The second approach to normalize trends in the extinction coefficient is to review relative

humidity data over a long period of record to derive climatological estimates for relative

humidity adjustment factors. These climatological estimates would then be used in Equations

6.1 and/or 6.2 to estimate visibility extinction coefficients. These estimates are more likely to

reflect “typical” relative humidity at different times of year and, thus, expected future visibility


The need to use climatological f(rh) data for tracking progress of measured data is

obvious. The measured relative humidity on the 20% worst days in a baseline period will be

different than the measured relative humidity in a future period. Therefore the only way to

normalize for relative humidity is to use a single climatological value. In the context of

modeling, there is a choice. We can use climatological data or we can use the modeled (or

measured) relative humidity data from the air quality model. Since the meteorological data is

held constant when predicting future year air quality, the modeled relative humidity values

would also be constant. This would be one way to normalize the data.

Throughout this section we make every attempt to be consistent with the “Guidance for

Tracking Progress Under the Regional Haze Program”(hereafter referred to as the “Tracking

Guidance”) (U.S. EPA, 2003b). It will be easier to interpret the modeling results and compare

them to future measured values, if the data are calculated in the same manner. We therefore

recommend using climatological f(rh) values to calculate baseline and future visibility.

Appendix A (Table A-1) from the Tracking Guidance displays the relationship between relative



n701 - n702 - n703 - n704 - n705 - n706 - n707 - n707 - n709 - n710 - n711 - n712 - n713 - n714 - n715 - n716 - n717 - n718 - n719 - n720 - n721 - n722 - n723 - n724 - n725 - n726 - n727 - n728 - n729 - n730 - n731 - n732 - n733 - n734 - n735 - n736 - n737 - n738 - n739 - n740 - n741 - n742 - n743 - n744 - n745 - n746 - n747 - n748 - n749 - n750


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