Gj. Frost et al., Photochemical modeling of OH levels during the first aerosol characterization experiment (ACE 1), J GEO RES-A, 104(D13), 1999, pp. 16041-16052
measurements of hydroxyl radical concentrations, [OH], in the lower free tr
oposphere and marine boundary layer, taken during the First Aerosol Charact
erization Experiment (ACE 1). This paper describes in more detail the model
ing results and the model-measurement comparisons which were presented by M
auldin et nl. [1997, 1998]. Modeled and observed [OH] agree to within the c
ombined uncertainties, with a median model overestimate of 32%. The model q
uantifies the sources and sinks of OH for low levels of nitric oxide (NO).
Predicted concentrations of the peroxy radicals HO2 and CH3O2 and total rad
ical production and Toss rates from these model calculations are presented.
Sensitivities to model input parameters and the uncertainty each contribut
es to the calculated [OH] are discussed. Model overpredictions of observed
[OH] within clouds can be corrected by including a loss of gas phase OH to
cloud droplets. The low [NO] conditions preclude HO2:! uptake alone from ex
plaining observed depletions of OH within clouds. We examined the dependenc
e of gas phase [OH] on its assumed mass accommodation coefficient, which th
e measurements suggest is 0.1 or greater for the cloud droplets encountered
in ACE 1.