A temperature dependent extreme value analysis of UK surface ozone, 1980–2019

Abstract

Elevated surface ozone during heatwaves and recent hot summers raises concerns over the potential for climate change to exacerbate ozone air pollution in the UK. In this paper, we perform a robust statistical analysis of four decades worth of daily maximum 8-h (MDA8) ozone measurements from the UK's Automated Urban and Rural Network. A temperature dependent extreme value model is developed to characterise the magnitude and frequency of extreme ozone events and to determine probabilities for ozone exceeding health thresholds, as defined in the UK's air quality index. Our model is found to describe the tails of the MDA8 ozone distributions well at all 119 monitoring sites considered. For the decade 2010–2019, we estimate that >90% of sites have a 1-year MDA8 ozone return level greater than the ‘moderate’ ozone threshold of 100 μg/m3. We also find that 33% of sites are currently expected to breach the UK government's national air quality objective that MDA8 ozone should not exceed 100 μg/m3 more than ten times per year. We estimate the present overall probability of MDA8 ozone exceeding 100 μg/m3 on a given day to be between <0.1% and 5.4%, depending on site, with averages of 2.7% (rural) and 1.6% (urban background locations). Our analysis reveals a significant decline over time in the likelihood of the UK experiencing extreme ozone episodes, with 1-year return levels in the 1980s now roughly comparable to 10-year return levels in the present. Similarly, probabilities of MDA8 ozone exceeding 100 μg/m3 have decreased by a factor of ∼2–6 since the 1980s in some locations. However, our results also highlight a strong positive temperature dependence to the risk of ozone exceedances. In consequence, increasingly hot summers due to climate change may offset some of these gains.