Alkenes such as 2-methyl-1,3-butadiene (isoprene) and the monoterpenes are emitted by various types of plants. Isoprene is the predominant VOC emitted by forest species such as poplar, oak, willow, sycamore and eucalyptus making up as much as 80 %. Several studies in recent years have shown isoprene to be a key VOC in tropospheric chemistry on both regional and global scales. Emissions appear to be higher than that of any other non-methane hydrocarbon. Terpenes are a classification of biogenic compounds; monoterpenes are a subgroup of terpenes some of which have structures which be thought as being loosely based on that of two isoprene molecules.
Monoterpenes can be either acyclic (e.g. myrcene), or contain one ring (d-limonene) or two ring structures (alpha and beta-pinene, 3-carene). The emissions from deciduous (hardwood/broadleaf) trees such as oak, poplar, aspens and willows comprise mainly of isoprene, whereas coniferous (softwood) woodland such as pine trees, cedars, redwood and firs emit predominantly monoterpene. There are several species which emit both isoprene and monoterpenes such as spruce and eucalyptus. Eucalyptus and pine are the trees which we studied in Portugal during the AEROBIC '96 Campaign.
Isoprene is almost exclusively emitted during daylight hours, hence reaction with hydroxyl radicals will be the major degradation path for isoprene in the troposphere. It has been estimated that the lifetime of isoprene in the troposphere is only a few hours, this is due to the relatively fast reaction between isoprene and the hydroxyl radical.
