Interaction between isoprene and ozone fluxes at ecosystem level in a poplar plantation and its impact at European level

Abstract:

The emissions of Biogenic volatile organic compounds (BVOCs) from vegetation, mainly in form of isoprenoids, play an important role in the tropospheric ozone (O₃) formation. The potential large expansion of isoprene emitter species (e.g. poplar) as biofuels feedstock might impact the ground level O₃ formation. Here we report the simultaneous observations, using the eddy covariance (EC) technique, of isoprene, O₃ and CO₂ fluxes in a short rotation coppice (SRC) of poplar. The impact of current poplar plantations and associated isoprene emissions on ground level ozone concentrations for Europe was evaluated using a chemistry transport model (CTM) LOTOS-EUROS. The isoprene fluxes showed a well-defined seasonal and daily cycle that mirrored with the stomata O₃ uptake. The isoprene emission and the stomata O₃ uptake showed significant statistical relationship especially at elevated temperature. Isoprene was characterized by a remarkable peak of emissions (e.g. 38 nmol m^-2s^-1) occurring for few days as a consequence of the rapid variation of the air and surface temperature. During these days the photosynthetic apparatus (i.e. the CO₂ fluxes) and transpiration rates did not show significant variation while we did observe a variation of the energy exchange and a reduction of the bowen ratio. The response of isoprene emissions to ambient O₃ concentration follows the common form of the hormetic dose–response curve with a considerable reduction of the isoprene emissions at [O₃] > 80 ppbv indicating a potential damping effect of the O₃ levels on isoprene.

Under the current condition the impact of SRC plantations on ozone concentrations / formation is very limited in Europe. Our findings indicate that, even with future scenarios with more SRC, or conventional poplar plantations, the impact on Ozone formation is negligible.