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Photosynthesis-dependent isoprene emission from leaf to planet in a global carbon-chemistry-climate model

by N. Unger, K. Harper, Y. Zheng, N. Y. Kiang, I. Aleinov, A. Arneth, G. Schurgers, C. Amelynck, A. Goldstein, A. Guenther, B. Heinesch, C. N. Hewitt, T. Karl, Q. Laffineur, B. Langford, K. A. McKinney, P. Misztal, M. Potosnak, J. Rinne, S. Pressley, N. Schoon, D. Serça show all authors
Atmospheric Chemistry and Physics ()
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Abstract

We describe the implementation of a biochemi-cal model of isoprene emission that depends on the elec-tron requirement for isoprene synthesis into the Farquhar– Ball–Berry leaf model of photosynthesis and stomatal con-ductance that is embedded within a global chemistry-climate simulation framework. The isoprene production is calculated as a function of electron transport-limited photosynthesis, intercellular and atmospheric carbon dioxide concentration, and canopy temperature. The vegetation biophysics mod-ule computes the photosynthetic uptake of carbon dioxide coupled with the transpiration of water vapor and the iso-prene emission rate at the 30 min physical integration time step of the global chemistry-climate model. In the model, the rate of carbon assimilation provides the dominant con-trol on isoprene emission variability over canopy tempera-ture. A control simulation representative of the present-day climatic state that uses 8 plant functional types (PFTs), pre-scribed phenology and generic PFT-specific isoprene emis-sion potentials (fraction of electrons available for isoprene synthesis) reproduces 50 % of the variability across differ-ent ecosystems and seasons in a global database of 28 mea-sured campaign-average fluxes. Compared to time-varying isoprene flux measurements at 9 select sites, the model au-thentically captures the observed variability in the 30 min Published by Copernicus Publications on behalf of the European Geosciences Union. 10244 N. Unger et al.: Photosynthesis-dependent isoprene emission from leaf to planet average diurnal cycle (R 2 = 64–96 %) and simulates the flux magnitude to within a factor of 2. The control run yields a global isoprene source strength of 451 TgC yr −1 that in-creases by 30 % in the artificial absence of plant water stress and by 55 % for potential natural vegetation.

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