Chemical and meteorological drivers of ozone extremes during the 2019 UK summer heatwaves

Andrea Mazzeo , Ryan Hossaini

Heatwave-associated air pollution episodes are increasingly recognized as a public health threat. Studies have observed a positive correlation between extreme temperature and extreme surface ozone concentrations. However, the underpinning chemical and physical drivers and their interconnections are not well quantified and may vary across regions and individual heatwave events. The 2019 UK summer offers a compelling case study, as three distinct heatwave events occurred, with record-breaking temperatures and widespread ozone exceedances above health-relevant thresholds. Here, we use the WRF-Chem chemistry-transport model to investigate the drivers of elevated UK ozone during the three heatwaves. We show WRF-Chem simulates observed ozone and temperature well over the full summer, including heatwave-associated ozone enhancements. Net chemical ozone production increased markedly over much of the UK, including enhancements of +4.2 ppb/hr relative to non-heatwave periods in the South-East of England (reaching 8.20 ppb/hr at midday). Large enhancements in biogenic isoprene emissions (up to 200% in some areas) during the heatwaves are shown to broadly correlate with formaldehyde enhancements observed from TROPOMI satellite data. Meteorologically, the heatwaves were characterised by low ventilation of the planetary boundary layer, affecting pollutant dispersal. Sensitivity analysis also demonstrates that the import of ozone and its precursors from outside of the UK was a key factor in elevating daily maximum 8-hour running mean (MDA8) ozone above the health-relevant 100 µg/m3 threshold. We conclude that as the frequency and intensity of heatwaves increases due to climate change, more extensive local monitoring of isoprene and other natural emissions is needed to characterise changes, and that efforts to reduce European-wide ozone precursor emissions are required to lower extreme surface ozone over the UK.