Photodissociation of particulate nitrate as a source of daytime tropospheric Cl2

Chlorine atoms (Cl) are highly reactive and can strongly influence the abundances of climate and air quality-relevant trace gases. Despite extensive research on molecular chlorine (Cl₂), a Cl precursor, in the polar atmosphere, its sources in other regions are still poorly understood. Here we report the highest ever measured daytime concentrations of Cl₂ (up to 1 ppbv) in a coastal area of Hong Kong, revealing a large daytime source of Cl₂(2.7 pptv s^-1 at noon). Field and laboratory experiments indicate that photodissociation of particulate nitrate by sunlight under acidic conditions (pH &lt;3.0) can activate chloride and account for the observed daytime Cl₂ production. The high Cl₂ concentrations significantly increased atmospheric oxidation. Given the ubiquitous existence of chloride, nitrate, and acidic aerosols, we propose that nitrate photolysis is a significant daytime chlorine source globally. This so far unaccounted for source of chlorine can have substantial impacts on atmospheric chemistry.氯原子（Cl）具有极高反应活性，可显著影响气候与空气质量相关痕量气体的丰度。尽管学界已针对极地大气中的氯前驱体——分子氯（Cl₂）开展了大量研究，但其他区域的Cl₂来源仍未得到充分阐明。本研究在香港沿海区域测得迄今最高的日间分子氯（Cl₂）浓度（最高达1 ppbv），并揭示了一处强日间Cl₂源（正午时达2.7 pptv s⁻¹）。野外与实验室实验表明，在酸性条件（pH<3.0）下，阳光对颗粒态硝酸盐的光解可激活氯化物，进而解释观测到的日间Cl₂生成过程。高浓度Cl₂显著增强了大气氧化能力。鉴于氯化物、硝酸盐与酸性气溶胶广泛存在于大气中，我们提出硝酸盐光解是全球范围内一种重要的日间氯源。这一此前未被纳入考量的氯源，可对大气化学过程产生显著影响。