BIOGENIC EMISSIONS IN EUROPE .1. ESTIMATES AND UNCERTAINTIES

Several biogenic volatile organic compound (VOC) emission algorithms have been used, together with meteorological data from the EMEP MSC-W ozone model, to generate estimates of the emissions of isoprene from European forests and agricultural crops over several summer periods. The most up-to-date estimate combines the recently updated isoprene emission factors from the United States with available knowledge of European tree species and emission factors. In some cases these European emission factors are significantly different from their U.S. equivalents because of differences in the tree species represented within a forest classification, especially with regard to spruce genera and Mediterranean oak genera. The new estimates have resulted in an approximate factor of 2-3 increase in isoprene emissions from northern Europe but a factor of 2 decrease in isoprene estimates for southern Europe. Overall, European isoprene emissions are estimated to be about 4000 kt C yr(-1), approximately 50-100% greater than previous estimates. Preliminary estimates are also made of the emissions of the so-called OVOC (other VOC) from forests and of soil NOx emissions. All of these estimates of biogenic emissions are subject to considerable uncertainty, not least because of a lack of knowledge of the species coverage in most European countries and of the appropriate emission factors which should be applied. Factors of 5-10 uncertainty are not unlikely for episodic ozone calculations. The implications of these uncertainties for the results of control strategy evaluations for rural ozone in Europe are assessed in a companion paper.