Atmospheric Environment 40 (2006) S152–S165 Process-based modelling of isoprenoid emissions from evergreen leaves of Quercus ilex (L.) R. Grote a,Ã , S. Mayrhofer a , R.J. Fischbach a , R. Steinbrecher a , M. Staudt b , J.-P. Schnitzler a a Institut fu ¨ r Meteorologie und Klimaforschung Bereich Atmospha ¨ rische Umweltforschung (IMK-IFU), Forschungszentrum Karlsruhe GmbH, Kreuzeckbahnstr. 19, D-82467 Garmisch-Partenkirchen, Germany b Centre d’Ecologie Fonctionelle et Evolutive du CNRS, 1919 Route de Mende, 34293 Montpellier CEDEX 5, France Received 20 June 2005; received in revised form 23 September 2005; accepted 10 October 2005 Abstract Monoterpenes play an important role in regulating the trace gas composition of the lower troposphere. Therefore, realistic estimates of the daily as well as seasonal variations of monoterpene emission source strength on the Earth surface are required. Monoterpenes are emitted by Holm oak (Quercus ilex L.) and other species lacking specific foliar terpene storage structures and their development is dependent on light and temperature. In the present work we describe a process-based emission model taking into account the physiological/phenological state of Holm oak leaves and biochemical processes leading to the formation of monoterpenes. The model ‘seasonal isoprenoid synthase model–biochemical isoprenoid biosynthesis model’ (SIM–BIM2) is developed based on a previous version which was used to simulate isoprene emissions from deciduous oaks. The current model considers additional enzymatic reactions in Holm oak chloroplasts that lead to the formation of monoterpenes. The comparison of simulated and measured biochemical properties as well as emission rates displayed that the ability of the model to dynamically adjust monoterpene biosynthesis capacity by modulating the amount of monoterpene synthase activities in dependence of the weather pattern led to realistic simulations of light-dependent monoterpene emission rates. Differences to simulation results obtained by a widely used alternative model [Guenther, A.B., Zimmerman, P.R., Harley, P.C., Monson, R.K., Fall, R., 1993. Isoprene and monoterpene emission rate variability—model evaluations and sensitivity analyses. Journal of Geophysical Research 98, 12609–12617] are discussed. r 2006 Elsevier Ltd. All rights reserved. Keywords: Monoterpene emission rates; Seasonality; SIM–BIM2; Monoterpene synthases; Model development; Quercus ilex 1. Introduction Terrestrial plants, in particular trees, are esti- mated to emit about 500 Tg of carbon (C) yr 1 as isoprene, and an equivalent amount of C as monoterpenes into the atmosphere (Guenther et al., 1995). Together with oxidizing compounds (ozone, NO x , HO  and NO 3 -radical) isoprenoids form secondary products and fine particles (Bie- senthal et al., 1997; Derwent et al., 1998). By decreasing HO  radical levels in the atmosphere isoprenoids indirectly contribute to global warming ARTICLE IN PRESS www.elsevier.com/locate/atmosenv 1352-2310/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2005.10.071 Ã Corresponding author. Tel.: +49 8821 183124; fax: +49 8821 73573. E-mail address: ruediger.grote@imk.fzk.de (R. Grote).