European surface ozone in the extreme summer 2003

Abstract. Measurements of ozone and other species from the European EMEP network in 2003 are presented. The European summer of 2003 was exceptionally warm and the surface ozone data for central Europe show the highest values since the end of the 1980s. The concentrations were particularly high in June and August 2003. In this paper we argue that a number of positive feedback effects between the weather conditions and ozone contributed to the elevated surface ozone. Firstly, direct measurements of isoprene as well as the increased temperature and solar radiation indicate that biogenic emissions in Europe were increased during summer 2003 with a potential for enhanced ozone formation. Secondly, we show that the anticyclonic conditions during the ozone episodes were accompanied by an extended residence time of air parcels in the atmospheric boundary layer, a low total ozone column and a reduced cloud cover, all favouring ozone formation. Thirdly, based on the Lagrangian dispersion model FLEXPART, we show that is very likely that extensive forest fires on the Iberian Peninsula, resulting from the drought and heat, contributed to the peak ozone values observed in North Europe in August. Additionally, forest fires in Siberia probably lead to an elevated background level of ozone and CO at northern latitudes during summer 2003 thereby increasing the level the peak ozone episodes sat on. Lastly, and most important, the heat wave presumably lead to less efficient ozone dry deposition due to stomata closure of the plants under drought stress. Due to climate change, situations like this may occur at a higher frequency in the future and may gradually overshadow the effect of reduced emissions from anthropogenic sources of VOC and NO_x. This scenario also holds for secondary PM.