Biogeosciences www.biogeosciences.net 1726-4170 1726-4189 7 6 2010 10.5194/bg-7-1809-2010 http://www.biogeosciences.net/7/1809/2010/ http://www.biogeosciences.net/7/1809/2010/bg-7-1809-2010.html http://www.biogeosciences.net/7/1809/2010/bg-7-1809-2010.pdf 1809 1832 2010-06-01 The leaf-level emission factor of volatile isoprenoids: caveats, model algorithms, response shapes and scaling Ü. Niinemets ylo.niinemets@emu.ee R. K. Monson A. Arneth P. Ciccioli J. Kesselmeier U. Kuhn S. M. Noe J. Peñuelas M. Staudt Institute of Agricultural and Environmental Sciences, Estonian University of Life Sciences, Kreutzwaldi 1, Tartu, 51014, Estonia Department of Ecology and Evolutionary Biology and Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80309-0334, USA Division of Physical Geography and Ecosystem Analysis, Lund University, Sölvegatan 12, Lund, 22362, Sweden Istituto di Metodologie Chimiche del CNR, Area della Ricerca di Roma 1, Monterotondo Scalo, 00016, Italy Max Planck Institute for Chemistry, Biogeochemistry Department, Joh.-J.-Becher Weg 27, Mainz, 55128, Germany Federal Research Station Agroscope Reckenholz-Taenikon, ART, Zuerich, Switzerland Global Ecology Unit CSIC-CEAB-CREAF, Facultat de Ciències, Universitat Autònoma de Barcelona, Bellaterra, 08193, Spain Centre d'Ecologie Fonctionnelle et Evolutive (CEFE-CNRS), 1919, Route de Mende, Montpellier cedex 5, 34293, France In models of plant volatile isoprenoid emissions, the instantaneous compound emission rate typically scales with the plant's emission potential under specified environmental conditions, also called as the emission factor, <i>E</i>_S. In the most widely employed plant isoprenoid emission models, the algorithms developed by Guenther and colleagues (1991, 1993), instantaneous variation of the steady-state emission rate is described as the product of <i>E</i>_S and light and temperature response functions. When these models are employed in the atmospheric chemistry modeling community, species-specific <i>E</i>_S values and parameter values defining the instantaneous response curves are often taken as initially defined. In the current review, we argue that <i>E</i>_S as a characteristic used in the models importantly depends on our understanding of which environmental factors affect isoprenoid emissions, and consequently need standardization during experimental <i>E</i>_S determinations. In particular, there is now increasing consensus that in addition to variations in light and temperature, alterations in atmospheric and/or within-leaf CO₂ concentrations may need to be included in the emission models. Furthermore, we demonstrate that for less volatile isoprenoids, mono- and sesquiterpenes, the emissions are often jointly controlled by the compound synthesis and volatility. Because of these combined biochemical and physico-chemical drivers, specification of <i>E</i>_S as a constant value is incapable of describing instantaneous emissions within the sole assumptions of fluctuating light and temperature as used in the standard algorithms. The definition of <i>E</i>_S also varies depending on the degree of aggregation of <i>E</i>_S values in different parameterization schemes (leaf- vs. canopy- or region-scale, species vs. plant functional type levels) and various aggregated <i>E</i>_S schemes are not compatible for different integration models. 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