Major comments:

1. For the introduction, we have adjusted it as required and added new content.

“1 Introduction

In China, mainly over the region east of 100°E and south of 40°N (Tie et al., 2009), PM_2.5 (particulate matter with an aerodynamic diameter equal to or less than 2.5 μm) has become the primary air pollutant (Wang, et al., 2017). Therefore，in September 2013, the Chinese government launched the China's first air pollution control action plan – ‘The Airborne Pollution Prevention and Control Action Plan (2013-2017)’ (State Council of the People’s Republic of China, 2013). By 2017, 64% of China’s cities are still suffering from the air pollution, especially Beijing-Tianjin-Hebei region and surrounding areas (Wang et al., 2019; Miao et al., 2019). Then, in July 2018, the Chinese government launched the second three-year action plan for air pollution control, the "blue sky defense plan", which demonstrates China's firm determination and new measures for air pollution control (State Council of the People’s Republic of China, 2018). After the implementation of air pollution control action plans, the air quality in many regions and cities in China has been significantly improved.

       Anthropogenic pollutant emissions and unfavorable meteorological conditions are commonly regarded as two key factors for haze pollution(Ding and Liu, 2014; Yim et al., 2014; Zhang et al., 2015). Air pollutants mainly come from the surface emission sources, and most of the pollutants are transported from the surface to the atmosphere through the atmospheric boundary layer (ABL) (Quan et al., 2020). Structural characteristics of ABL are the key meteorological conditions which influences the formation and maintenance of heavy pollution episodes (Wang et al., 2015; Cheng et al., 2016; Wang et al., 2016; Tang et al., 2016; Wang et al., 2019).

       Most of the previous studies focused on exploring the impact on the heavy air pollution in Eastern China (EC) for the meteorological conditions in ABL. However, the thermodynamic and dynamic structures of troposphere can affect the meteorological conditions in ABL (Cai et al., 2020). The convection and diffusion in the ABL were suppressed by a relatively stable structure of ‘warm cover’ in the middle troposphere, leading to the ABL height decreases, which were favourable for the formation and persistence of heavily polluted weather (Quan et al.,2013; Wang et al.,2015; Cai et al., 2020).

This study investigated whether the thermodynamic structure of the troposphere and its intensity changes can be used as a "strong warning signal" for the changes of PM_2.5 concentration in heavily polluted weathers, and whether this strong signal exists in the time scales of seasonal, interannual and interdecadal. In order to explore the interaction between the free troposphere and the ABL and the impact on the heavy air pollution in Eastern China (EC), this study extended the meteorological conditions for heavy air pollution from the boundary layer to the middle troposphere. We identify a precursory ‘strong signals’ hidden in the free troposphere for frequent haze pollution in winter in EC. ”

2. Meteorological conditions in February 2014 were worse than that in January 2013. In February 2014，a rarely persistent air pollution weather process occurred in central and eastern China, this process had caused severe air pollution in more than 50 cities, with an impact area of 2.07 million square kilometers. Among the 25 regional ultra-long pollution events (≥6 days) during the winters of 2012–2018, the ‘most serious’ pollution events during February 18–27 was long lasting, contained heavy pollution, produced low visibility, and had a broad impact on northern China. In the Beijing area during February 20–26, the regional average PM_2.5 concentration was continued to exceed the ‘most-serious’ pollution level, and with a peak value of up to 456 µg m^-3.

3. Due to the limited space of the article, some quantitative analysis will be given in the future, such as the contribution of each meteorological element to polluted weather. This study focused on exploring whether the thermodynamic structure of the troposphere and its intensity changes can be used as a "strong warning signal" for the changes of PM_2.5 concentration in heavily polluted weathers, and whether this strong signal exists in the time scales of seasonal, interannual and interdecadal.

Minor comments:

1. Line 26: "In addition to". We have adjusted it as required.

1. Line 43: Delete "with excessive concentrations of PM _2.5". We have adjusted it as required.

1. Line 87: the North China Plain. Please check it throughout the manuscript. We have removed it.

1. Line 88-89: Change to "for the long-lasting and heavy haze pollution". This statement needs to be rephrased. "sulfur-dioxide pollutants” is not appropriate. We have adjusted it as required.

1. Line 98: What do you mean by "long heavy air pollution "? We have adjusted it as required, "persistent heavy air pollution".

1. The labels in the contour plot in Fig.3-4 are overlaid and need to be optimized. We have adjusted it as required.

1. All the abbreviations should be defined for the first time. Please check throughout the article. We have adjusted it as required.

We are grateful to the referee for the encouraging comments and careful reviews which helped to improve the quality of our paper. In the followings we quoted each review question in the square brackets and presented our response after each paragraph.

Review Comment: Anthropogenic pollutant emissions and unfavorable meteorological conditions are commonly regarded as two key factors for haze pollution. This study investigated whether the structure of atmospheric thermodynamics in the troposphere and its intensity variation could act as a ‘strong forewarning signal’ for surface PM_2.5 concentration variations. It is a very interesting topic and significant for air pollution control. However, I think the current analysis is not sufficient to support the conclusion. Thus, some quantitative estimation and mechanisms illustration is suggested before publication. The detailed reason and suggestions are listed below.

Reply: Thank you for the encouraging comments.

