Comparisons are made between a steady state photochemical model and the first airborne measurements of hydroxyl radical concentrations, , in the lower free troposphere and marine boundary layer, taken during the First Aerosol Characterization Experiment (ACE 1). This paper describes in more detail the modeling results and the model-measurement comparisons which were presented by Mauldin et al. <1997, 1998>. Modeled and observed agree to within the combined uncertainties, with a median model overestimate of 32%. The model quantifies the sources and sinks of OH for low levels of nitric oxide (NO). Predicted concentrations of the peroxy radicals HO2 and CH3O2 and total radical production and loss rates from these model calculations are presented. Sensitivities to model input parameters and the uncertainty each contributes to the calculated are discussed. Model overpredictions of observed within clouds can be corrected by including a loss of gas phase OH to cloud droplets. The low conditions preclude HO2 uptake alone from explaining observed depletions of OH within clouds. We examined the dependence of gas phase on its assumed mass accommodation coefficient, which the measurements suggest is 0.1 or greater for the cloud droplets encountered in ACE 1. ¿ 1999 American Geophysical Union