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.

This study starts with a comprehensive delineation of an elevated foehn-induced O3 air pollution event in Beijing. Such a delineation includes the spatial-temporal distribution of surface O3 in Beijing, the synoptic background, the evolution of vertical air structure, the backward trajectory analysis, cross-section progression, and climatological status. The manuscript is clearly structured and well-written, with figures that are clear and easy to interpret. I would recommend its publication at ACP with the following issues addressed.

1. The authors pointed out the distinct impacts of shallow and elevated foehn on O3 air pollution. Could the authors please add somewhere an illustration of the different progression and structure between shallow and elevated foehn?
2. One factor for post-foehn O3 pollution is increased temperature. The author also mentioned in some places in the manuscript that foehn could decrease PM2.5. Therefore, I presume that foehn has different or even opposite impacts on O3 and PM2.5, since the former is more sensitive to temperature while the latter is more affected by diffusion and transport processes. Could the authors please add some clarifications about the possible different impacts of foehn on PM2.5 and O3?
3. So the foehn is a more localized meteorological impact on O3, as compared to synoptic-scale meteorology. The authors mentioned in the end to explore the contribution of elevated foehn to O3 generation under different synoptic backgrounds. Could the authors here illustrate more of this point for the events introduced in this paper? This could be important since there are numerous publications showing the importance of synoptic-scale weather on regional O3 pollution in the NCP region.