Review of the IMPROVE Equation for Estimating Ambient Light Extinction Coefficients

The Interagency Monitoring of Protected Visual Environments (IMPROVE) protocols for reconstructing ambient light extinction from measured aerosol species is the basis for evaluating compliance under the Regional Haze Rule. We review the assumptions involved in computing reconstructed light extinction using the IMPROVE protocol. This review includes examining the biases in the measurements of aerosol composition, the assumed chemical forms of aerosol species, particle hygroscopicity, and assumed mass scattering efficiencies. We present a thorough survey of estimates of mass scattering efficiencies from recent peer-reviewed literature. Furthermore, we use IMPROVE nephelometry and composition data to estimate mass scattering efficiencies using a variety of methods. The current mass scattering efficiencies applied in the IMPROVE equation are then interpreted in the context of this survey and results derived from the IMPROVE data analyses. Finally, a summary of provisional recommendations for refinements to the IMPROVE equation and a discussion of important uncertainties to consider in the assumptions is presented. Although tentative recommendations of refinements to the IMPROVE equation are presented, final refinements to the IMPROVE equation await future discussions of the results presented here.