Estimates of total organic and inorganic chlorine in the lower stratosphere from in situ and flask measurements during AASE II

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Abstract
Aircraft sampling has provided extensive in situ and flask measurements of organic chlorine species in the lower stratosphere. The recent Airborne Arctic Stratospheric Expedition II (AASE II) included two independent measurements of organic chlorine species using whole air sample and real‐time techniques. From the whole air sample measurements we derive directly the burden of total organic chlorine (CCly) in the lower stratosphere. From the more limited real‐time measurements we estimate the CCly burden using mixing ratios and growth rates of the principal CCly species in the troposphere in conjunction with results from a two‐dimensional photochemical model. Since stratospheric chlorine is tropospheric in origin and tropospheric mixing ratios are increasing, it is necessary to establish the average age of a stratospheric air parcel to assess its total chlorine (ClTotal) abundance. Total inorganic chlorine (Cly) in the parcel is then estimated by the simple difference, Cly = ClTotal ‐ CCly. The consistency of the results from these two quite different techniques suggests that we can determine the CCly and Cly in the lower stratosphere with confidence. Such estimates of organic and inorganic chlorine are crucial in evaluating the photochemistry controlling chlorine partitioning and hence ozone loss processes in the lower stratosphere.