A Satellite-Based\nIndicator for Diagnosing Particulate\nNitrate Sensitivity to Precursor Emissions: Application to East Asia,\nEurope, and North America

Particulate nitrate is a major component of fine particulate\nmatter\n(PM<sub>2.5</sub>) and a key target for improving air quality. Its\nformation is varyingly sensitive to emissions of nitrogen oxides (NO<sub><i>x</i></sub> ≡ NO + NO<sub>2</sub>), ammonia (NH<sub>3</sub>), and volatile organic compounds (VOCs). Diagnosing the dominant\nsensitivity is critical for effective pollution control. Here, we\nshow that satellite observations of the NO<sub>2</sub> column and\nthe NH<sub>3</sub>/NO<sub>2</sub> column ratio can effectively diagnose\nthe dominant sensitivity regimes in polluted regions of East Asia,\nEurope, and North America, in different seasons, though with reduced\nperformance in the summer. We demarcate the different sensitivity\nregimes using the GEOS-Chem chemical transport model and apply the\nmethod to satellite observations from the OMI (NO<sub>2</sub>) and\nIASI (NH<sub>3</sub>) in 2017. We find that the dominant sensitivity\nregimes vary across regions and remain largely consistent across seasons.\nSensitivity to NH<sub>3</sub> emissions dominates in the northern\nNorth China Plain (NCP), the Yangtze River Delta, South Korea, most\nof Europe, Los Angeles, and the eastern United States. Sensitivity\nto NO<sub><i>x</i></sub> emissions dominates in central\nChina, the Po Valley in Italy, the central United States, and the\nCentral Valley in California. Sensitivity to VOCs emissions dominates\nonly in the southern NCP in the winter. These results agree well with\nthose of previous local studies. Our satellite-based indicator provides\na simple tool for air quality managers to choose emission control\nstrategies for decreasing PM<sub>2.5</sub> nitrate pollution.