A reevaluation of airborne HO_x observations from NASA field campaigns

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 NO_x (NO + NO₂) levels and at high solar zenith angles. This analysis shows that much of the model discrepancy at high NO_x during SUCCESS can be attributed to modeling observations at timescales too long to capture the nonlinearity of HO_x (OH+HO₂) chemistry under highly variable conditions for NO_x. Discrepancies at high NO_x 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 NO_x because of both the chemical nonlinearity between HO_x and NO_x and the increased sensitivity of HO_x to changes in sources at high NO_x. In contrast, discrepancies at high solar zenith angles will persist until an adequate nighttime source of HO_x 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 NO_x 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 HO_x radicals.