Multiphase and Heterogeneous O 3 Chemistry on Aerosol and Infrastructural Surfaces Drives Cl 2 Sources in a Coastal City
Katelyn L. Richard, Michael P. Vermeuel, Emily Franklin, Rose K. Rossell, Adam De Groodt, Trey A. Maddaleno, Andrew Hallward‐Driemeier, Yuwei Zhao, Mitchell J. Rogers, Drew R. Gentner, Rachel O’Brien, Róisín Commane, Dylan B. Millet, Delphine K. FarmerAbstract
Molecular chlorine (Cl 2 ) is a key source of chlorine atoms (Cl∙) in the atmosphere, which can alter local oxidation chemistry and enhance both ozone (O 3 ) and particulate matter. While Cl 2 levels in some coastal cities can be substantial and are enhanced by particulate nitrate, the presence and impact of Cl 2 in US cities is poorly understood. In summer 2023, we measured gas phase Cl 2 mixing ratios (4 and 32.5 m above ground level, a.g.l) and fluxes (32.5 m a.g.l) in Mineola, New York. Cl 2 mixing ratios and fluxes consistently peaked during the afternoon, with averages (± standard deviation) of 24 ± 10 ppt and 170 ± 100 ppt·cm·s −1 , respectively, at 32.5 m from 12 to 4 p.m. local time. Strong correlations between Cl 2 emissions, solar radiation, hypochlorous acid, and O 3 suggest photochemical multiphase chemistry is a prominent source of Cl 2 to the region. Positive fluxes indicate emission of Cl 2 from the surface to the atmosphere and are consistent with sources exceeding the expected multiphase production rate of Cl 2 on aerosol surfaces. We hypothesize that rooftops, fences, roads and other urban surfaces act as reservoirs for chloride (Cl − ) from deteriorating building materials, industrial processes, and dry deposition of sea spray particles and chlorine‐containing gases, enabling heterogeneous formation of gas phase Cl 2 upon interaction with atmospheric pollutants such as O 3 . Deterioration of building materials may also enable surface chemistry that could impact urban chlorine chemistry, even in areas without strong coastal sea spray influence.