Establishing the Dependence of [HO₂]/[OH] on Temperature, Halogen Loading, O₃, and NOx Based on in Situ Measurements from the NASA ER-2

In situ observations of OH and HO 2 from the Airborne Southern Hemisphere Ozone Experiment/Measurements for Assessing the Effects of Stratospheric Aircraft (ASHOE/MAESA), Stratospheric TRacers of Atmospheric Transport (STRAT), and Polar Ozone Loss in the Arctic Region in Summer (POLARIS) NASA ER-2 field campaigns are used to examine the partitioning of HO x in the lower stratosphere (tropopause to ∼21 km) and upper troposphere (∼10 km to tropopause). These measurements span a latitude range from 70°S to 90°N and a variety of atmospheric conditions as a result of seasonal changes and altitude. The response of the observed [HO 2 ]/[OH] to changes in temperature, [O 3 ], [CO], [NO], [ClO], and [BrO] is investigated. The measured ratio is accurately described (∼±10%) by a steady-state model constrained by the measured mixing ratios of O 3, CO, NO, ClO, and BrO, where the model is valid for conditions of HO x cycling much faster than HO x production and loss. The concentration of HO 2 depends on [OH], which, to first order, has been observed to be a simple function of the solar zenith angle in the lower stratosphere. 1 The partitioning between OH and HO 2 is controlled by the local chemistry between the HO x radicals and O 3, CO, NO, ClO, and BrO. The response of [HO x ] to changes in [NO x ] and [O 3 ] is demonstrated. Further observations are necessary to illustrate the response of HO x to changes in halogen concentrations. A quantitative understanding of [HO 2 ]/[OH] is important, since many of the reactions that control this ratio are directly involved in catalytic removal of O 3 in the lower stratosphere and production of O 3 in the upper troposphere.