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

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).