**6. Conclusions, Policy Implications, and Discussions**

This study investigates the different impacts of the coordinated development of the BTH region on industrial energy and pollution intensities based on the DID method and theh quantile DID method. The panel data cover industrial energy consumption and three wastes, which are industrial wastewater, sulfur dioxide, and dust emissions, from all 13 cities in the BTH region and 17 cities in Henan Province for the period 2007–2017.

The study finds that, first, based on the DID method results, the dummy variable is significantly negative at the one percent level, indicating that China's BTH coordinated development strategy on average tends to restrain regional industrial energy intensity in the BTH region. Second, based on the quantile DID method results, the BTH coordinated development strategy tends to restrain industrial energy intensity in lower quantile level (0.1–0.4) cities; still, it tends to promote industrial energy intensity in higher quantile level (0.7–0.9) cities. Third, the impacts of BTH coordinated development strategy on the environment vary among industrial wastewater, sulfur dioxide, and dust emissions. The BTH coordinated development strategy tends to decrease industrial wastewater and sulfur dioxide emission intensities because the coefficients are statistically significant at the 10 percent level and the 5 percent level but have no significant impact on dust emission intensity though the coefficient is positive.

The findings of this study can generate some policy implications. First, China should pay more attention to the green transfer and clean energy use of industries from Beijing instead of transferring outdated production capacities that may increase industrial fossil energy consumption and related pollution emissions. More attention should be given to cities that have a high energy intensity. Second, it is necessary to control dust emission during the integrated development of transportation by enforcing strict standards and by establishing an ecological traffic system, especially in the construction of expressways and high-speed railways. Third, on the premise of respecting the rules of market economy and the rules of fairness in world trade, relevant subsidy policies can be formulated with the goal of promoting environmental protection talents and of upgrading energy conservation and emission reduction technologies in Hebei, so as to change the serious unsustainability and environmental pollution problems. Fourth, standards for wastewater and sulfur dioxide emissions from some industries such as oil refining, steel making, and metallurgy can be tightened more, and governments can continue making efforts to replace coal with electricity or gas for heating and can extend the action to a wider region to reduce industrial sulfur dioxide and dust emissions, like some small towns and rural areas in Hebei province. Fifth, except for stimulating upgrade of regional industrial structures to service industry domination, the governments of some Hebei cities, like Tangshan, Handan, and Zhangjiakou, should provide more funds for energy conservation

and emission reduction-related research. Sixth, the government should strengthen Beijing's role in driving industrial and technological upgrade in neighboring cities. For example, the new Free Trade Zone in Beijing established in 2020 will focus on service trade and will develop high-end industries such as the digital economy; Beijing ranked first in scientific research level on the Nature Index for five consecutive years. All of these advantages can be put to good use in the coordinated development of the BTH region. Seventh, relying on some major projects like the Xiong'an New District, Hebei free trade area, and the Beijing–Zhangjiakou 2022 Olympic Winter Games, Hebei cities should accurately position their regional comparative advantages, such as existing resources, talents, and industrial bases, to undertake related high-end industrial relocation from Beijing; to absorb the radiation effect to accelerate the upgrade of local industry and technology, thus forming the ability to retain talent and capital; and then further to own the ability to attract talent and capital, fundamentally changing the Beijing siphon phenomenon. However, it is worth noting that the central government's decision to build the Xiong'an New Area in Hebei has downgraded the national importance of Tianjin's Binhai New Area, which is the second state-level pilot zone in China after the Pudong district of Shanghai [11]. Relevant policies should be formulated to balance the relationship between the two state-level new areas so that they can complement each other. Eighth, we should specifically develop the four regional central cities, including Shijiazhuang, Tangshan, Baoding, and Handan, which will be models of environmental protection, industrial upgrade, and green transportation for neighboring node cities. Ninth, strengthening the integration of customs clearance in the BTH region can shorten customs clearance time and can reduce transportation cost of goods imported from Beijing via air transportation and those imported from Tianjin via sea transportation. At last, as the current environmental policy loses the interest of enterprises, the participation enthusiasm of industrial transfer and green upgrade is not high. The coordinated environmental regulation should be strengthened to reduce energy consumption while maintaining corporate interests, like establishing a carbon emission trading market in Hebei Province.

Although the above research gives some findings and policy implications on this topic, it should be acknowledged that, due to the limitation of the latest data, for which the time series is till 2017, whether the effects of the BTH coordinated development strategy on industrial energy and related pollutant emission intensities will continue, to this day, is unknown. Except for the DID method, other empirical techniques and data if available can be used to test some of the major aspects of the BTH coordinated development strategy, such as the impact of investment in the Xiong'an new area on energy consumption and related pollutions. If more relevant data can be obtained to form a larger control group, the DID results based on propensity score matching (PSM) can be also used to test the robustness.

**Author Contributions:** Conceptualization, C.H. and Y.W.; methodology, X.S.; software, Y.W. and X.S.; validation, B.H. and X.S.; formal analysis, Y.W. and C.H.; data curation, C.H.; writing—original draft preparation, C.H. and Y.W.; writing—review and editing, C.H. and X.S.; supervision, B.H. and X.S.; project administration, B.H.; funding acquisition, B.H. All authors have read and agreed to the published version of the manuscript.

**Funding:** We are grateful for the financial support provided by the National Social Science Foundation Major Project of China (No. 19ZDA100), by the National Development and Reform Commission of China (No. 221100004; 240200004), by International Poverty Reduction Center in China (No. 240200003), by the Natural Science Foundation of China (No. 71828401), by the Social Science Fund of Beijing Education Commission (No. SM201910011011; PXM2019\_014213\_000007), and by the Beijing Social Science Foundation (No. 17SRC012).

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