Evaluating the impact of different spaceborne land cover distributions on isoprene emissions and their trends using the MEGAN model.

The biogenic volatile organic compounds (BVOCs) are emitted globally at about 1100 Tg per year of which almost half of the share is entailed by isoprene. Isoprene is highly reactive in the atmosphere, and its degradation products impact the atmospheric composition through the generation of ozone (in presence of NOx typical of polluted areas) and secondary organic aerosols, and may pose a risk to human health. Isoprene is mainly emitted by plant foliage, with trees being the major contributors due to their relatively high emission factors.

In the modelling framework of biosphere-atmosphere interactions, the representation of land cover and vegetation distributions is a key aspect. We use the state-of-the-art biogenic emission model MEGAN (Guenther et al. 2012) coupled with a multi-layer canopy model MOHYCAN (Müller et al. 2008) to estimate isoprene emissions on the global scale. In its current standard version, the model uses a static plant functional type (PFT) distribution obtained from the Community Land Model (CLM4) for 2000. Our objective is to replace the static map by time-dependent PFT distributions based on satellite global land cover maps, and estimate the resulting biogenic emissions over 2001-2018. To this purpose, we use either the MODIS land cover dataset (Friedl and Sulla-Menashe, 2019), or the MODIS dataset modified to account for tree cover changes from Hansen et al. (2013). Comparisons with the ESA-CCI dataset (Poulter et al. 2015) and the FAOSTAT (www.fao.org) database are performed and the trends over large forested regions are discussed. The comparisons show a large variability in the representation of the tree cover by the available remotely-sensed datasets, leading to different spatial distributions and temporal variability in the estimated isoprene emissions. This gives a measure of the uncertainty associated to this input parameter. This work is conducted in the frame of the ALBERI project that aims at assessing links between biogenic emissions and remotely-sensed photosynthesis indicators, funded by BELSPO through the STEREO III programme.