Size-Dependent Nighttime Formation of Particulate Secondary Organic Nitrates in Urban Air

Particulate secondary organic nitrates play a key role in understanding secondary organic aerosol production, ozone formation, and the atmospheric nitrogen cycle. However, the formation of particulate secondary organic nitrates in ambient air remains poorly understood. In this study, the nighttime formation processes of particulate secondary organic nitrates were investigated based on size-resolved aerosols measured in urban air of China with a soot particle long-time-of-flight aerosol mass spectrometer. The results show a bimodal size distribution of particulate secondary organic nitrates, peaking at similar to 350 nm in condensation mode (100-400 nm) and similar to 750 nm in droplet mode (400-2,500 nm), respectively. The nighttime formation processes of particulate secondary organic nitrates in the two size modes were respectively governed by temperature-dependent condensation and aqueous-phase processing. In particular, the mass concentration of particulate secondary organic nitrates in condensation mode was positively correlated with nitrate radical production and negatively correlated with temperature, suggesting that the formation processes were associated with the gas-particle conversion of nitrate radical oxidation products. In contrast, the enhanced particulate secondary organic nitrates in droplet mode were predominantly contributed by aqueous-phase processing, as indicated by the strong positive correlation with aerosol liquid water content and typical fragment ions from aqueous processing products (r = 0.52-0.59, P < 0.01). Our results highlight the potential of a size-dependent mechanism to elucidate the formation processes of particulate secondary organic nitrates.