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 \(HO_2\) and \(H_2O_2\) 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 \(HO_2\) 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 \(HO_2\) by aerosol in the model using a standard reactive uptake coefficient parameterization with \(\gamma(HO_2)\) 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. \(HO_2\) uptake by aerosol is then a major \(HO_x\) and \(HO_y\) sink, decreasing mean OH and \(HO_2\) concentrations in the Arctic troposphere by 32% and 31% respectively. Better rate and product data for \(HO_2\) uptake by aerosol are needed to understand this role of aerosols in limiting the oxidizing power of the Arctic atmosphere.