| Summary: | Model-measurement comparisons of HO x in extremely clean air ([NO]<3 ppt) are reported. Measurements were made during the second Southern Ocean Photochemistry Experiment (SOAPEX-2), held in austral summer 1999 at the Cape Grim Baseline Air Pollution Station in north-western Tasmania, Australia. The free-radical chemistry was studied using a zero-dimensional box-model based upon the Master Chemical Mechanism (MCM). Two versions of the model were used, with different levels of chemical complexity, to explore the role of hydrocarbons upon free-radical budgets under very clean conditions. The "detailed" model was constrained to measurements of CO, CH 4 and 17 NMHCs, while the "simple" model contained only the CO and CH 4 oxidation mechanisms, together with inorganic chemistry. The OH and HO 2 (HO x ) concentrations predicted by the two models agreed to within 5–10%. The model results were compared with the HO x concentrations measured by the FAGE (Fluorescence Assay by Gas Expansion) technique during four days of clean Southern Ocean marine boundary layer (MBL) air. The models overestimated OH concentrations by about 10% on two days and about 20% on the other two days. HO 2 concentrations were measured during two of these days and the models overestimated the measured concentrations by about 40%. Better agreement with measured HO 2 was observed by using data from several MBL aerosol measurements to estimate the aerosol surface area and by increasing the HO 2 uptake coefficient to unity. This reduced the modelled HO 2 overestimate by ~40%, with little effect on OH, because of the poor HO 2 to OH conversion at the low ambient NO x concentrations. Local sensitivity analysis and Morris One-At-A-Time analysis were performed on the "simple" model, and showed the importance of reliable measurements of j(O 1 D) and [HCHO] and of the kinetic parameters that determine the efficiency of O( 1 D) to OH and HCHO to HO 2 conversion. A 2σ standard deviation of 30–40% for OH and 25–30% for HO 2 was estimated for the model ...
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