Premise PM8. Ability of models to predict future concentrations of the components of
PM2.5 is limited by a variety of factors. Our ability to characterize emissions on a day to day
basis or on a source-specific basis is limited. Fully characterizing meteorological conditions on
any given day is also problematic. Further, for regulatory models to be tractable, they must
characterize chemical and physical processes by simplifying them in some reasonable manner.
In some cases, most notably for secondary organic particulate matter, the extent to which current
simplifications are reasonable is uncertain. These limitations (and others) add to the uncertainty
of the model’s ability to accurately predict concentrations of PM2.5 and its components at a given
time and location.
The preceding paragraph has several implications for using models to demonstrate future
attainment of a NAAQS for PM2.5 or assessing uniform rate of progress for regional haze. It
suggests that we should focus on composite responses of the model averaged over several days
to help circumvent the problem of not knowing all of the details on an individual day. This
composite response then needs to be related to the form of the air quality goal in some manner.
Limitations in available models and their underlying data bases also suggest that the guidance
should recognize a need for performing other, corroboratory analyses to confirm conclusions
reached with a model.
1.4 What Topics Are Covered In This Guidance?
This guidance addresses two broad topics: Part I, “How do I use results of models and
other analyses to help demonstrate attainment?”, and Part II, “How should I apply air quality
models to produce results needed to help demonstrate attainment?”. Part I is divided into 8
sections (i.e., Sections 2-9). Part II consists of 9 sections (Sections 10-18).
Part I ( “How do I use results of models and other analyses to help demonstrate
attainment?”) begins in Section 2 with an overview of the procedure for using modeling results
to help demonstrate attainment of the 8-hour ozone NAAQS and the annual and 24-hour
NAAQS for PM2.5 . Section 2 also summarizes the uniform rate of progress modeling analysis.
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