Precipitation in light extinction reconstruction

The U.S. Environmental Protection Agency (EPA) published the Regional Haze Rule (RHR) in 1999. The RHR default goal is to reduce haze linearly to natural background in 2064 from the baseline period of 2000-2004. The EPA default method for estimating natural and baseline visibility uses the Interagency Monitoring of Protected Visual Environments (IMPROVE) formula. The IMPROVE formula predicts the light extinction coefficient from aerosol chemical concentrations measured by the IMPROVE network. The IMPROVE light scattering coefficient formula using data from 1994-2002 underestimated the measured light scattering coefficient by 700 Mm(-1), on average, on days with precipitation. Also, precipitation occurred as often on the clearest as haziest days. This led to estimating the light extinction coefficient of precipitation, averaged over all days, as the light scattering coefficient on days with precipitation (700 Mm(-1)) multiplied by the percent of precipitation days in a year. This estimate added to the IMPROVE formula light extinction estimate gives a real world estimate of visibility for the 20% clearest, 20% haziest, and all days. For example, in 1993, the EPAs Report to Congress projected visibility in Class I areas would improve by 3 deciviews by 2010 across the haziest portions of the eastern United States because of the 1990 Clean Air Act Amendments. Omitted was the light extinction coefficient of precipitation. Adding in the estimated light extinction coefficient of precipitation, the estimated visibility improvement declines to <1 deciview.