Data analysis and figure making for: Multiphase and heterogeneous O₃ chemistry on aerosol and infrastructural surfaces drives Cl₂ sources in a coastal city

Gas phase chlorine (Cl2) is photolyzed to form chlorine radicals (Cl•) that impact atmospheric oxidation chemistry, enhancing ozone (O3) and particulate matter. Some coastal regions demonstrate increase in Cl2 with enhanced local particulate nitrate, but generally, Cl2 formation is poorly studied globally. We measured Cl2 mixing ratios and fluxes in the greater New York City region in the summer of 2023. Cl2 mixing ratios and fluxes reached maximum hourly averages (± standard deviation) of 24±10 ppt and 170±100 ppt∙cm∙s-1, respectively, at midday. Cl2 mixing ratios correlate with solar radiation, hypochlorous acid (HOCl) and O3, suggesting multiphase photchemistry as a dominant source of Cl2 in the region. Further, positive fluxes demonstrate emission of Cl2 from within a couple miles of the field site. We hypothesize that infrastructural surfaces provide additional surface area and are a potential source of Cl-, enabling gas-phase Cl2 formation from these surfaces in addition to previously observed Cl2 formation from aerosol surfaces.