Midwest Wintertime PM2.5 Response to NOx and NH3 Emissions Controls

Wintertime regional air pollution episodes are responsible for the majority of PM2.5 NAAQS violations in the Midwest U.S., with ammonium nitrate and ammonium sulfate responsible for much of the episodic increases in both rural and urban areas. While policymakers have a working conceptual understanding of these episodes, contributions of sectoral reactive nitrogen emissions and their spatial distributions to the regional aerosol thermodynamic regime during these episodes have not been documented to date. The future frequency and intensity of these episodes is also uncertain, as the stagnant inversions that drive them are expected to increase as global climate changes, while NOx and SO2 precursor emissions continue to decline in response to federal policies. Future emissions control policies will further need to account for expected emissions changes due to recent and proposed policies. Here, we employ the Community Multiscale Air Quality model (CMAQ) to simulate regional chemical transport during the 2009 LADCO Winter Nitrate Study using a 2009 emissions inventory and a 2016 proxy inventory reflecting ongoing reductions in point and area source emissions. We identify distinct thermodynamic regimes across the region and throughout the season. We quantify speciated PM2.5 sensitivities to total and sectoral NOx and NH3 reductions and their spatial footprints using emissions perturbations and sensitivities calculated by the Direct Decoupled Method. Results indicate non-linear changes to PM2.5 emissions sensitivities due to ongoing emissions reductions since 2009.