Budget of atmospheric nitrous acid (HONO) during the haze and clean periods in Shanghai: Importance of heterogeneous reactions

Jialiang Feng; Ensi Ren; Ming Hu; Qingyan Fu; Yusen Duan; Cheng Huang; Yue Zhao; Shunyao Wang

doi:10.1016/j.scitotenv.2023.165717

Budget of atmospheric nitrous acid (HONO) during the haze and clean periods in Shanghai: Importance of heterogeneous reactions

Sci Total Environ. 2023 Nov 20:900:165717. doi: 10.1016/j.scitotenv.2023.165717. Epub 2023 Jul 22.

Authors

Jialiang Feng¹, Ensi Ren¹, Ming Hu², Qingyan Fu², Yusen Duan², Cheng Huang³, Yue Zhao⁴, Shunyao Wang⁵

Affiliations

¹ School of Environmental and Chemical Engineering, Shanghai University, Shanghai 200444, China.
² Shanghai Environmental Monitoring Center, Shanghai 200235, China.
³ State Environmental Protection Key Laboratory of the Cause and Prevention of Urban Air Pollution Complex, Shanghai Academy of Environmental Sciences, Shanghai, China.
⁴ School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China. Electronic address: yuezhao20@sjtu.edu.cn.
⁵ School of Environmental and Chemical Engineering, Shanghai University, Shanghai 200444, China. Electronic address: syw@shu.edu.cn.

PMID: 37482358
DOI: 10.1016/j.scitotenv.2023.165717

Abstract

Nitrous acid (HONO) plays a significant role in radical cycling and atmospheric oxidative chemistry. While the source and evolution of HONO in the Yangtze River Delta (YRD) region of China after 2018 remains largely unknown, this work monitored HONO and other air pollutants throughout 2019 at an urban site (Pudong, PD) and a suburban site (Qingpu, QP) in Shanghai. Episodes with high HONO mixing ratios but different PM_2.5 levels, namely haze and clean episodes, were chosen for HONO budget analysis. Using an observation-based photochemical box model, relative importance of different sources and sinks of HONO were evaluated. Gas-phase reaction of NO with OH was found to be one of the most important daytime HONO formation sources, especially during the QP_haze period (accounting for 40.3 % of daytime HONO formation). In particular, heterogeneous conversion of NO₂ on ground and aerosol surface was found to be the dominant source for nocturnal HONO. Photo-enhanced NO₂ conversion on ground surface plays an important role in daytime HONO production (19.4 % in PD_haze vs. 27.6 % in PD_clean, and 19.8 % in QP_haze vs. 25.9 % in QP_clean). In addition, photo-enhanced NO₂ conversion at the aerosol surface during haze episodes made more significant contributions to HONO formation compared to the clean periods (20.9 % in PD_haze vs. 17.1 % in PD_clean, and 19.7 % in QP_haze vs. 11.2 % in QP_clean). The role of multiphase reactions was found to be increasingly important in HONO generation with enhanced relative humidity (RH) during daytime. Significant unknown HONO source was further analyzed and found to be positively related with photolytic as well as multiphase pathways. Overall, our study sheds light on the budget of HONO in one of the biggest megacities in east China, which would help developing future mitigation strategies for urban HONO and atmospheric oxidation capacity.

Keywords: Atmospheric oxidation capacity; Budget analysis; Haze episodes; Multiphase pathways; Nitrous acid; Observation-based photochemical box model.