QUANTIFYING THE FINE-SCALE (1KMX1KM) EXPOSURE AND EFFECTS OF OZONE .1. METHODOLOGY AND APPLICATION FOR EFFECTS ON FORESTS

Surface ozone concentrations show very large local variability. This, in rural areas, is largely a consequence of the degree to which the su rface air is decoupled from the bulk of the atmosphere by nocturnal an d winter stability effects and of the aerodynamic roughness and topogr aphy of the countryside. Procedures have been developed to simulate th ese effects and have been used with O-3 data from a regional monitorin g network to quantify exposure of vegetation to mean and peak O-3 conc entrations. The index of exposure to potentially phytotoxic O-3 concen trations is the AOT40 (accumulated exposure over a threshold of 40 ppb ) expressed in ppb.h. The accumulated exposure of crop and forest surf aces to ozone is shown to increase approximately linearly with altitud e and along a gradient: from 3000 ppb.h in north west Scotland to 7000 ppb.h in the south of England. The area of UK forest which exceeds th e critical level for ozone effects (10(4) ppb.h above 40 ppb) totals 2 .0 x 10(4) km(2) and represents 47% of afforested land. The areas affe cted include most of southern and central England and Wales and large plantation forests of Northumberland and the Scottish borders.