Atmos. Chem. Phys. Discuss., 10, 6955-6994, 2010
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
Chemistry of hydrogen oxide radicals (HOx) in the Arctic troposphere in spring
J. Mao1, D. J. Jacob1,2, M. J. Evans3, J. R. Olson4, X. Ren5, W. H. Brune6, J. M. S. Clair7, J. D. Crounse7, K. M. Spencer7, M. R. Beaver7, P. O. Wennberg8,9, M. J. Cubison10, J. L. Jimenez10, A. Fried11, P. Weibring11, J. G. Walega11, S. R. Hall12, A. J. Weinheimer12, R. C. Cohen13, G. Chen4, J. H. Crawford4, L. Jaeglé14, J. A. Fisher2, R. M. Yantosca1, P. Le Sager1,*, and C. Carouge1
1School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA
2Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA, USA
3School of Earth and the Environment, University of Leeds, Leeds, LS2 9JT, UK
4Science Directorate, NASA Langley Research Center, Hampton, VA, USA
5Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, FL, USA
6Department of Meteorology, Pennsylvania State University, University Park, PA, USA
7Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA, USA
8Division of Geological and Planetary Sciences, California Institute of Technology,\newline Pasadena, CA, USA
9Division of Engineering and Applied Science, California Institute of Technology, Pasadena, CA, USA
10Department of Chemistry and Biochemistry and Cooperative Institute for Research in the Environmental Sciences (CIRES),University of Colorado at Boulder, Boulder, CO, USA
11Earth Observing Laboratory, National Center for Atmospheric Research, Boulder, CO, USA
12Atmospheric Chemistry Division, National Center for Atmospheric Research, Boulder, CO, USA
13Department of Chemistry and Department of Earth and Planetary Science, University of California Berkeley, Berkeley, CA, USA
14Department of Atmospheric Sciences, University of Washington, Seattle, Washington, USA
*now at: KNMI, Chemistry and Climate Division, De Bilt, The Netherlands

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. 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 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 48% and 45% respectively. Circumpolar budget analysis in the model shows that transport of peroxides from northern mid-latitudes contributes 50% of the HOy source above 6 km, and cloud chemistry and deposition of H2O2 account together for 40% of the HOy sink below 3 km. 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.

Citation: Mao, J., Jacob, D. J., Evans, M. J., Olson, J. R., Ren, X., Brune, W. H., Clair, J. M. S., Crounse, J. D., Spencer, K. M., Beaver, M. R., Wennberg, P. O., Cubison, M. J., Jimenez, J. L., Fried, A., Weibring, P., Walega, J. G., Hall, S. R., Weinheimer, A. J., Cohen, R. C., Chen, G., Crawford, J. H., Jaeglé, L., Fisher, J. A., Yantosca, R. M., Le Sager, P., and Carouge, C.: Chemistry of hydrogen oxide radicals (HOx) in the Arctic troposphere in spring, Atmos. Chem. Phys. Discuss., 10, 6955-6994, doi:10.5194/acpd-10-6955-2010, 2010.
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