Investigating ozone build-up in the east of England during the July 2015 heat wave

Ozone (O₃) precursors controls have reduced peak episodes in the UK, yet some regions still exceed the 50 ppbv (8-h) limit under certain meteorological conditions. This study uses O₃-tagging within WRF-Chem to quantify NOₓ contributions to near-surface O₃ across the East of England during a short heatwave (30 June-5 July 2015), where O₃ peaked at 116 ppbv. Despite underestimating peak O₃, the model effectively captures the episode's evolution and spatial extent (MB: -2 ppbv, r: 0.78), depicting O₃ increases on the hottest days and their decline. During the peak O₃ event, Western Europe contributed ∼60 % of daily mean O₃, while domestic NOₓ emissions accounted for only ∼4.6 %. South-easterly winds transported O₃ and precursors from BENELUX, France, Germany, the North Sea, and the English Channel. Near-surface wind convergence (-0.00002 s^-1) with weaker westerlies redirected domestic O₃ to the central UK, intensifying continental influence. Conversely, south-south-westerly flows amplified domestic O₃ contributions. Analysis of the processes contributing to O₃ build-up highlights that transboundary transport played a significant role during the peak event. Boundary layer growth contributed by entraining O₃-rich air (up to 10.3 ppb h^-1 across the boundary layer top), along with precursors from the residual layer, facilitating their downward mixing to surface levels. This process set the stage for in-situ photochemical O₃ production, which peaked at midday, contributing to a total daily O₃ production of 42 ppbv on average across the region. Our findings underscore the necessity of coordinated international efforts to manage transboundary pollution, alongside targeted local measures to reduce O₃ precursor emissions.