Declining ozone exposure of European vegetation under climate change and reduced precursor emissions

Abstract. The impacts of changes in ozone precursor emissions as well as climate change on the future ozone exposure of the vegetation in Europe were investigated. The ozone exposure is expressed as AOT40 (Accumulated exposure Over a Threshold of 40 ppb O₃) as well as POD_Y (Phytotoxic Ozone Dose above a threshold Y). A new method is suggested to express how the length of the period during the year when coniferous and evergreen trees are sensitive to ozone might be affected by climate change. Ozone precursor emission changes from the RCP4.5 scenario were combined with climate simulations based on the IPCC SRES A1B scenario and used as input to the Eulerian Chemistry Transport Model MATCH from which projections of ozone concentrations were derived. The ozone exposure of vegetation over Europe expressed as AOT40 was projected to be substantially reduced between the periods 1990–2009 and 2040–2059 to levels which are well below critical levels used for vegetation in the EU directive 2008/50/EC as well as for crops and forests used in the LRTAP convention, despite that the future climate resulted in prolonged yearly ozone sensitive periods. The reduction in AOT40 was mainly driven by the emission reductions, not changes in the climate. For the toxicologically more relevant POD₁ index the projected reductions were smaller, but still significant. The values for POD₁ for the time period 2040–2059 were not projected to decrease to levels which are below critical levels for forest trees, represented by Norway spruce. This study shows that substantial reductions of ozone precursor emissions have the potential to strongly reduce the future risk for ozone effects on the European vegetation, even if concurrent climate change promotes ozone formation.