Quantifying temporal trends in ground level ozone concentration in the UK.

There is a high interest in quantifying temporal trends in surface ozone concentrations as they serve to quantify the impacts of the anthropogenic precursor reductions and to assess the effects of emission control strategies. In this paper ozone trends for nearly 2 decades (1993 to 2011) at both rural and urban sites have been analysed, using ground level ozone data from 5 urban and 15 rural sites, which are part of the UK AURN. This study analyses ozone trends at various percentiles, in addition to traditional mean trends using quantile regression, TheilSen function, and changepoint analysis. Ozone trends show significant variability at different statistical metrics (e.g., mean, median, maximum and selected quantiles). Maximum trends were negative, whereas median and mean trends were positive during the study period (1993-2011) at both rural and urban sites. Urban and rural trends show different rates of change and indicate that urban decrement (the difference in ozone concentration between rural and urban areas) has been decreasing over the period. Ozone trends were negative during the last 8 years (2004-2011), which could have been caused by the stabilisation of NOx concentration during this period. Furthermore, 3 changepoints were detected in the temporal trend using Pruned Exact Linear Time (PELT) search algorithm, which provides further insight into the ozone temporal trends.