Inorganic chlorine partitioning in the summer lower stratosphere: Modeled and measured [ClONO2]/[HCl] during POLARIS

We examine inorganic chlorine (Cl-y) partitioning in the summer lower strat osphere using in situ ER-2 aircraft observations made during the Photochemi stry of Ozone Loss in the Arctic Region in Summer (POLARIS) campaign. New s teady state and numerical models estimate [ClONO2]/[HCl] using currently ac cepted photochemistry. These models are tightly constrained by observations with OH (parameterized as a function of solar zenith angle) substituting f or modeled HO2 chemistry. We find that inorganic chlorine photochemistry al one overestimates observed [ClONO2]/[HCl] by approximately 55-60% at mid an d high latitudes. On the basis of POLARIS studies of the inorganic chlorine budget, [ClO]/[ClONO2], and an intercomparison with balloon observations, the most direct explanation for the model-measurement discrepancy in Cl-y p artitioning is an error in the reactions, rate constants, and measured spec ies concentrations linking HCl and ClO (simulated [ClO]/[HCl] too high) in combination with a possible systematic error in the ER-2 ClONO2 measurement (too low). The high precision of our simulation (+/-15% 1 sigma for [ClONO 2]/[HCl], which is compared with observations) increases confidence in the observations, photolysis calculations, and laboratory rate constants. These results, along with other findings, should lead to improvements in both th e accuracy and precision of stratospheric photochemical models.