Changing aerosol chemistry is redefining HONO sources
Yusheng Zhang, Yongchun Liu, Wei Ma, Chenjie Hua, Feixue Zheng, Chaofan Lian, Weigang Wang, Men Xia, Zhixin Zhao, Jinwen Li, Jiali Xie, Zongcheng Wang, Yuzheng Wang, Xin Chen, Ying Zhang, Zemin Feng, Chao Yan, Biwu Chu, Wei Du, Veli‐Matti Kerminen, Federico Bianchi, Tuukka Petäjä, Douglas R. Worsnop, Markku Kulmala
Abstract
Heterogeneous reactions of NO2 on particulate matter have been considered an important source of HONO (Nitrous acid) in the troposphere, whereas its contribution is controversial due to the lack of uptake coefficient of NO2 (γNO2) on the surfaces of ambient particulate matter (PM). Here we investigate the the γNO2 to form HONO and its evolution based on long-term comprehensive field observations (2019–2023) in Beijing and a random forest model with Shapley additive explanations. The γNO2 on ambient PM is on the order of 10−6, decreasing markedly from 3.07 ± 5.99 × 10−6 in 2019 to 1.43 ± 3.22 × 10−6 in 2023. This decrease is driven by the increase in aerosol pH, linked to increased ratio of NH4NO3 to (NH4)2SO4, resulting from an unbalanced desulfurization and denitrification. This study implies that the role of the heterogeneous reaction of NO2 on aerosol surfaces in HONO production is declining in Beijing, providing valuable insights into the atmospheric chemistry in urban environments. Long-term observations show that a rising fraction of ammonium nitrate in PM2.5 reduces aerosol acidity, suppressing the heterogeneous conversion from NO2 to HONO. This weakens its contribution to the HONO source and atmospheric oxidation capacity.