Primary sources of HONO vary during the daytime: Insights based on a field campaign

Dongyang Chen; Li Zhou; Song Liu; Chaofan Lian; Weigang Wang; Hefan Liu; Chunyuan Li; Yuelin Liu; Lan Luo; Kuang Xiao; Yong Chen; Yang Qiu; Qinwen Tan; Maofa Ge; Fumo Yang

doi:10.1016/j.scitotenv.2023.166605

Primary sources of HONO vary during the daytime: Insights based on a field campaign

Sci Total Environ. 2023 Dec 10:903:166605. doi: 10.1016/j.scitotenv.2023.166605. Epub 2023 Aug 26.

Authors

Dongyang Chen¹, Li Zhou², Song Liu¹, Chaofan Lian³, Weigang Wang³, Hefan Liu⁴, Chunyuan Li¹, Yuelin Liu¹, Lan Luo⁵, Kuang Xiao⁵, Yong Chen⁵, Yang Qiu⁶, Qinwen Tan⁴, Maofa Ge³, Fumo Yang¹

Affiliations

¹ College of Architecture and Environment, Sichuan University, Chengdu 610065, China; College of Carbon Neutrality Future Technology, Sichuan University, Chengdu 610065, China; Sichuan University Yibin Park, Yibin Institute of Industrial Technology, Yibin 644600, China.
² College of Architecture and Environment, Sichuan University, Chengdu 610065, China; College of Carbon Neutrality Future Technology, Sichuan University, Chengdu 610065, China; Sichuan University Yibin Park, Yibin Institute of Industrial Technology, Yibin 644600, China. Electronic address: lizhou@scu.edu.cn.
³ State Key Laboratory for Structural Chemistry of Unstable and Stable Species, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China.
⁴ Chengdu Academy of Environmental Sciences, Chengdu 610000, China.
⁵ Sichuan province Chengdu Ecological Environment Monitoring Center Station, Chengdu 610066, China.
⁶ Department of Industrial Engineering, The Pittsburgh Institute, Sichuan University, Chengdu 610065, China.

PMID: 37640078
DOI: 10.1016/j.scitotenv.2023.166605

Abstract

Nitrous acid (HONO) is an established precursor of hydroxyl (OH) radical and has significant impacts on the formation of PM_2.5 and O₃. Despite extensive research on HONO observation in recent years, knowledge regarding its sources and sinks in urban areas remains inadequate. In this study, we monitored the atmospheric concentrations of HONO and related pollutants, including gaseous nitric acid and particulate nitrate, simultaneously at a supersite in downtown Chengdu, a megacity in southwestern China during spring, when was chosen due to its tolerance for both PM_2.5 and O₃ pollution. Furthermore, we employed the random forest model to fill the missing data of HONO, which exhibited good predictive performance (R² = 0.96, RMSE = 0.36 ppbv). During this campaign, the average mixing ratio of HONO was measured to be 1.0 ± 0.7 ppbv. Notably, during periods of high O₃ and PM_2.5 concentrations, the mixing ratio of HONO was >50 % higher compared to the clean period. We developed a comprehensive parameterization scheme for the HONO budget, and it performed well in simulating diurnal variations of HONO. Based on the HONO budget analysis, we identified different mechanisms that dominate HONO formation at different times of the day. Vehicle emissions and NO₂ heterogeneous conversions were found to be the primary sources of HONO during nighttime (21.0 %, 30.2 %, respectively, from 18:00 to 7:00 the next day). In the morning (7:00-12:00), NO₂ heterogeneous conversions and the reaction of NO with OH became the main sources (35.0 %, 32.2 %, respectively). However, in the afternoon (12:00-18:00), the heterogeneous photolysis of HNO₃ on PM_2.5 was identified as the most substantial source of HONO (contributing 52.5 %). This study highlights the significant variations in primary HONO sources throughout the day.

Keywords: Heterogeneous conversion; Nitrous acid; Random Forest model; Solar radiation.