There are several data completeness issues that may cause problems within the modeling
analysis. First, a site may have less than 3 years of complete data. This will be a problem for the
purpose of modeling and for tracking progress. States should work with their EPA Regional
Office and Federal Land Managers to determine how to estimate baseline visibility for these
Another issue that is a more specific problem for modeling occurs when data is missing
during the meteorological time period that was modeled. It is likely that most States will only be
modeling a single year (or less). Therefore it is possible that some or all of the ambient data at
one or more Class I areas is missing during that year. Without ambient data, it is impossible to
identify the 20% best and worst days (used to calculate modeled RRFs).
80The f(rh) values in the Tracking Guidance were developed for the “old” IMPROVE
algorithm and should not be used to calculate visibility using the “new” IMPROVE algorithm.
81Bering Sea Wilderness (Alaska) is the only Class I area that has no IMPROVE monitor
and no representative IMPROVE site. On-site IMPROVE or representative IMPROVE data can
be found for the other 155 sites.
82It may be informative to compare model response at the representative IMPROVE site
versus the actual location of the Class I area. Large differences in the model response may
indicate that the “representative” site may not adequately represent visibility at the Class I area.
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