Secondary processes driven by multi-factor interactions dominate the aerosol nitroaromatic compound pollution during winter in China

Abstract. Previous observational and chamber studies have highlighted the significant promoting effects of relative humidity (RH) or aerosol liquid water (ALW) on the formation of aerosol nitroaromatic compounds (NACs). However, the interpretability of this pattern needs further validation in large-scale field observations. This study presents the simultaneous investigation of the compositions, abundances, and potential origins of NACs in PM2.5 across 11 Chinese cities during winter, with a focus on the key factors controlling their formation. Nitrophenols (NPs) and nitrocatechols (NCs) were identified as the main NAC groups, with their relative dominance varying by city. Higher total NAC concentrations were observed in northern cities, likely due to intensified biomass and coal combustion. While secondary processes dominated wintertime NAC formation across all investigated cities, the average proportion of secondarily formed NACs was lower in the north (87 %) than in the south (93 %). This north-south disparity was more pronounced during polluted periods (82 % vs. 96 %). Furthermore, insignificant promoting effect of RH or ALW was found for most NACs except nitrosalicylic acids. The constraining effects from O3, ⚫OH, and solar radiation on NAC formation were stronger in northern China due to higher levels of light-absorbing air pollution (generally severer haze in the north), potentially offsetting the promoting effects of RH or ALW. These findings suggest that the RH- or ALW-promoted NAC formation may not be universally interpretable in real atmospheric environments, where multi-factor interactions play a critical role. This study highlights the necessity of considering complex field conditions in future research on NAC formation mechanisms.