Photosynthesis-dependent isoprene emission from leaf to planet in a global carbon-chemistry-climate model
We describe the implementation of a biochemical model of isoprene\nemission that depends on the electron requirement for isoprene synthesis\ninto the Farquhar-Ball-Berry leaf model of photosynthesis and stomatal\nconductance that is embedded within a global chemistry-climate\nsimulation framework. The isoprene production is calculated as a\nfunction of electron transport-limited photosynthesis, intercellular and\natmospheric carbon dioxide concentration, and canopy temperature. The\nvegetation biophysics module computes the photosynthetic uptake of\ncarbon dioxide coupled with the transpiration of water vapor and the\nisoprene emission rate at the 30 min physical integration time step of\nthe global chemistry-climate model. In the model, the rate of carbon\nassimilation provides the dominant control on isoprene emission\nvariability over canopy temperature. A control simulation representative\nof the present-day climatic state that uses 8 plant functional types\n(PFTs), prescribed phenology and generic PFT-specific isoprene emission\npotentials (fraction of electrons available for isoprene synthesis)\nreproduces 50% of the variability across different ecosystems and\nseasons in a global database of 28 measured campaign-average fluxes.\nCompared to time-varying isoprene flux measurements at 9 select sites,\nthe model authentically captures the observed variability in the 30 min\naverage diurnal cycle (R-2 = 64-96 %) and simulates the flux magnitude\nto within a factor of 2. The control run yields a global isoprene source\nstrength of 451 TgC yr(-1) that increases by 30% in the artificial\nabsence of plant water stress and by 55% for potential natural\nvegetation.