Normalizing trends in regional haze- f(rh) factors. It is clear from equations 6.1 and

6.2 that relative humidity can have a substantial effect on estimated extinction coefficients, as

well as on the relative importance of changes in different components of particulate matter can

have on trends in regional haze. Because of the importance of relative humidity as a determinant

of regional haze, it is necessary to normalize any apparent trend in the estimated extinction

coefficient for differences in relative humidity. This enables us to assess whether an emissions

control strategy will reduce regional haze, without confounding effects of different relative

humidity during the base and future periods.

There are two obvious potential ways to normalize trends in visibility for changes in

relative humidity. The first is to assume that the same day to day changes in relative humidity

which were observed in the base period calculations will occur in future years. Thus, one would

use the relative humidity observations made on a specific day together with measured

components of particulate matter on that day to compute the day specific visibility extinction

coefficient on that day. Subject to the uncertainties posed by the empirically derived coefficients

in Equation (6.1), this approach is the most likely to identify the 20% best and worst visibility

days during the base period at a Class I site. However, the approach could lead to misleading

conclusions if humidity observations were missing for some days or if the humidity observations

are atypical in some way. Further, if a State or regional planning organization wanted to perform

visibility calculations in a number of Class I areas, they would need to obtain hourly relative

humidity data for each area.


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