ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-10-2491-2010 Interannual variability of tropospheric composition: the influence of changes in emissions, meteorology and clouds Voulgarakis A. 1 4 Savage N. H. 2 Wild O. 3 Braesicke P. 1 Young P. J. 1 5 Carver G. D. 1 Pyle J. A. 1 Centre for Atmospheric Science, University of Cambridge, UK Met Office, Exeter, UK Lancaster Environment Centre, Lancaster University, UK now at: NASA Goddard Institute for Space Studies & Columbia University, Center for Climate Systems Research, New York, USA now at: NOAA Earth System Research Laboratory, Boulder, Colorado, USA 11 03 2010 10 5 2491 2506 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/10/2491/2010/acp-10-2491-2010.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/10/2491/2010/acp-10-2491-2010.pdf

We have run a chemistry transport model (CTM) to systematically examine the drivers of interannual variability of tropospheric composition during 1996–2000. This period was characterised by anomalous meteorological conditions associated with the strong El Niño of 1997–1998 and intense wildfires, which produced a large amount of pollution. On a global scale, changing meteorology (winds, temperatures, humidity and clouds) is found to be the most important factor driving interannual variability of NO<sub>2</sub> and ozone on the timescales considered. Changes in stratosphere-troposphere exchange, which are largely driven by meteorological variability, are found to play a particularly important role in driving ozone changes. The strong influence of emissions on NO<sub>2</sub> and ozone interannual variability is largely confined to areas where intense biomass burning events occur. For CO, interannual variability is almost solely driven by emission changes, while for OH meteorology dominates, with the radiative influence of clouds being a very strong contributor. Through a simple attribution analysis for 1996–2000 we conclude that changing cloudiness drives 25% of the interannual variability of OH over Europe by affecting shortwave radiation. Over Indonesia this figure is as high as 71%. Changes in cloudiness contribute a small but non-negligible amount (up to 6%) to the interannual variability of ozone over Europe and Indonesia. This suggests that future assessments of trends in tropospheric oxidizing capacity should account for interannual variability in cloudiness, a factor neglected in many previous studies.

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