Critical Levels of Ozone Over the United Kingdom: Mapping Aggregate Exceedances Over Moderate to High Thresholds

The critical level for ozone, above which it has a detectable effect on bio-logical targets, is potentially to be set by the United Nations Economic Commission for Europe at 300 nL·h/L hours per annum over 40 nL/L. It is therefore important to determine the aggregate exccedancc over 40 nL/L throughout the United Kingdom. Over most of the UK, ozone concentrations are unknown so we rely on our understanding of the atmospheric processes and on the statistical properties of ozone concentrations to interpolate between monitoring sites. This paper describes the application of statistical models derived for storm severity data to the ozone data for the United Kingdom. Aggregate excess distributions were fitted to data from all rural monitoring sites using a Weibull model with a 40 nL/L threshold. At this threshold the scale parameter has a spatial interpretation, but, with higher thresholds, there were problems with missing data and small scale spatial effects were not detected. The approach appears successful for all except very large aggregate exceedances which deviate from the Weibull predictions.