HOCl Formation Driven by Photochemical Processes Enhanced Atmospheric Oxidation Capacity in a Coastal Atmosphere

Area of Science:

• Atmospheric Chemistry
• Environmental Science

Background:

• Chlorine (Cl) radicals significantly influence atmospheric oxidation and pollutant transformation.
• Hypochlorous acid (HOCl) is a key chlorine precursor, but its formation and impact are not fully understood.

Purpose of the Study:

• To investigate the formation mechanisms and atmospheric impacts of hypochlorous acid (HOCl).
• To quantify HOCl concentrations and identify key contributing factors in a coastal urban environment.

Main Methods:

• Field observations of HOCl concentrations in Southeast China during autumn 2022.
• Machine learning analysis to determine factors influencing HOCl distribution.
• Chemical mechanism analysis to understand HOCl production and its subsequent effects.

Main Results:

• High daytime HOCl concentrations (average peak 181 ppt) were observed.
• Key factors influencing HOCl included chlorine (Cl₂), ozone (O₃), nitrate, temperature, and iron.
• Nitrate photolysis and aerosol iron photochemistry contributed to Cl radical production, driving HOCl formation.
• HOCl photolysis released OH and Cl radicals, increasing RO radicals by ~10% and enhancing ozone generation.

Conclusions:

• HOCl plays a significant role in atmospheric chemistry, particularly in coastal urban areas.
• Controlling ozone levels may help mitigate chlorine radical production and its negative air quality impacts.