Observational constraints on the formation of Cl2 from the reactive uptake of ClNO2 on aerosols in the polluted marine boundary layer

We use observations from the 2015 Wintertime Investigation of Transport, Emissions, and Reactivity (WINTER) aircraft campaign to constrain the proposed mechanism of Cl-2 production from ClNO2 reaction in acidic particles. To reproduce Cl-2 concentrations observed during WINTER with a chemical box model that includes ClNO2 reactive uptake to form Cl-2, the model required the ClNO2 reaction probability, gamma (ClNO2), to range from 6 x 10(-6) to 7 x 10(-5), with a mean value of 2.3 x 10(-5) (1.8 x 10(-5)). These field-determined gamma (ClNO2) are more than an order of magnitude lower than those determined in previous laboratory experiments on acidic surfaces, even when calculated particle pH is <= 2. We hypothesize this is because thick salt films in the laboratory enhanced the reactive uptake ClNO2 compared to that which would occur in submicron aerosol particles. Using the reacto-diffusive length-scale framework, we show that the field and laboratory observations can be reconciled if the net aqueous-phase reaction rate constant for ClNO2 (aq) + Cl-(aq) in acidic particles is on the order of 10(4) s(-1). We show that wet particle diameter and particulate chloride mass together explain 90% of the observed variance in the box model-derived gamma (ClNO2), implying that the availability of chloride and particle volume limit the efficiency of the reaction. Despite a much lower conversion of ClNO2 into Cl-2, this mechanism can still be responsible for the nocturnal formation of 10-20 pptv of Cl-2 in polluted regions, yielding an atmospherically relevant concentration of Cl atoms the following morning.