Establishing the dependence of [HO2]/[OH]] on temperature, halogen loading, O-3, and NOx based on in situ measurements from the NASA ER-2

In situ observations of OH and HO2 from the Airborne Southern Hemisphere Oz one Experiment/Measurements for Assessing the Effects of Stratospheric Airc raft (ASHOE/MAESA), Stratospheric TRacers of Atmospheric Transport (STRAT), and Polar Ozone Loss in the Arctic Region in Summer (POLARIS) NASA ER-2 fi eld campaigns are used to examine the partitioning of HOchi in the lower st ratosphere (tropopause to similar to 21 km) and upper troposphere (similar to 10 km to tropopause). These measurements span a latitude range from 70 d egreesS to 90 degreesN and a variety of atmospheric conditions as a result of seasonal changes and altitude. The response of the observed [HO2]/[OH] t o changes in temperature, [O-3], [CO], [NO], [CIO], and [BrO] is investigat ed. The measured ratio is accurately described (similar to+/-10%) by a stea dy-state model constrained by the measured mixing ratios of O-3, CO, NO, CI O, and BrO, where the model is valid for conditions of HOchi, cycling much faster than HOchi production and loss. The concentration of HO2 depends on [OH], which, to first order, has been observed to be a simple function of t he solar zenith angle in the lower stratosphere.(1) The partitioning betwee n OH and HO2 is controlled by the local chemistry between the HOchi radical s and O-3, CO, NO, CIO, and BrO. The response of [HOchi] to changes in [NOc hi] and [O-3] is demonstrated. Further observations are necessary to illust rate the response of HOchi to changes in halogen concentrations. A quantita tive understanding of [HO2]/ [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.