ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-9-2793-2009 The CO<sub>2</sub> inhibition of terrestrial isoprene emission significantly affects future ozone projections Young P. J. 1 2 4 Arneth A. 3 5 Schurgers G. 3 Zeng G. 1 2 6 Pyle J. A. 1 2 Centre for Atmospheric Science, Department of Chemistry, University of Cambridge, Cambridge, CB2 1EW, UK National Centre for Atmospheric Science, UK Department of Physical Geography and Ecosystems Analysis, Centre for GeoBiosphere Science, Lund University, Sölvegatan, Lund, Sweden now at: NOAA Earth System Research Laboratory, Boulder, Colorado, USA currently at: Physics Department, Helsinki University, Helsinki, Finland now at: National Institute of Water and Atmospheric Research, Lauder, New Zealand 27 04 2009 9 8 2793 2803 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/9/2793/2009/acp-9-2793-2009.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/9/2793/2009/acp-9-2793-2009.pdf

Simulations of future tropospheric composition often include substantial increases in biogenic isoprene emissions arising from the Arrhenius-like leaf emission response and warmer surface temperatures, and from enhanced vegetation productivity in response to temperature and atmospheric CO<sub>2</sub> concentration. However, a number of recent laboratory and field data have suggested a direct inhibition of leaf isoprene production by increasing atmospheric CO<sub>2</sub> concentration, notwithstanding isoprene being produced from precursor molecules that include some of the primary products of carbon assimilation. The cellular mechanism that underlies the decoupling of leaf photosynthesis and isoprene production still awaits a full explanation but accounting for this observation in a dynamic vegetation model that contains a semi-mechanistic treatment of isoprene emissions has been shown to change future global isoprene emission estimates notably. Here we use these estimates in conjunction with a chemistry-climate model to compare the effects of isoprene simulations without and with a direct CO<sub>2</sub>-inhibition on late 21st century O<sub>3</sub> and OH levels. The impact on surface O<sub>3</sub> was significant. Including the CO<sub>2</sub>-inhibition of isoprene resulted in opposing responses in polluted (O<sub>3</sub> decreases of up to 10 ppbv) vs. less polluted (O<sub>3</sub> increases of up to 10 ppbv) source regions, due to isoprene nitrate and peroxy acetyl nitrate (PAN) chemistry. OH concentration increased with relatively lower future isoprene emissions, decreasing methane lifetime by ~7 months (6.6%). Our simulations underline the large uncertainties in future chemistry and climate studies due to biogenic emission patterns and emphasize the problems of using globally averaged climate metrics (such as global radiative forcing) to quantify the atmospheric impact of reactive, heterogeneously distributed substances.

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