| Abstract
| - In situ observations of OH and HO2 from the Airborne Southern Hemisphere Ozone Experiment/Measurementsfor Assessing the Effects of Stratospheric Aircraft (ASHOE/MAESA), Stratospheric TRacers of AtmosphericTransport (STRAT), and Polar Ozone Loss in the Arctic Region in Summer (POLARIS) NASA ER-2 fieldcampaigns are used to examine the partitioning of HOx in the lower stratosphere (tropopause to ∼21 km) andupper troposphere (∼10 km to tropopause). These measurements span a latitude range from 70°S to 90°Nand a variety of atmospheric conditions as a result of seasonal changes and altitude. The response of theobserved [HO2]/[OH] to changes in temperature, [O3], [CO], [NO], [ClO], and [BrO] is investigated. Themeasured ratio is accurately described (∼±10%) by a steady-state model constrained by the measured mixingratios of O3, CO, NO, ClO, and BrO, where the model is valid for conditions of HOx cycling much fasterthan HOx production and loss. The concentration of HO2 depends on [OH], which, to first order, has beenobserved to be a simple function of the solar zenith angle in the lower stratosphere. The partitioning betweenOH and HO2 is controlled by the local chemistry between the HOx radicals and O3, CO, NO, ClO, and BrO.The response of [HOx] to changes in [NOx] and [O3] is demonstrated. Further observations are necessary toillustrate the response of HOx to changes in halogen concentrations. A quantitative understanding of [HO2]/[OH] is important, since many of the reactions that control this ratio are directly involved in catalytic removalof O3 in the lower stratosphere and production of O3 in the upper troposphere.
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