Chlorine enhances nocturnal heterogeneous uptake of NO 2 in coastal atmosphere under sea-land breeze circulation

Abstract

Abstract. Heterogeneous uptake of NO2 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 NO2 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 NO2 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 (NO3-) increased significantly during SLB days, with high mean value of NO2 uptake rate constant (kNO2) reaching 9.70×10-6 s-1. Machine learning revealed that chlorine was the most important influencing factor for the enhanced kNO2. Incorporating this kNO2 into the chemical box model substantially resolved the underestimation of HONO concentration and NO3- production under SLB conditions. Notably, nocturnal NO2 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 NO2 uptake in atmospheric reactive nitrogen cycling and provides valuable insights for nocturnal chemistry in complex coastal environments.