Chemical and meteorological drivers of Ozone extremes during the heatwave of summer 2019 in the UK.

Heatwaves, defined as prolonged periods of excessively hot weather, are increasingly recognized as a public threat due to their significant impact on human health and the environment. While the intrinsic impact of heatwaves on public health and the environment is well-recognised, a growing interest is emerging in the impact that these extreme weather events have on air pollution. Moreover, climate change has increased the frequency and intensity of these events raising concern about the potential impact on air pollution, and in particular on O₃ levels. While the positive correlation between O₃ and Temperature has been extensively analysed worldwide and in different time scales, the underpinning processes, their occurrence and their combination are still unclear. The summer of 2019 in the UK offers a compelling case study, as three distinct heatwave events occurred, characterized by record-breaking temperatures, including a peak of 38.7°C, and widespread O₃ exceedances across both urban and rural areas. This study uses the WRF-Chem chemistry-transport model to simulate the entire summer of 2019, focusing on heatwave events and their influence on O₃ formation. The analysis identifies key meteorological and chemical drivers, such as temperature, VOC emissions, and stagnation of air masses, which exacerbate O₃ levels during the heat waves. The results indicate that the increased presence of biogenic isoprene played a significant role in O₃ formation, particularly during heatwaves, with urban areas experiencing higher peaks due to a combination of temperature, emissions, and weak air movement.  With the frequency and intensity of heatwaves increasing due to climate change, our findings underscore the importance of considering both anthropogenic and natural emissions in future air quality management to protect public health.