Atmos. Chem. Phys. Discuss., 12, 30755-30804, 2012
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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.
Preindustrial to present day changes in tropospheric hydroxyl radical and methane lifetime from the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP)
V. Naik1, A. Voulgarakis2, A. M. Fiore3, L. W. Horowitz4, J.-F. Lamarque5, M. Lin4,6, M. J. Prather7, P. J. Young8,9,*, D. Bergmann10, P. J. Cameron-Smith10, I. Cionni11, W. J. Collins12,**, S. B. Dalsøren13, R. Doherty14, V. Eyring15, G. Faluvegi16, G. A. Folberth12, B. Josse17, Y. H. Lee16, I. A. MacKenzie14, T. Nagashima18, T. P. C. van Noije19, D. A. Plummer20, M. Righi15, S. T. Rumbold12, R. Skeie13, D. T. Shindell16, D. S. Stevenson14, S. Strode21, K. Sudo22, S. Szopa23, and G. Zeng24
1UCAR/NOAA Geophysical Fluid Dynamics Laboratory, Princeton, New Jersey, USA
2Department of Physics, Imperial College, London, UK
3Department of Earth and Environmental Sciences and Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York, USA
4NOAA Geophysical Fluid Dynamics Laboratory, Princeton, New Jersey, USA
5National Center for Atmospheric Research, Boulder, Colorado, USA
6Atmospheric and Oceanic Sciences, Princeton University, New Jersey, USA
7Department of Earth System Science, University of California, Irvine, California, USA
8Cooperative Institute for Research in the Environmental Sciences, University of Colorado-Boulder, Boulder, Colorado, USA
9Chemical Sciences Division, NOAA Earth System Research Laboratory, Boulder, Colorado, USA
10Lawrence Livermore National Laboratory, Livermore, California, USA
11ENEA, Bologna, Italy
12Hadley Centre for Climate Prediction, Met Office, Exeter, UK
13CICERO, Center for International Climate and Environmental Research-Oslo, Oslo, Norway
14School of Geosciences, University of Edinburgh, Edinburgh, UK
15Deutsches Zentrum für Luft- und Raumfahrt, Institut für Physik der Atmosphäre, Oberpfaffenhofen, Germany
16NASA Goddard Institute for Space Studies, New York City, New York, USA
17Météo-France, CNRM/GMGEC/CARMA, Toulouse, France
18National Institute for Environmental Studies, Tsukuba-shi, Ibaraki, Japan
19Royal Netherlands Meteorological Institute, De Bilt, The Netherlands
20Canadian Centre for Climate Modeling and Analysis, Environment Canada, Victoria, British Columbia, Canada
21NASA Goddard Space Flight Center, Greenbelt, Maryland, USA and Universities Space Research Association, Columbia, MD, USA
22Department of Earth and Environmental Science, Graduate School of Environmental Studies, Nagoya University, Nagoya, Japan
23Laboratoire des Sciences du Climat et de l'Environment, Gif-sur-Yvette, France
24National Institute of Water and Atmospheric Research, Lauder, New Zealand
*now at: Lancaster Environment Centre, Lancaster University, Lancaster, UK
**now at: Department of Meteorology, University of Reading, UK

Abstract. We have analysed results from 17 global models, participating in the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP), to explore trends in hydroxyl radical concentration (OH) and methane (CH4) lifetime since preindustrial times (1850) and gain a better understanding of their key drivers. For the present day (2000), the models tend to simulate higher OH abundances in the Northern Hemisphere versus Southern Hemisphere. Evaluation of simulated carbon monoxide concentrations, the primary sink for OH, against observations suggests low biases in the Northern Hemisphere that may contribute to the high north-south OH asymmetry in the models. A comparison of modelled and observed methyl chloroform lifetime suggests that the present day global multi-model mean OH concentration is slightly overestimated. Despite large regional changes, the modelled global mean OH concentration is roughly constant over the past 150 yr, due to concurrent increases in OH sources (humidity, tropospheric ozone, and NOx emissions), together with decreases in stratospheric ozone and increase in tropospheric temperature, compensated by increases in OH sinks (methane abundance, carbon monoxide and non-methane volatile organic carbon (NMVOC) emissions). The large intermodel diversity in the sign and magnitude of OH and methane lifetime changes over this period reflects differences in the relative importance of chemical and physical drivers of OH within each model. For the 1980 to 2000 period, we find that climate warming and a slight increase in mean OH leads to a 4.3 ± 1.9% decrease in the methane lifetime. Analysing sensitivity simulations performed by 10 models, we find that preindustrial to present day climate change decreased the methane lifetime by about 4 months, representing a negative feedback on the climate system. Further, using a subset of the models, we find that global mean OH increased by 46.4 ± 12.2% in response to preindustrial to present day anthropogenic NOx emission increases, and decreased by 17.3 ± 2.3%, 7.6 ± 1.5%, and 3.1 ± 3.0% due to methane burden, and anthropogenic CO, and NMVOC emissions increases, respectively.

Citation: Naik, V., Voulgarakis, A., Fiore, A. M., Horowitz, L. W., Lamarque, J.-F., Lin, M., Prather, M. J., Young, P. J., Bergmann, D., Cameron-Smith, P. J., Cionni, I., Collins, W. J., Dalsøren, S. B., Doherty, R., Eyring, V., Faluvegi, G., Folberth, G. A., Josse, B., Lee, Y. H., MacKenzie, I. A., Nagashima, T., van Noije, T. P. C., Plummer, D. A., Righi, M., Rumbold, S. T., Skeie, R., Shindell, D. T., Stevenson, D. S., Strode, S., Sudo, K., Szopa, S., and Zeng, G.: Preindustrial to present day changes in tropospheric hydroxyl radical and methane lifetime from the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP), Atmos. Chem. Phys. Discuss., 12, 30755-30804, doi:10.5194/acpd-12-30755-2012, 2012.
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