Diagnosing the Sensitivity of Particulate Nitrate to Precursor Emissions Using Satellite Observations of Ammonia and Nitrogen Dioxide

Particulate nitrate is a major component of fine particulate matter (PM2.5). Its formation may be varyingly sensitive to emissions of ammonia (NH3), nitrogen oxides (NOx equivalent to NO + NO2), and volatile organic compounds (VOCs), depending on local conditions. Diagnosing these sensitivities is critical for successful air quality management. Here, we show that satellite measurements of tropospheric NH3 and NO2 columns can be used as a quick indicator of the dominant sensitivity regime through the NH3/NO2 column ratio together with the NO2 column. We demonstrate the effectiveness of this indicator with the GEOS-Chem chemical transport model and define thresholds to separate the different sensitivity regimes. Applying the method to wintertime IASI and OMI observations in East Asia reveals that surface nitrate is dominantly VOC-sensitive in the southern North China Plain (NCP), NOx-sensitive in most of the East China Plain, and NH3-sensitive in the northern NCP, southern China, and Korea. We present a novel application of satellite remote sensing to better understand the causes of particulate nitrate pollution. Particulate nitrate is a major air pollutant throughout the urbanized world. It is produced by atmospheric oxidation of emitted nitrogen oxides (NOx) but may be more sensitive to emissions of ammonia (NH3) or volatile organic compounds (VOCs). Understanding which of NH3, NOx, or VOC emissions is most important in driving nitrate formation is critical for air quality management. We show that satellite measurements of the NH3/NO2 column ratio along with NO2 columns is an effective indicator to determine the dominant sensitivity regime (NH3-,NOx-,orVOC-sensitive ${\text{NH}}_{3}-,{\text{NO}}_{\mathrm{x}}-,\text{or}\,\text{VOC}-\text{sensitive}$). We develop this approach using an atmospheric chemistry model and apply it to wintertime satellite observations in East Asia. The approach should be applicable to other continents, seasons, and a broader range of satellite instruments, providing valuable insights for particulate nitrate reduction strategies. Reducing particulate nitrate pollution requires understanding its local sensitivities to NH3, NOx, and volatile organic compound emissionsSatellite observation of the NH3/NO2 column ratio is an effective indicator for diagnosing these sensitivitiesIASI NH3 and OMI NO2 observations reveal varying regimes of nitrate sensitivity across wintertime East Asia