Observational models. Receptor models are potentially useful for flagging potential

importance of local sources, if any, in influencing measurements made in a Class I area. This

could lead to a refined treatment of a local source, either through a more finely resolved nested

grid model application or plume-in-grid modeling. Trajectory models may also be useful for

identifying the types of meteorological conditions most often corresponding to observed “worst”

and “best” visibility in various Class I areas. This, in turn, may enable States to draw inferences

about the orientation of areas containing sources most likely to influence visibility in a Class I

area on days with “poor” and “good” visibility. Grid model based techniques such as tagging,


DDM, or source apportionment may also be useful in identifying areas and emissions sources

most responsible for visibility impairment on the worst or best days.

Refinements to the recommended uniform rate of progress analysis. If a strategy for

meeting the glidepath appears generally successful, but the glidepath is not met in a limited

number of Class I areas, States may consider refining the recommended uniform rate of progress

analysis in some manner. Refinements are best made if they are based on local

observations/analyses which suggest that some of the underlying assumptions in the

recommended assessment may not be applicable. We list some potential refinements which

could be considered. The list is intended to illustrate types of additional analyses which could be


- Use an alternative light extinction equation such as the revised IMPROVE equation or

an area-specific version.

-Available speciated data and other information may be reviewed to see whether the

outcome of the test is being influenced by including one or more days with extraordinary

events (e.g., a nearby major forest fire lasting a number of days or transported dust

events). If convincing arguments can be made that the event is a “natural” one,

excluding these days from the calculations should be discussed with the appropriate U.S.

EPA regional office.


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