Elevated foehn exacerbates surface ozone pollution in summer Beijing

Abstract. While several studies have evaluated the impact of shallow foehn on air pollution, the effects of elevated foehn on O₃ pollution remain poorly understood. Here, we investigate the role of elevated foehn in summer O₃ pollution in Beijing through detailed case analysis and a long-term climatological evaluation. The case study reveals that elevated foehn exacerbates next-day O₃ pollution through three primary mechanisms: first, by increasing boundary layer temperature, thereby enhancing photochemical O₃ formation; second, by reducing the residual/boundary layer height, thereby inhibiting vertical diffusion of pollutants; and third, by slowing boundary layer winds, thereby suppressing horizontal dispersion. A ten-year climatological evaluation of 54 identified elevated foehn events strongly supports these mechanisms. On average, these events led to a post-foehn afternoon boundary layer temperature increase exceeding 3 °C, an afternoon boundary layer height reduction of more than 100 m, and a decrease in afternoon boundary layer wind speed of more than 1.0 m s^-1 compared to the pre-foehn days. Consequently, 87 % of elevated foehn events were associated with a worsening of O₃ pollution. Post-foehn daily maximum 8-hour average O₃ concentrations frequently surpassed the national pollution threshold (160 μg m^-3), with an average increase of 20 %–60 % (varying by site and higher in urban areas) compared to preceding days. These results demonstrate a robust and deterministic exacerbating effect of elevated foehn on surface O₃ pollution, suggesting that elevated foehn can serve as a reliable meteorological precursor for O₃ pollution warnings in summer Beijing.