Citation:

Impact/Purpose:

To calculate visibility conditions under the EPA's Regional Haze rule, State and local agencies, and their technical analysts need both measurements of particulate matter concentrationss as well as information about the relative humidity (RH) conditions to which hygroscopic particles are exposed. An EPA workgroup has defined a method for the use of monthly average RH weighting factors f(RH) to satisfy this requirement. The technical members of this multi-agency workgroup have provided input into the development of these datasets and the methodology for their use in the Regional Haze program. A technical paper describing the work and the resulting f(RH) data was presented at the AWMA conference this past spring.

Description:

The EPA's Regional Haze Rule directs that visibility conditions in 156 national parks and wilderness areas should improve during the next sixty years. Since visibility varies with air quality, the contrast between objects, the angle of the sun, and other factors, visibility impairment is measured in the Regional Haze Rule from the calculated light extinction, based on 24-hour concentrations of ambient particulate matter species. Sulfate and nitrate particulate species scatter more light when the relative humidity is high, so a relative humidity weighting factor is included in the calculation. However, relative humidity is not routinely measured in the 156 national parks and wilderness areas. Therefore, ten years of hourly relative humidity data at 375 weather stations across the United States (from both urban and rural sites) were interpolated to estimate monthly average values of the light extinction weighting factors for sulfate and nitrate anywhere in the country. The database and a Java based software tool for using the database is now available through the EPA's HazeCalc freeware tool for calculating visibility conditions. Activity this past year included the review of the methodology used in the calculation of the f(RH) function through both a "formal, public comment period review" of the Regional Haze guidance documentation for tracking progress and the continued technical scrutiny by the technical users of the relative humidity information in other Federal Agencies. The technical experts from the other Federal Agencies are continuing to use this material in their analysis of Regional Haze conditions. This f(RH) Dataset, and this freeware to use it, play an important role in these Regional Haze studies. We believe both the technical and general public comments on this Dataset and the associated methodology for this f(RH) HazeCalc freeware properly cover the needed review of this product. (Note: Product will not be available until late 2005).

URLs/Downloads:

Record Details: