Analysis and updates to the IMPROVE Equation for estimating light extinction.

: 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.