Review on “Tracing the origins of stratospheric ozone intrusions and their impacts on Central and Eastern China: a long-term study of direct and indirect pathways” by Meng et al.

General comments

This paper focuses on the impact of stratospheric intrusions (SI) on tropospheric ozone variations in China concerning the origins and transport pathways of these ozone-rich stratospheric airmasses. They rely on pure trajectory simulations using the FLEXPART model, through which a forward trajectory history of air parcels reaching the lower troposphere is obtained. Overall, the paper is well organized and written and fit the scope of the ACP journal. However, there are some issues that need to be addressed before it turned into a publication, especially in the context of the trajectory simulations and their validation and interpretation. More details of the trajectory settings, sensitivity analysis and validation should be provided in this paper. It would be better that the authors take more time to address these problems and the characteristics of the two SI types. So, I would like to suggest a major revision and hope to get a high-quality paper concerning the importance of SI in tropospheric ozone.

Major issues

1. The trajectory simulations rely heavily on the stratospheric tracer definition. In this paper, a PV value of 1.5 PVU was chosen to represent the dynamic tropopause and the stratospheric origin of airmasses. In my opinion, this value cannot sufficiently resolve the STE processes over such a broad region, though several researchers have used a threshold of 1.5 or 1.8 PVU. For example, tropopause height is higher in warm months than in cold months, but the seasonal variations in tropopause height in South China derived from the 1.5-PVU dynamic tropopause are nearly constant throughout the year. In Fig. 7, the vertical distribution of stratospheric air parcels reaching the middle troposphere is southward placed than those reaching the PBL and ground surface. Such a distribution is opposite to the mechanism of stratospheric airmass transportation through tropopause folding where the ozone-rich airmasses are transported southward and downward (as shown in Fig. 10). In this perspective, the choice of 1.5 PVU seems to be not appropriate and may include airmasses in the upper troposphere (not the stratosphere) reaching the lower troposphere. I suggest that the authors should address the performances of different PV thresholds to track stratospheric origins through trial-and-error analysis. Sensitivity tests are necessary to be performed in order to obtain reliable results over China.

2. The validation of the filtered stratospheric trajectories and the associated vertical profiles of ozone are essentially important to infer the key features of those SI events. However, a detailed validation is lacking in this manuscript, which may greatly impair the accuracy and robustness of the results. Given the rareness of SI reaching the lower troposphere (Fig. 5a), it is warranted to examine the responses of surface ozone and other pollutants to the SI inferred from the forward trajectory simulations, providing direct observational evidence.

3. The authors claimed that they provided a “long-term” result of SI reaching middle-to-low troposphere in China. Only intrusions from May to August in 2019 are provided in this paper, while actually SI peak in later winter and early spring. The expression of “long-term” is not appropriate and may be biased because only several months in 2019 were included. A complete examination throughout a year, at least in 2019, are essentially important to reveal the seasonality of SI.

4. Similar to Point 3, the authors only considered the Eurasia region in their domain-filling settings. As known, stratospheric air can travel thousands of kilometers during their downward descending. Thus, SI that initiate outside the in Eurasia are likely to be missed and hence the stratospheric impact is underestimated to some extent. Please address this point and assess the sensitivity of the domain setting.

5. The direct and indirect SI are divided according to the descending time. It is not clear how the descending time is determined. Is it the time of stratospheric air reaching the middle troposphere or the lower troposphere? From Fig. 3, I guess that the authors calculated the time when stratospheric air reaches the middle troposphere, since there are no direct intrusion cases reaching the boundary layer. Often, the direct intrusions refer to those descending rapidly and vigorously into the lower troposphere even the ground level, inducing strong perturbations to chemical species such as ozone, since the dilution and mixing of stratospheric air is reduced in shorter period. However, in this paper, the direct intrusions are weaker than the indirect ones, which is opposite to previous studies, e.g., the Fig. 4 of Cristofanelli et al. (2006). In this angle, the authors should rethink the way to separate direct and indirect SI in their trajectory samples.

6. The TOST ozone reanalysis profile data, with a horizontal resolution 5 degrees, rely heavily on the available ozonesonde observations. These data are not sufficient and appropriate for regional-scale analysis of vertical ozone distributions, especially over those ozonesonde-lacking regions.

7. Line268-270: In Fig. 3, there is no contribution of SI to ozone below 2 km, and hence the calculation of near-surface ozone associated with direct SI (green boxes in Fig. 6) is totally mismatched. Moreover, even for those indirect intrusions (red boxes in Fig. 6), highest value was not seen in LP. I suggest that the authors should pay more attention to the interpretation of the results.

8. There are too few samples of direct SI in Fig. 6 to perform a statistical analysis.

9. Fig.9-10. More ground-based observations, either chemical composition or meteorological data, are necessary to validate the occurrence of this SI.

Reference:

Cristofanelli, P., Bonasoni, P., Tositti, L., Bonafè, U., Calzolari, F., Evangelisti, F., Sandrini, S., and Stohl, A.: A 6-year analysis of stratospheric intrusions and their influence on ozone at Mt. Cimone (2165 m above sea level), J. Geophys. Res., 111, D03306, https://doi.org/10.1029/2005JD006553, 2006.

This study investigates the impacts of stratospheric ozone intrusion on tropospheric ozone in central and eastern China, using air quality observations, global ozone (re)analysis data, and customized Lagrangian simulations. The results are interesting and the manuscript is well organized  in general, however, there are still some problems need to be improved before a consideration for publication.

Abstract L16: SIs

Data: Why the former ERA-Interim data was used in this study, instead of the more advanced ERA5 or MERRA2?

Model configuration: Why the year 2019 was chosen to simulate? As I know, the SI’s contribution to tropospheric ozone in China is significantly larger than normal years. Anyway, to discuss the SI’s contribution with simulations only in one year will lead to large uncertainties due to the interannual differences.

It is better to describe briefly about the domain-filling technology here.

Figure 1: The colormap used in this figure should be updated to make the signals on a-b and k-m clearer.

Figure 2: The vertical ozone gradients also represent the position of the tropopause. Seen from this figure, the 1.5 PVU may not a proper choice for all the selected regions.

3.2: Why the 2 and 10 days forward trajectories can indicate the direct and indirect transports? The reason should be well explained and the sensitivity of the results to the selected length should be tested.

Figure 3 and 4: How the contributions of stratospheric ozone directly/indirectly transported to the troposphere were estimated? As an important result of the paper, detailed description of the methos should be clearly described.

Figure 7: It is better to mark the location of each region.

P12 L305: Is the CSAm1 related to the southern Asia High or collocated with the subtropical jet?

Figure 9: The colormap should be improved for this figure to make the information more visable.