Global sensitivity analysis of the GEOS-Chem chemical transport model: ozone and hydrogen oxides during ARCTAS (2008)

Abstract. Developing predictive capability for future atmospheric oxidation capacity requires a detailed analysis of model uncertainties and sensitivity of the modeled oxidation capacity to model input variables. Using oxidant mixing ratios modeled by the GEOS-Chem chemical transport model and measured on the NASA DC-8 aircraft, uncertainty and global sensitivity analyses were performed on the GEOS-Chem chemical transport model for the modeled oxidants hydroxyl (OH), hydroperoxyl (HO₂), and ozone (O₃). The sensitivity of modeled OH, HO₂, and ozone to model inputs perturbed simultaneously within their respective uncertainties were found for the flight tracks of NASA's Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) A and B campaigns (2008) in the North American Arctic. For the spring deployment (ARCTAS-A), ozone was most sensitive to the photolysis rate of NO₂, the NO₂ + OH reaction rate, and various emissions, including methyl bromoform (CHBr₃). OH and HO₂ were overwhelmingly sensitive to aerosol particle uptake of HO₂ with this one factor contributing upwards of 75 % of the uncertainty in HO₂. For the summer deployment (ARCTAS-B), ozone was most sensitive to emission factors, such as soil NO_x and isoprene. OH and HO₂ were most sensitive to biomass emissions and aerosol particle uptake of HO₂. With modeled HO₂ showing a factor of 2 underestimation compared to measurements in the lowest 2 km of the troposphere, lower uptake rates (γ_HO2 < 0. 055), regardless of whether or not the product of the uptake is H₂O or H₂O₂, produced better agreement between modeled and measured HO₂.