The Interagency Monitoring of Protected Visual Environments (IMPROVE) protocols for reconstructing ambient light extinction from measured aerosol species are the basis for tracking progress under the Regional Haze Rule. The original IMPROVE algorithm, developed in the mid-1990s, is used to estimate light extinction from the contribution of major aerosol species. The equation also includes hygroscopic and light extinction efficiencies specific to each aerosol species. In 2007 the equation was revised to reflect the most current scientific understanding. Recent research suggests the IMPROVE equation is currently under-predicting measured light scattering coefficients, especially after 2011. In addition, comparisons of reconstructed and measured PM_(2.5) mass suggest that mass reconstruction inherent in the IMPROVE equation is also under-predicting measured mass during this same time period. These results indicate that a review of the current IMPROVE algorithms to reconstruct mass and haze is necessary. As part of this review the assumptions in the algorithms will be evaluated to understand the causes of the under-prediction. A summary of provisional recommendations for refinements to the IMPROVE equation and a discussion of important uncertainties to consider in the assumptions will be discussed. 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.
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