EGUsphere EGUsphere EGUsphere EGUsphere Copernicus Publications Göttingen, Germany 10.5194/egusphere-2026-3380 Chlorine enhances nocturnal heterogeneous uptake of NO<sub>2</sub> in coastal atmosphere under sea-land breeze circulation Lin Ziyi 1 2 3 Yong Xiuwen 1 2 4 Li Lingjun 1 2 3 Chen Yuping 1 2 3 Xu Lingling 1 2 3 Ji Xiaoting 1 2 3 Yang Chen 1 2 3 Zhang Keran 1 2 3 Zhang Feng 1 2 Chen Ziying 1 2 3 Chen Gaojie 1 2 3 Fan Xiaolong 1 2 Li Mengren 1 2 Chen Jinsheng https://orcid.org/0000-0002-0285-6094 1 2 3 State Key Laboratory of Advanced Environmental Technology, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, China Fujian Key Laboratory of Atmospheric Ozone Pollution Prevention, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, China University of Chinese Academy of Sciences, Beijing 100049, China Fujian Agriculture and Forestry University, Fuzhou, 350002, China 01 07 2026 2026 1 20 Copyright: © 2026 Ziyi Lin et al. 2026 This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/ This article is available from https://egusphere.copernicus.org/preprints/2026/egusphere-2026-3380/ The full text article is available as a PDF file from https://egusphere.copernicus.org/preprints/2026/egusphere-2026-3380/egusphere-2026-3380.pdf

Heterogeneous uptake of NO<sub>2</sub> serves as a significant source for reactive nitrogen species, playing an important role in atmospheric chemistry. Laboratory studies have demonstrated that chlorine can promote the heterogeneous NO<sub>2</sub> uptake, yet this effect under real ambient conditions remains poorly elucidated. Based on comprehensive field observations, a machine learning technique, and a multiphase chemical box model in the coastal city of Xiamen, China, this study reveals the enhancement effect of chlorine (Cl) on NO<sub>2</sub> uptake and quantifies the impact of this enhanced uptake on reactive nitrogen species during nocturnal sea-land breeze (SLB) periods. Compared with non-SLB days, nocturnal concentrations of nitrous acid (HONO) and particulate nitrate (NO<sub>3</sub><sup>-</sup>) increased significantly during SLB days, with high mean value of NO<sub>2</sub> uptake rate constant (kNO<sub>2</sub>) reaching 9.70&times;10<sup>-6</sup> s<sup>-1</sup>. Machine learning revealed that chlorine was the most important influencing factor for the enhanced kNO<sub>2</sub>. Incorporating this kNO<sub>2</sub> into the chemical box model substantially resolved the underestimation of HONO concentration and NO<sub>3</sub><sup>-</sup> production under SLB conditions. Notably, nocturnal NO<sub>2</sub> uptake dominated HONO formation (83.9 %), while making a substantial contribution (47.9 %) to nitrate formation. This study highlights the critical role of chlorine-enhanced NO<sub>2</sub> uptake in atmospheric reactive nitrogen cycling and provides valuable insights for nocturnal chemistry in complex coastal environments.