Modeling biogenic emissions of isoprene: exploration of model drivers, climate control algorithms, and use of global satellite observations

An improved global budget for isoprene emissions from terrestrial vegetation sources is fundamental to a better understanding of the oxidative capacity of the lower atmosphere and changes in the concentration of major greenhouse gases. In this study, we present a biosphere modeling analysis designed to ascertain the interactions of global data drivers for estimating biogenic isoprene emissions. We have integrated generalized isoprene emission algorithms into a process-based simulation model of ecosystem carbon fluxes, the NASA-CASA (Carnegie–Ames–Stanford Approach) model. This new modeling approach for predicting isoprene emissions operates on scales designed to directly link regional and global satellite data sets with estimates of ecosystem carbon cycling, hydrology, and related biogeochemistry. The NASA-CASA model results indicate that the annual isoprene flux from terrestrial plant sources is 559 TgC. Three ecosystem types, broadleaf evergreen forest, dry tropical forest, and wooded grassland (savanna), account for approximately 80% of these global vegetation isoprene emissions. Based on analyses to improve understanding of the relative influence of climatic (e.g., light and temperature) versus biotic (NPP) controllers on predicted isoprene emission estimates, it appears that the largest portion of total biogenic flux to the global atmosphere is emitted from ecosystems that are mainly light-limited for isoprene emissions. These modeling results imply that, along with better process understanding of base emission factor controls for volatile organic compounds, improvements in global fields of solar surface radiation fluxes in warm climate zones will be needed to reduce major uncertainties in isoprene source fluxes.