Testing fast photochemical theory during TRACE‐P based on measurements of OH, HO₂, and CH₂O

Measurements of several short‐lived photochemical species (e.g., OH, HO 2 , and CH 2 O) were obtained from the DC‐8 and P3‐B aircraft during the NASA Transport and Chemical Evolution over the Pacific (TRACE‐P) campaign. To assess fast photochemical theory over the east Asian coast and western Pacific, these measurements are compared to predictions using a photochemical time‐dependent box model constrained by coincident measurements of long‐lived tracers and physical parameters. Both OH and HO 2 are generally overpredicted by the model throughout the troposphere, which is a different result from previous field campaigns. The calculated‐to‐observed ratio of OH shows an altitude trend, with OH overpredicted by 80% in the upper troposphere and by 40–60% in the middle troposphere. Boundary layer and lower tropospheric OH ratios decrease from middle tropospheric values to 1.07 for the DC‐8 and to 0.70 for the P3‐B. HO 2 measured on the DC‐8 is overpredicted by a median of 23% and shows no trend in the agreement with altitude. Three subsets of data which compose 12% of the HO 2 measurements represent outliers with respect to calculated‐to‐observed ratios: stratospherically influenced air, upper tropospheric data with NO > 135 pptv, and data from within clouds. Pronounced underpredictions of both HO 2 and OH were found for stratospherically influenced air, which is in contrast to previous studies showing good agreement of predicted and observed HO x in the stratosphere. Observational evidence of heterogeneous uptake of HO 2 within low and middle tropospheric clouds is presented, though there is no indication of significant HO 2 uptake within higher‐altitude clouds. Model predictions of CH 2 O are in good agreement with observations in the median for background concentrations, but a large scatter exists. Factors contributing to this scatter are examined, including the limited availability of some important constraining measurements, particularly CH 3 OOH. Some high concentrations of CH 2 O near the coast are underpredicted by the box model as a result of the inherent neglect of transport effects of CH 2 O and its precursors via the steady state assumption; however, these occurrences are limited to ∼1% of the data. For the vast majority of the atmosphere, transport is unimportant in the budget of CH 2 O, which may be considered to be in steady state.