Chemistry of hydrogen oxide radicals (HO_x) in the Arctic troposphere in spring

Abstract. We use observations from the April 2008 NASA ARCTAS aircraft campaign to the North American Arctic, interpreted with a global 3-D chemical transport model (GEOS-Chem), to better understand the sources and cycling of hydrogen oxide radicals (HO_x≡H+OH+peroxy radicals) and their reservoirs (HO_y≡HO_x+peroxides) in the springtime Arctic atmosphere. We find that a standard gas-phase chemical mechanism overestimates the observed HO₂ and H₂O₂ concentrations. Computation of HO_x and HO_y gas-phase chemical budgets on the basis of the aircraft observations also indicates a large missing sink for both. We hypothesize that this could reflect HO₂ uptake by aerosols, favored by low temperatures and relatively high aerosol loadings, through a mechanism that does not produce H₂O₂. We implemented such an uptake of HO₂ by aerosol in the model using a standard reactive uptake coefficient parameterization with γ(HO₂) values ranging from 0.02 at 275 K to 0.5 at 220 K. This successfully reproduces the concentrations and vertical distributions of the different HO_x species and HO_y reservoirs. HO₂ uptake by aerosol is then a major HO_x and HO_y sink, decreasing mean OH and HO₂ concentrations in the Arctic troposphere by 32% and 31% respectively. Better rate and product data for HO₂ uptake by aerosol are needed to understand this role of aerosols in limiting the oxidizing power of the Arctic atmosphere.