Part II of the guidance describes how to apply air quality models. The recommended
procedure for applying a model has nine steps. The results of this process are then used to apply
the modeled attainment test to support an attainment demonstration, as described in Part I of the
1Modeling is not used to determine reasonable progress goals and does not determine
whether reasonable progress has been met. Modeling is one part of the reasonable progress
analysis. The modeling results are used to determine if future year visibility at Class I areas are
estimated to be on a (glide)path towards reaching natural background. This is called a uniform
rate of progress analysis or “glidepath” analysis. The uniform rate of progress analysis is
described in more detail in section 6.
2While this guidance document is primarily directed at modeling applications in
nonattainment areas, it may also be useful as a guide for modeling in maintenance areas or to
support other rules or sections of the Clean Air Act.
3The Regional Haze rule assumes that all 50 States either contain a Class I area or impact
visibility within a Class I area. Therefore, a regional haze State Implementation Plan (SIP) is
required for all States (40 CFR 50.308(b)).
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