| Summary: | We examine inorganic chlorine (Cl_y,) partitioning in the summer lower stratosphere using in situ ER-2 aircraft observations made during the Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) campaign. New steady state and numerical models estimate [ClONO_2]/[HCl] using currently accepted photochemistry. These models are tightly constrained by observations with OH (parameterized as a function of solar zenith angle) substituting for modeled HO_2 chemistry. We find that inorganic chlorine photochemistry alone overestimates observed [ClONO_2]/[HCl] by approximately 55–60% at mid and high latitudes. On the basis of POLARIS studies of the inorganic chlorine budget, [ClO]/[ClONO_2], and an intercomparison with balloon observations, the most direct explanation for the model-measurement discrepancy in Cl_y, partitioning is an error in the reactions, rate constants, and measured species concentrations linking HCl and ClO (simulated [ClO]/[HCl] too high) in combination with a possible systematic error in the ER-2 ClONO_2 measurement (too low). The high precision of our simulation (±15% 1σ 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 the accuracy and precision of stratospheric photochemical models. © 2001 American Geophysical Union. Received March 14, 2000; revised July 24, 2000; accepted August 8, 2000. Paper number 2000JD900494. We thank NASA's Upper Atmosphere Research Program as well as the researchers pilots, crew, and managers of NASA's ER-2 program for supporting the POLARIS campaign. This work was funded under NASA grants NAG-1-2007 and NAG-2-1300. We thank J. J. Margitan for the ER-2 O_3 data. P. B. Voss is grateful for support from NASA's Global Change Fellowship Program. Published - jgrd7993.pdf
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