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Constraints for the photolysis rate and the equilibrium constant of ClO‐dimer from airborne and balloon‐borne measurements of chlorine compounds
Author(s) -
Kleinböhl Armin,
Khosravi Maryam,
Urban Joachim,
Canty Timothy,
Salawitch Ross J.,
Toon Geoffrey C.,
Küllmann Harry,
Notholt Justus
Publication year - 2014
Publication title -
journal of geophysical research: atmospheres
Language(s) - English
Resource type - Journals
eISSN - 2169-8996
pISSN - 2169-897X
DOI - 10.1002/2013jd021433
Subject(s) - chemistry , equilibrium constant , photodissociation , reaction rate constant , analytical chemistry (journal) , dimer , photochemistry , kinetics , physics , organic chemistry , quantum mechanics
We analyze measurements of ClO across the terminator taken by the Airborne Submillimeter Radiometer (ASUR) in the activated vortices of the Arctic winters of 1995/1996, 1996/1997, and 1999/2000 to evaluate the plausibility of various determinations of the ClO‐dimer photolysis cross section and the rate constant controlling the thermal equilibrium between ClO‐dimer and ClO. We use measured ClO during sunlit conditions to estimate total active chlorine (ClO x ). As the measurements suggest nearly full chlorine activation in winter 1999/2000, we compare ClO x estimates based on various photolysis frequencies of ClO‐dimer with total available inorganic chlorine (Cl y ), estimated from an N 2 O‐Cl y correlation established by a balloon‐borne MkIV interferometer measurement. Only ClO‐dimer cross sections leading to the fastest photolysis frequencies in the literature (including the latest evaluation by the Jet Propulsion Laboratory) give ClO x mixing ratios that overlap with the estimated range of available Cl y . Slower photolysis rates lead to ClO x values that are higher than available Cl y . We use the ClO x calculated from sunlit ClO measurements to estimate ClO in darkness based on different equilibrium constants, and compare it with ASUR ClO measurements before sunrise at high solar zenith angles. Calculations with equilibrium constants published in recent evaluations of the Jet Propulsion Laboratory give good agreement with observed ClO mixing ratios. Equilibrium constants leading to a higher ClO/ClO x ratio in darkness yield ClO values that tend to exceed observed abundances. Perturbing the rates for the ClO + BrO reaction in a manner that increases OClO formation and decreases BrCl formation leads to lower ClO values calculated for twilight conditions after sunset, resulting in better agreement with ASUR measurements.