Regional scale risk assessment of ozone and forests

The ozone exposure risk for vegetation in Lombardy (Northern Italy) has been assessed by the AOT40 exposure index, based on data taken from the existing local monitoring networks covering 5 growing seasons (1994 to 1998). One-square kilometer exposure maps were obtained by using geostatistic techniques (ordinary kriging) followed by an altitude detrendization of measurement’s temporal series to account for the domain’s large topographic heterogeneity. Risk areas (Level I maps) were identified using a GIS and overlaying the ozone-criticallevel exceedance maps on the distribution maps of forests and sensitive species. The critical ozone exposure level of 10 000 ppb h, adopted by UN/ECE protocols, is exceeded over the whole Lombardy Territory over the 6-month growing season. The highest risk areas are the northwest pre-alpine and alpine belt, directly impacted by the photo-oxidant plume generated by the Milan urban area. Difficulties met in creating a proper Level II risk assessment for forests in mountain areas have been bypassed by comparing ozone exposures with summer climate features. Soil water availability was assumed not to be a significant modifying factor in the mountains of this region because of the frequent summer rains, whereas the opposite held true for wind ventilation which is generally weak. Field surveys have reported foliar injuries attributable to ozone in different species of forest trees and shrubs, which provide further evidence of potentially phytotoxic ozone levels