1. Fig 1 and Fig 2 demonstrate the key role of “warm cover” in the haze process. However, the illustration of the connection of “warm cover” with the Tibetan Plateau has lacked. The “warm cover” shown in Figure S1 is below 900 hPa, which is similar to the height of the PBL top. It results in a very stable ABL and further improves the surface PM_2.5 concentration. However, the “warm cover” induced by Tibetan Plateau is about 600 hPa, which is 4 km. The mechanisms of the impact of “warm cover” in such altitude on PBL is needed to be illustrated in the manuscript.

   Reply: Many thanks for the referee’s discussion. We agree with the suggestion. Following this comment, the content of Section 3.3 have adjusted (lines 188-193 and Figure 5) with following sentences:

   “The concept of interdecadal variations of the tropospheric ‘warm cover’ has been proposed in this work. Under the background of climate change, it is worth considering whether the variational tendency of the structure of the plateau’s heat source induces variations of the tropospheric thermal structure in downstream areas of the Plateau, leading to the interdecadal variations of the frequency of haze events seen in Eastern China since the 21th century. Thermal anomalies of the TP also play an important role in the variations of the frequency of haze events in EC apart from the anthropogenic pollutant emission related to the rapid industrialization of China. The observational and modeling studies have demonstrated that the interannual variations in the thermal forcing of TP are positively correlated with the incidences of wintertime haze over EC (Xu et al., 2016). The TP induced changes in atmospheric circulation, increasing atmospheric stability and driving frequent haze events in EC (Xu et al., 2016). In this study, the data analysis concerning the interannual variations of the TP’s apparent heat source and air temperature in wintertime at the TP with the altitudes above 3000 meters showed that since the 1960s the heat source in areas vulnerable to TP climate change strengthen continuously as the surface temperature increased (Fig. 5a). Furthermore, the TP’s apparent heat and air temperature of the middle troposphere over EC presented the significant positive, which is similar to ‘warm cover’ structure characteristic (Fig. 5b). Therefore, we considered that the ‘warm cover’ change in the middle troposphere over EC was closely related to TP’s apparent heat and the surface temperature. The TP induced changes in thermodynamic structure of atmospheric provided favorable climatic backgrounds driving air pollution events in EC.”

   We have accordingly cited the following article in the revised manuscript:

   Xu, X. D., Zhao, T. L., Liu, F., Gong, S. L., Kristovich, D., Lu, C., Guo, Y., Cheng, X. H, Wang, Y. J., and Ding, G.: Climate modulation of the Tibetan Plateau on haze in China, Atmos. Chem. Phys., 16, 1365–1375, https://doi.org/10.5194/acp-16-1365-2016, 2016.

2. Fig 3 shows that the “upper warming and bottom cooling” vertical structure in Autumn and Winter favors haze formation. It is interesting. However, the analysis is on the seasonal scale and did not directly support the haze formation on the daily scale.

   Reply: Based on the study of the Great Smog of London in 1952 and the heavy pollution of Beijing in February 2014, it is found that the abnormal ‘warm cover’ in the middle troposphere connected to both severe air pollution events ( Sect. 3.1 and in Sect. 3.2). This study attempts to explore that whether such the similar structural characteristic of thermodynamic structure, i.e. the abnormal ‘warm cover’ in the middle troposphere, also exist from the perspective of different time scales, we have further analyzed the PM_2.5 concentrations and the number of haze days with seasonal and interdecadal variations of the thermodynamic structures in the atmosphere. We found that the thermal vertical structure of atmospheric showed a ‘upper warming and bottom cooling’ vertical structure under heavy pollution conditions. The concept of the tropospheric ‘warm cover’ has been confirmed on the seasonal and climatic scale.

   Following this comment, we have added these in the revised Abstract (line 32) as follows:

   "The anomalous structure of the troposphere’s “warm cover” not only exist in heavy haze pollution on the daily scale, but also provide seasonal and interdecadal ‘strong signals’ for frequently occurring regional haze pollution."

3. Fig 4 compares the interdecadal change of thermal structure in EC and eastern TP with haze days. However, the anthropogenic emissions in EC have increased several times from 1961 to 2018. It is hard to attribute the increase of haze days to the change of TP thermal structure.

   Reply: The pollutant emission with high intensity was the internal cause of frequent air pollution in EC, and the adverse weather conditions were often the key ‘inducement’ for the accumulation of air pollutants in the atmosphere. Although the variation trends of air pollution in EC depend on the air pollutant emissions,, the air pollution, including its intensity and duration, are closely related to meteorological conditions. Thus, we analyzed the anomalous thermodynamic structure (air temperature anomalies) from the perspective of meteorological conditions, in order to reveal the influence difference of the background field of meteorological conditions on regional atmospheric dispersion conditions. Furthermore, we found that the TP induced changes in thermodynamic structure of atmospheric provided favorable climatic backgrounds driving air pollution events in EC.

4. I guess the impact of TP thermal structure on air pollution may cover a large part of EC. Maybe large-scale haze processes could be tried.

   Reply: Yes, the observational and modeling studies have demonstrated that the interannual variations in the thermal forcing of TP are positively correlated with the incidences of wintertime haze over EC (Xu et al., 2016). The TP induced changes in atmospheric circulation, increasing atmospheric stability and driving large-scale haze events in EC (Xu et al., 2016).

   References:

   Xu, X. D., Zhao, T. L., Liu, F., Gong, S. L., Kristovich, D., Lu, C., Guo, Y., Cheng, X. H, Wang, Y. J., and Ding, G.: Climate modulation of the Tibetan Plateau on haze in China, Atmos. Chem. Phys., 16, 1365–1375, https://doi.org/10.5194/acp-16-1365-2016, 2016.