Chemistry of hydrogen oxide radicals (HOₓ) 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 (HOₓ≡H+OH+peroxy radicals) and their reservoirs (HOy≡HOₓ+peroxides) in the springtime Arctic atmosphere. We find that a standard gas-phase chemical mechanism overestimates the observed HO₂ and H₂O₂ concentrations. Computation of HOₓ and HOy 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 HOₓ species and HOy reservoirs. HO₂ uptake by aerosol is then a major HOₓ and HOy 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.