Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 10 3 2010 10.5194/acpd-10-6955-2010 http://www.atmos-chem-phys-discuss.net/10/6955/2010/ http://www.atmos-chem-phys-discuss.net/10/6955/2010/acpd-10-6955-2010.html http://www.atmos-chem-phys-discuss.net/10/6955/2010/acpd-10-6955-2010.pdf 6955 6994 2010-03-11 Chemistry of hydrogen oxide radicals (HO_x) in the Arctic troposphere in spring J. Mao mao@fas.harvard.edu D. J. Jacob M. J. Evans J. R. Olson X. Ren W. H. Brune J. M. S. Clair J. D. Crounse K. M. Spencer M. R. Beaver P. O. Wennberg M. J. Cubison J. L. Jimenez A. Fried P. Weibring J. G. Walega S. R. Hall A. J. Weinheimer R. C. Cohen G. Chen J. H. Crawford L. Jaeglé J. A. Fisher R. M. Yantosca P. Le Sager C. Carouge School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA, USA School of Earth and the Environment, University of Leeds, Leeds, LS2 9JT, UK Science Directorate, NASA Langley Research Center, Hampton, VA, USA Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, FL, USA Department of Meteorology, Pennsylvania State University, University Park, PA, USA Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA, USA Division of Geological and Planetary Sciences, California Institute of Technology,\newline Pasadena, CA, USA Division of Engineering and Applied Science, California Institute of Technology, Pasadena, CA, USA Department of Chemistry and Biochemistry and Cooperative Institute for Research in the Environmental Sciences (CIRES),University of Colorado at Boulder, Boulder, CO, USA Earth Observing Laboratory, National Center for Atmospheric Research, Boulder, CO, USA Atmospheric Chemistry Division, National Center for Atmospheric Research, Boulder, CO, USA Department of Chemistry and Department of Earth and Planetary Science, University of California Berkeley, Berkeley, CA, USA Department of Atmospheric Sciences, University of Washington, Seattle, Washington, USA now at: KNMI, Chemistry and Climate Division, De Bilt, The Netherlands 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. Such a mechanism could involve HO₂ aqueous-phase reaction with sulfate (58% of the ARCTAS submicron aerosol by mass) to produce peroxymonosulfate (HSO₅^−) that would eventually convert back to sulfate and return water. 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. 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