Atmospheric Nitrogen-Containing Organic Aerosols in Wintertime Sichuan Basin of China: A Key Role of Aqueous-Phase Process in Secondary Production and Optical Absorption

Nitrogen-containing organic aerosols (NOAs) significantly affect air quality and climate, but their formation mechanisms remain unclear. Here, we investigated the chemical characteristics and light absorption of water-soluble NOA (WSOAN) in PM2.5 from three Sichuan Basin cities of China, Chengdu, Yibin, and Nanchong, during winter 2023, to elucidate both regional consistence and intercity differences of WSOAN formation. Our results showed that the water-soluble organic nitrogen (WSON) of PM2.5 was 0.8-1.0 μg N m-3, and WSOAN accounted for 15.4-38.4% of WSOA. Four WSOAN subclass fragments (amines, amides, amino acids, and imidazoles (IMs)) were identified. The concentration of IMs increased by 1.3-1.4 times during the regional heavy haze event (HHE). Aqueous secondary formation dominated WSOAN production during HHE, contributing 45-58% of total WSON. Chengdu showed a much lower apparent IMs yield than Yibin and Nanchong, mainly due to stronger aerosol acidity and higher levels of dicarbonyls and aerosol liquid water. WSOA light absorption at λ365 nm increased linearly with WSON, with a larger slope in Yibin and Nanchong, indicating a stronger light-absorbing of WSOAN in the two cities. Our study revealed abundant NOA pollution in inland China, which is strongly light-absorbing and largely produced from aerosol aqueous-phase reactions.