humidity and f(rh) values. The f(rh) values were calculated from data reported by Tang (1996).

The f(rh) values are 1.00 up to 36% relative humidity and grow to a maximum value of 7.40 at a

relative humidity of 95%76. The Tracking Guidance contains monthly climatological average

f(rh) values for each Class I area. These values were calculated using 10 years of meteorological


For calculations with the new IMPROVE equation, three separate f(rh) curves are

needed. These can be found in (Hand, 2006). The monthly f(rh) values for both the old and new

IMPROVE equations have been incorporated into precalculated daily extinction and deciview

data that can be found on the Visibility Information Exchange Web System (VIEWS) website ( ). The use of precalculated f(rh) and visibility data will

make it more simple for States and RPOs to calculate visibility changes and will also provide for

more consistency.

6.4 How Do I Apply A Model To Calculate Changes in Visibility ?

The purpose of a modeling assessment of uniform rate of progress is to determine if a

proposed control strategy will result in uniform rate of progress being met when measured

concentrations of particulate matter are used to estimate visibility impairment at some future

date. The analysis we recommend has 6 steps.

1. For each Class I area, rank visibility (in deciviews) on each day with observed

speciated PM2.5 data plus PM10 data for each of the 5 years comprising the base period.



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