A reevaluation of airborne HOx observations from NASA field campaigns

[ 1] In situ observations of tropospheric HO2 obtained during four NASA airborne campaigns ( SUCCESS, SONEX, PEM-Tropics B and TRACE-P) are reevaluated using the NASA Langley time-dependent photochemical box model. Special attention is given to previously diagnosed discrepancies between observed and predicted HO2 which increase with higher NOx (NO + NO2) levels and at high solar zenith angles. This analysis shows that much of the model discrepancy at high NOx during SUCCESS can be attributed to modeling observations at timescales too long to capture the nonlinearity of HOx (OH+ HO2) chemistry under highly variable conditions for NOx. Discrepancies at high NOx during SONEX can be moderated to a large extent by complete use of all available precursor observations. Differences between kinetic rate coefficients and photolysis frequencies available for previous studies versus current recommendations also explain some of the disparity. Each of these causes is shown to exert greater influence with increasing NOx because of both the chemical nonlinearity between HOx and NOx and the increased sensitivity of HOx to changes in sources at high NOx. In contrast, discrepancies at high solar zenith angles will persist until an adequate nighttime source of HOx can be identified. It is important to note that other data sets from ground-based field studies show a similar discrepancy between observed and predicted HO2 for high NOx environments, and that the analysis presented here cannot resolve differences from those additional ground studies. Nevertheless, results from this study highlight important considerations in the application of box models to observationally based predictions of HOx radicals.