| 23 Mar 2026
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.
Overview:
Liao et al. conducts a detailed case study, accompanied by a longitudinal observational assessment, of lofted foehn impacts on surface ozone through boundary layer modifications in Beijing during the summer. In doing so, the authors utilize various observational datasets including ground-monitoring and lidar ozone measurements, radar wind profiles, and radiosondes. The authors focus on ozone and winds before and after a single lofted foehn event (occurring overnight on August 29) that doesn’t result in surface warming as typical foehn events. The authors also assess back trajectories and conduct WRF-chem simulations to confirm elevated foehn impacts on residual layer warming. Finally, a climatological analysis of radiosonde data from 2015 – 2024 reveals that elevated foehn winds caused 87% of identified nocturnal residual layer warming (n = 63, 6.85% of summer nights) with elevated surface ozone concentrations the next day after every one of the events. The work mainly concludes elevated foehn increase surface ozone concentrations through three mechanisms: 1) increasing boundary layer temperature leading to increased ozone production; 2) reduction of residual/boundary layer height causing a subsequent decrease in ozone vertical diffusion; and 3) slowing boundary layer winds resulting in reduction of horizontal dispersion.
Major comments:
The paper effectively uses observations to characterize foehn influence on the nocturnal residual layer and traces those impacts through to the following day’s convective boundary layer. The associated layer changes (increased temperature, reduced boundary layer height, and weaker winds) are well documented in both observations and model simulations. However, the physical justifications for mechanisms 1 and 3 warrant further support and considerations of nuances.
Regarding mechanism 1, the authors use changes in ozone, which is a function of production and loss, both of which can be independently impacted by temperature. While elevated temperatures are known to enhance ozone production, temperature also modulates chemical loss pathways and this is not addressed. Reduced PAN transport into the study region (due reduction in vertical mixing as mentioned in mechanism 2), could suppress ozone loss and independently contribute to the observed surface ozone increases. The current attribution of higher ozone concentrations solely to enhanced production is incomplete. The paper would benefit from discussion of temperature-sensitive loss processes, or at minimum acknowledgement of them as a potential contributing factor.
Mechanism 3 is more directly supported by the observations, though a brief clarification would strengthen it specifically, whether the reduced boundary layer winds are an elevated foehn-driven signal or a signature of the converging synoptic systems (specified on page 6) within which the foehn occurs.
Specific comments:
Figure 7c seems to have the middle hollow marker (~06, 2024-08-30) cut off. Also, please include time zone (e.g. local time vs UTC) as well as for other time relevant plots (e.g. Fig 6).
Figure 10b is missing longitude and latitude coordinates.
Figure 11b seems to have a cut off marker in the fourth box-whisker.
For all site-relevant figures, it would be helpful to delineate the specified sites (CXT, SDZ, and YQ); e.g. Figure 3b and c and Figure 12.