Atmos. Chem. Phys. Atmospheric Chemistry and Physics, vol. 9 (2009) pp. 2793-2803
Simulations of future tropospheric composition often include substantial increases in biogenic isoprene emis-sions arising from the Arrhenius-like leaf emission response and warmer surface temperatures, and from enhanced veg-etation productivity in response to temperature and atmo-spheric CO 2 concentration. However, a number of recent laboratory and field data have suggested a direct inhibition of leaf isoprene production by increasing atmospheric CO 2 con-centration, notwithstanding isoprene being produced from precursor molecules that include some of the primary prod-ucts of carbon assimilation. The cellular mechanism that un-derlies 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 2 -inhibition on late 21st century O 3 and OH levels. The impact on surface O 3 was significant. Including the CO 2 -inhibition of isoprene re-sulted in opposing responses in polluted (O 3 decreases of up to 10 ppbv) vs. less polluted (O 3 increases of up to 10 ppbv) source regions, due to isoprene nitrate and peroxy acetyl ni-trate (PAN) chemistry. OH concentration increased with rel-atively lower future isoprene emissions, decreasing methane lifetime by ∼7 months (6.6%). Our simulations underline the Correspondence to: P. J. Young (email@example.com) large uncertainties in future chemistry and climate studies due to biogenic emission patterns and emphasize the prob-lems 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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