A Satellite-Based Indicator for Diagnosing Particulate Nitrate Sensitivity to Precursor Emissions: Application to East Asia, Europe, and North America

Particulate nitrate is a major component of fine particulate matter (PM_2.5) and a key target for improving air quality. Its formation is varyingly sensitive to emissions of nitrogen oxides (NO_<i>x</i> ≡ NO + NO₂), ammonia (NH₃), and volatile organic compounds (VOCs). Diagnosing the dominant sensitivity is critical for effective pollution control. Here, we show that satellite observations of the NO₂ column and the NH₃/NO₂ column ratio can effectively diagnose the dominant sensitivity regimes in polluted regions of East Asia, Europe, and North America, in different seasons, though with reduced performance in the summer. We demarcate the different sensitivity regimes using the GEOS-Chem chemical transport model and apply the method to satellite observations from the OMI (NO₂) and IASI (NH₃) in 2017. We find that the dominant sensitivity regimes vary across regions and remain largely consistent across seasons. Sensitivity to NH₃ emissions dominates in the northern North China Plain (NCP), the Yangtze River Delta, South Korea, most of Europe, Los Angeles, and the eastern United States. Sensitivity to NO_<i>x</i> emissions dominates in central China, the Po Valley in Italy, the central United States, and the Central Valley in California. Sensitivity to VOCs emissions dominates only in the southern NCP in the winter. These results agree well with those of previous local studies. Our satellite-based indicator provides a simple tool for air quality managers to choose emission control strategies for decreasing PM_2.5 nitrate pollution.