The Changing Sensitivity of Wintertime Particulate Nitrate to Precursor Emissions Diagnosed via Satellite Observations of Ammonia and Nitrogen Dioxide over the Midwestern United States

Abstract. Particulate nitrate (PN) is a critical component of fine particulate matter (PM_2.5). During wintertime, the contribution of PN to PM_2.5 over the Midwestern United States (MWUS), an agriculturally intensive region, has increased over the past decade and now contributes up to 40 % of the particle mass. PN formation is controlled by nitrogen oxides (NO_x=NO + NO₂), ammonia (NH₃), and volatile organic compounds (VOCs). To best control wintertime PM_2.5 burden, it is critical to determine PN formation sensitivity to precursor gases, but this is not well constrained. Prior efforts to diagnose PN sensitivity have been limited on both spatial and temporal scales. Satellite tropospheric column NH₃/NO₂ ratios cover large areas and long timeframes, and they have been shown to be effective in diagnosing PN sensitivity over East Asia, Europe, and the Eastern United States. Here, we expand this approach to quantify spatially and temporally resolved multidecadal wintertime PN formation sensitivity to NH₃, NO_x, and VOCs in the MWUS from 2007 to 2023 via satellite observations and GEOS-Chem sensitivity simulations. More than half of the total diagnosed pixels are classified as NO_x-sensitive in 2007, and this increases to 89.0 % by 2023. VOCs do not control MWUS PN formation. The shift in PN formation sensitivity is explained by relatively flat trends in satellite NO₂ column densities (0.48 ± 0.60 % yr^-1) in combination with increases in satellite NH₃ column densities (1.3 ± 0.3 % yr^-1). Our work indicates that targeting NO_x emissions is chemically effective for reducing wintertime PN and PM_2.5 burden.