3 at violating monitoring sites.
c) Annual PM2.5- Choose time periods from each quarter which reflect the variety
of meteorological conditions which represent average concentrations for that
quarter and year
d) Regional Haze- .Choose time periods which reflect the variety of
meteorological conditions which represent visibility impairment on the 20% best
and 20% worst days in the Class I areas being modeled.
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