Atmospheric gaseous HCl influenced by sea-land breeze circulation in a southeast Chinese coastal city

Abstract

As a vital atmospheric halogen, gaseous hydrochloric acid (HCl) exerts a key influence on various physicochemical processes, particularly in coastal regions. Sea-Land Breeze (SLB) is a common local mesoscale circulation in coastal area that alters local weather conditions and further affects the diffusion and transport of air pollutants. However, the fate of HCl under SLB circulation remains poorly understood, which hinders a comprehensive understanding of coastal halogen chemistry and its associated atmospheric impacts. Here, a measurement campaign conducted in Xiamen, China, during winter 2023 revealed substantial levels of gaseous HCl, with concentrations ranging from 3.9 ppt to 290.4 ppt. Average HCl concentrations were 102.2 ppt and 71.4 ppt under SLB and non-SLB (NSLB) conditions, respectively. The integration of field observations and machine learning methods indicated that gas-particle partitioning could be a key driver of elevated HCl levels. In addition, high RH and abundant particulate nitrate concentrations on SLB days were the dominant factors affecting the HCl formation and reactions between HCl and OH radicals generated atomic chlorine at significant rates, up to 3.6×10³ molecules/(cm³·s). The potential adverse health effects from chlorine-containing oxygenated organic molecules (Cl-OOMs) were greater under SLB conditions than on NSLB days. The observed elevation in HCl concentrations through sea-land air exchange can indicate an important chlorine cycling pathway in the coastal urban environment, further explaining the potential influence of chlorine chemistry on atmospheric oxidizing capacity and health effects in coastal cities.

Graphical abstract

Keywords

Gaseous hydrochloric acid / Sea land breeze / Key influencing factors / Machine learning / Potential health effects

Highlight

	● High HCl concentrations reached 290.4 ppt during SLB days.
	● Random Forest model can identify the influencing factors of HCl concentration.
	● High RH and NO3− affect HCl concentration in SLB days.

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