Evolution of inorganic chlorine partitioning in the Arctic polar vortex

Abstract

The first simultaneous, in situ atmospheric measurements of ClO, ClOOCl, ClONO2, and HCl, which together nearly compose total inorganic chlorine, Cly, were obtained using the NASA ER‐2 aircraft, deployed from Kiruna, Sweden, during the SOLVE/THESEO mission. These chlorine measurements, along with Cly inferred from in situ measurements of organic chlorine source gases, offer an unprecedented opportunity to observe chlorine activation and recovery in the polar winter stratosphere and evaluate the inorganic chlorine budget, i.e., a test of the quantitative agreement between the sum of ClOx (ClO + 2 × ClOOCl), ClONO2, and HCl with Cly. Inside the Arctic vortex, inorganic chlorine is rarely fully activated with ClOx/Cly ranging from approximately 0.25 to 0.90. An apparent discrepancy in the inorganic chlorine budget for much of the midwinter correlates well with back trajectory solar zenith angle minimum and is consistent with the presence of Cl2, which was not observable. Evaluation of the midwinter inorganic chlorine budget, excluding data where Cl2 is present, yields (ClOx + ClONO2 + HCl)/Cly = 0.94 ± 0.07. In contrast, a shortfall in the late winter inorganic chlorine budget is the result of an apparent low bias in the HCl measurements. A diurnal box model constructed to analyze chlorine recovery rates highlights the importance of not only the Cl + CH4 reaction in HCl formation, but also OH + ClO and potentially HO2 + ClO. We find that significant HCl production accompanied net ClONO2 production in the inorganic chlorine recovery phase of this Arctic winter.