Analysis and updates to the IMPROVE Equation for estimating light extinction

The Clean Air Act Amendments of 1977 included protections for visibility in Class I areas, such as national parks and wilderness areas. Visibility degradation (haze) is determined from speciated aerosol mass concentrations measured by the Interagency Monitoring of Protected Visual Environments (IMPROVE) network. Mass concentrations are converted into reconstructed light extinction values using an algorithm that considers the mass extinction efficiencies (MSEs) and water uptake of different major aerosol species. This algorithm is evaluated against measured light scattering (b_sp) using nephelometers that are co-located at several IMPROVE sites. Evaluating the performance of the algorithm is critical because of its use in tracking visibility trends for the U.S. Environmental Protection Agency's (EPA's) Regional Haze Rule (RHR). Previous evaluations (2001-2016) demonstrated that the current IMPROVE reconstructed light extinction equation (Second IMPROVE Equation) suggested in the RHR guidance results in spatial and temporal biases in visibility trends, leading to greater uncertainty in reaching national visibility goals. Extending the evaluation through 2024 suggests that using the current algorithm continues to introduce biases in visibility trends, primarily due to the parameterized dependence of MSEs on mass concentrations in the equation. We recommend a revised algorithm (Third IMPROVE Equation) that removes this MSE-mass dependence and updates aerosol composition assumptions to be consistent with the most recent science. The equation is more appropriate for representing long-term trends in regulatory visibility metrics across the IMPROVE network.Implications: IMPROVE data are central to the EPA's Regional Haze Rule (RHR) for tracking visibility trends in Class I areas. The current light-extinction equation (Second IMPROVE Equation) improved fits with early 2000s measurement data but introduced long-term biases. We present a revised equation, based on the First IMPROVE Equation, that improves consistency with measured light scattering and long-term trends. Implementing the revision would minimally affect RHR tracking metrics. This work provides timely input to ongoing EPA evaluations of RHR metrics and implementation guidance.