Sulfate and Carboxylate Suppress the Formation of ClNO2 at Atmospheric Interfaces

We report measurements of the nitryl chloride (ClNO2) branching fraction following reactive uptake of N2O5 to mixed organic and inorganic solutions representative of atmospheric interfaces. For sodium chloride containing solutions, mixed with either sodium sulfate (Na2SO4) or sodium acetate (NaAc), the ClNO2 branching fraction (ΦClNO2 ) is suppressed relative to a sodium chloride only solution. In the case of the sulfate-chloride solution, ΦClNO2 is reduced from 0.85 ± 0.03 (0.5 M NaCl) to 0.32 ± 0.14 upon the addition of 2.0 M Na2SO4. In the case of the acetate-chloride solution, ΦClNO2 is reduced to 0.18 ± 0.03 upon the addition of 0.5 M NaAc. In contrast, no statistically significant suppression in ΦClNO2 was observed for the addition of sodium perchlorate up to 3.0 M, implying that an increase in ionic strength of the solution does not necessitate a reduction in ΦClNO2 . We suggest that the reduction in ΦClNO2 may result from a direct reaction between SO4 2– (and Ac–) with NO2 + (or NO2 +NO3 –) which competes with the NO2 + + Cl– reaction that produces ClNO2. The dependence of ΦClNO2 on SO4 2– and Ac– is compared with both a time-dependent reaction-diffusion model and recent field observations, suggesting that the reaction rate of SO4 2– (or Ac–) with NO2 + would need to be similar in magnitude to the rate of the NO2 + + Cl– reaction to explain the observed suppression in ΦClNO2 . We show that the dependence of ΦClNO2 on particulate sulfate and carboxylate can be readily incorporated into existing parametrizations of ClNO2 heterogeneous chemistry. The results presented here indicate that anions which are ubiquitous in atmospheric aerosol, yet commonly considered to be unreactive, may regulate the production of reactive gases such as ClNO2.