Observational Constraints on Tropospheric Chlorine Cycling

Abstract

The multiphase photochemical cycling of chlorine has a widespread influence on the chemical composition of the troposphere, affecting climate, the oxidant budget, acid deposition to the biosphere, and the chemistry of primary and secondary pollutants such as nitrogen oxides, nitrate aerosols, and ozone. There have been relatively few observations to constrain chlorine chemistry in the troposphere, often resulting in either its exclusion from global chemistry climate models or inclusion with limited verification. This work catalogues critical constraints on tropospheric chlorine chemistry utilizing data collected in 2015 during the Wintertime Investigation of Transport, Emissions, and Reactivity (WINTER) aircraft campaign. Using WINTER data, the contribution of various oxidants, including Cl atoms from nitryl chloride (ClNO2) photolysis, on a regional basis are quantified, and I show that multiphase reactions between gas‐phase NOx reservoirs and chlorine containing aerosol particles controls greater than 70% of the oxidizing capacity of polluted air during winter. I use observations from WINTER in a chemical box model to constrain a laboratory-based mechanism of chlorine gas (Cl2) production from ClNO2 uptake on acidic particles. To reproduce Cl2 concentrations observed during WINTER, the model required the ClNO2 reaction probability to be more than an order of magnitude lower than those determined in previous laboratory experiments on acidic surfaces. I show the availability of Cl- and particle volume limit the efficiency of the reaction in the atmosphere, compared to the laboratory. By 2015, when the WINTER campaign took place, strict emissions controls on NOx and SO2, which impact anthropogenic emissions of chlorine compounds, particle pH, and ClNO2 formation, had already been enacted across the United States. Using deposition data collected from precipitation samples across the US, I show that the mean wet deposition flux of non-sea salt chloride has decreased at sites throughout the Northeastern US over a 20-year span and are well correlated with anthropogenic HCl emissions decreases, suggesting that industry emissions controls have decreased the tropospheric atmospheric Cly burden and implying that its impacts on the oxidant budget were larger in the past than they are today.