| Summary: | 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 H_2O_2. We implemented such an uptake of HO_2 by aerosol in the model using a standard reactive uptake coefficient parameterization with γ(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. © Author(s) 2010. This work is distributed under the Creative Commons Attribution 3.0 License. Received: 3 March 2010 – Published in Atmos. Chem. Phys. Discuss.: 11 March 2010. Revised: 13 June 2010 – Accepted: 21 June 2010 – Published: 1 July 2010. The authors would like to thank Scot T. Martin, Hongyu Liu, Charles E. Miller, Richard A. Ferrare, Karl D. Froyd and Daniel M. Murphy for helpful discussions. We also would like to thank Yuhang Wang for providing the TOPSE dataset, Huisheng Bian for providing Fast-JX updates and Dirk Richter for contributing to the HCHO measurement. J. Mao also thanks David M. ...
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