Trends in ozone concentration distributions in the UK since 1990: Local, regional and global influences

Data from the UK automatic monitoring network have been analysed to investigate average trends in ozone concentrations, concentration distributions and exceedences of the public information threshold (180 μg m−3) at 13 rural sites (over the period 1990-2006) and at five urban sites (over the period 1993-2006) throughout the UK. The data indicate that the observations at a given location in the UK can be influenced by a combination of global (hemispheric)-, regional- and local-scale effects. As a result, the observed trend in the ozone metrics is determined from the net trend of these three influences, the relative contributions of which can vary both spatially and temporally. Specifically, the data reflect the following three major influences: (i) a gradual increase in the hemispheric baseline ozone concentration resulting from global-scale effects, thereby influencing the baseline levels of ozone brought into the UK from the Atlantic Ocean; (ii) substantial short-term elevations in ozone concentrations during summertime episodes, resulting from the formation of additional ozone from regional-scale photochemical processing of emitted VOC and NOx over north-west Europe, with such events tending to be more frequent and intense towards the south and east of the UK. On average, the severity of such events has progressively decreased since about 1990, as a result of EU controls of anthropogenic VOC and NOx emissions; and (iii) local-scale removal of ozone by direct reaction with emitted NO has gradually decreased, as a result of the control of NOx emissions. This has resulted in a general increasing influence on ozone concentrations since about 1990, which is most apparent at urban sites, where NOx emissions are higher, but also influences the observations at the majority of rural locations.