**5. Conclusions**

To reach the carbon peak and carbon neutrality targets, reducing carbon emissions and reducing particulate matter concentration are important ways to improve atmospheric quality. China has introduced a policy of coal to gas and electricity to combat frequent smog and severe air pollution caused by coal-fired heating. Based on the panel data of nineteen cities in The Beijing–Tianjin–Hebei region and surrounding areas from 2016 to 2020, the PSM-DID model and the time-varying DID model were used to test the effectiveness and dynamic sustainability of the policy of coal to gas and electricity on air pollution. The empirical analysis results show that (1) the propensity score matching method (PSM) can be used to match the influencing factors of air quality in nineteen cities in Beijing–Tianjin– Hebei and surrounding areas and to overcome the endogenous problem among variables. It provides new evidence to study the relationship between the policy of changing coal to gas and electricity and the influencing factors of air pollution, and it enriches the research methods of the existing literature on the influence of coal to gas and electricity. (2) On the whole, the fitting effect of DID model is at a normal level; that is, the air pollution

levels in the Beijing–Tianjin–Hebei urban agglomeration that participated in the Coal to Gas and Electricity policy did decrease, indicating that the coal to gas power policy has indeed achieved the expected effect. One possible reason is that the government plays a leading role in air pollution; the coal to gas policy involves the coordinated development of the Beijing–Tianjin–Hebei city, indicating that the choice of pilot cities is obviously representative. (3) Regarding the significance level, the policy had a relatively large impact on SO2 and PM10, while the effects on PM2.5 and CO were relatively weak. This result shows the urgent need for more accurate improvement plans for various pollutants. The weak reduction capacity of PM2.5 may be due to the fact that PM2.5 had shown a downward trend before the implementation of the Coal to Gas and Electricity policy in the Beijing– Tianjin–Hebei region; other air joint prevention and control policies had shown a more powerful governance effect. The weak impact on carbon emissions suggests that other precise solutions for carbon emissions are needed to achieve dual-carbon targets. (4) The dynamic sustainability results of the policy of changing coal to gas and electricity show that the virtual variables in the first two weeks have a significant negative effect, proving that the policy has a strong impact in the initial stage. The effect of the last two weeks was very modest, with a significant negative effect only in SO2. This shows that the effect of the policy at the end stage or near the end stage of the implementation is greatly reduced and is far less obvious than at the early stage of the policy. It shows that the one-size-fits all policy does not have a good sustainable effect, which may be due to the smaller subsidy intensity in the later stage, the increased cost for residents using gas and electricity, or the inherent living habits resulting in the smaller policy intensity.

In the end, given the current situation in Beijing, Tianjin, and Hebei, we propose the following recommendations to improve air quality: First, during the implementation of the policy, the air quality in Beijing, Tianjin, and Hebei was improved, and specific action arrangements have been planned for the future. However, there are different difficulties and effects between regions, Hebei is regarded as a key area to undertake the transfer industries of Beijing, which can hardly bear the impact of the coal ban on its economy, so as to Tianjin, while the air pollution in Hebei and Tianjin will also infect Beijing. So, establishing a regional joint prevention-and-control mechanism for air pollution; building one picture of the responsibility of replacing coal with gas and electricity in Beijing, Tianjin, and Hebei; promoting the upgrading of regional industrial structure; and balancing the regional central budget are the most crucial missions that should be completed in the following days. Second, although the Coal to Gas and Electricity policy has reduced the AQI index, it only had a large impact on SO2 and PM10, and was relatively weak on PM2.5 and CO. Therefore, to increase investment in R&D, strategies for reducing individual pollutants are urgently needed. The policy of coal to gas and electricity has had little impact on carbon emissions. To achieve the dual-carbon target, we need to cooperate to achieve a precise emission reduction strategy. Last but not least, we find air quality continues to deteriorate after the end of the policy, which means that a policy that imposes uniformity in all cases is not sustainable. In reality, the implementation of policy is often difficult to accomplish in one stroke. Not only does the policy need to prevent the emergence of a gas shortage under the law of objective economics and slowly promote the policy according to the local economic situation and residents' income and habits, but it also must enhance service guarantees, strive for more subsidies and more alternative energy like biomass fuel, wind energy, and solar energy for the local people. So, we will continue to pay attention to that in our research.

**Author Contributions:** Conceptualization, J.Z. and L.G.; methodology, J.Z.; software, J.Z. and L.G.; validation, Z.D.; formal analysis, J.Z. and Z.D.; investigation, J.Z.; resources, Z.D.; data curation, Z.D.; writing—original draft preparation, J.Z.; writing—review and editing, J.Z., W.W. and L.G.; visualization, W.W.; supervision, J.Z. and Y.T.; project administration, W.W.; funding acquisition, W.W. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research is funded by the Annual Project of Philosophy and Social Sciences of Henan Province (Grant No.2021CJJ135); Science and Technology Innovation Project of Beijing Forestry University (Grant No.2021SPS01); The Social Science Foundation Cultivation Program of Yanshan University (Grant No.118/0370044).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** All data used during the study are available from the corresponding author by request.

**Conflicts of Interest:** The authors declare no conflict of interest.

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