Constraining the chlorine monoxide (CIO)/chlorine peroxide (CIOOCI) equilibrium...

Santee, M. (2010), Constraining the chlorine monoxide (CIO)/chlorine peroxide (CIOOCI) equilibrium constant from Aura Microwave Limb Sounder measurements of nighttime CIO, Proc. Natl. Acad. Sci., 107, 6588-6593, doi:10.1073/pnas.

The primary ozone loss process in the cold polar lower stratosphere hinges on chlorine monoxide (CIO) and one of its dimers,chlorine peroxide (CIOOCI).Recently,analyses of atmospheric observations
have suggested that the equilibrium constant. <eq.governing the
balance between CIOOCIformation and thermaldecomposition in darkness is lower than that in the current evaluation of kinetics data. Measurements of CIO at night. when CIOOCI is unaffected by photolysis, provide a useful means of testing quantitative un­ derstanding of the CIO/CIOOCI relationship. Here we analyze nighttime CIO measurements from the National Aeronautics and Space Administration Aura Microwave limb Sounder (MLS) to infer an expression for Keq. Although the observed temperature depen­ dence of the nighttime CIO is in line with the theoretical CIO/ OOOCIequilibrium relationship, none of the previously published expressions for 1<eq consistently produces CIO abundances that match the MLS observations well under all conditions.Employing a standard expression for Keq.A x exp(B/T).we constrain the para­ meter A to currently recommended values and estimate 8 using a nonlinear weighted least squares analysis of nighttime MLS CIO data. 00 measurements at multiple pressure levels throughout the periods of peak chlorine activation inthree Arctic and four Ant­ arctic winters are used to estimate B. Our derived 8 leads to values
of 1<eq that are 1.4 times smaller at stratospherically relevant
temperatures than currently rec:ommended,consistent with earlier studies. Our results are in better agreement with the newly updated (2009) kinetics evaluation than with the previous (2006) recommendation.

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Atmospheric Composition Modeling and Analysis Program (ACMAP)
Tropospheric Composition Program (TCP)