OH and HO2 chemistry in clean marine air during SOAPEX-2

[摘要] Model-measurement comparisons of HO_x in extremely cleanair ([NO]<3 ppt) are reported. Measurements weremade during the second Southern Ocean Photochemistry Experiment(SOAPEX-2), held in austral summer 1999 at the Cape Grim BaselineAir Pollution Station in north-western Tasmania, Australia.The free-radical chemistry was studied using a zero-dimensionalbox-model based upon the Master Chemical Mechanism (MCM). Twoversions of the model were used, with different levels of chemicalcomplexity, to explore the role of hydrocarbons upon free-radicalbudgets under very clean conditions. The "detailed" model wasconstrained to measurements of CO, CH₄ and 17NMHCs, while the "simple" model contained only the CO andCH₄ oxidation mechanisms, together with inorganicchemistry. The OH and HO₂ (HO_x)concentrations predicted by the two models agreed to within5–10%.The model results were compared with the HO_xconcentrations measured by the FAGE (Fluorescence Assay by GasExpansion) technique during four days of clean Southern Oceanmarine boundary layer (MBL) air. The models overestimatedOH concentrations by about 10% on two days and about 20%on the other two days. HO₂ concentrations were measuredduring two of these days and the models overestimated the measuredconcentrations by about 40%. Better agreement with measuredHO₂ was observed by using data from several MBL aerosolmeasurements to estimate the aerosol surface area and byincreasing the HO₂ uptake coefficient to unity. Thisreduced the modelled HO₂ overestimate by ~40%, withlittle effect on OH, because of the poor HO₂ toOH conversion at the low ambient NO_x concentrations.Local sensitivity analysis and Morris One-At-A-Time analysis wereperformed on the "simple" model, and showed the importance ofreliable measurements of j(O¹D) and [HCHO] and ofthe kinetic parameters that determine the efficiency ofO(¹D) to OH and HCHO to HO₂conversion. A 2σ standard deviation of 30–40% forOH and 25–30% for HO₂ was estimated for the modelcalculations using a Monte Carlo technique coupled with LatinHypercube Sampling (LHS).

[学科分类] 大气科学