Investigation of an oxidant-based methodology for AOT40 exposure assessment in the UK

The levels of ozone (O3) at a given location in the UK have been reported to be determined by a combination of global (northern hemispheric), regional and local-scale effects. These effects therefore also potentially influence values of the O3 exposure metric, AOT40 (the accumulated ozone exposure over a threshold of 40 ppb). A corresponding background oxidant metric, AOXBT40, is defined, which quantifies the value of AOT40 corrected for the effects of local oxidant sources and the chemical coupling of O3 with emitted NOx (i.e. effectively a remote rural background AOT40 value). Data from the UK automatic monitoring network have been analysed to determine values of AOT40 and AOXBT40 at 39 UK sites for the example year of 2009. The spatial variation of AOXBT40 over the UK is consistent with contributions deriving both directly from the northern hemispheric baseline O3 level and from episodic events during which the baseline is supplemented by regional-scale oxidant formation; and the approximate relative importance of these inputs is estimated. The fractional decrement in AOT40 (AOT40/AOXBT40), shows a well-defined dependence on the average NOx level for the hours that contribute to AOXBT40, consistent with the expected impact of the local-scale chemical coupling of O3 and NOx. The AOXBT40 concept can therefore be used to help characterise the factors that control AOT40 on different spatial scales, and its potential applications in empirical O3 exposure modelling activities and model evaluation are briefly discussed.