**1. Introduction**

Industrial pollution is one of the world's most serious environmental problems. In China, industries are the largest source of pollution, especially for air contamination. Any form of pollution that can be directly traced back to industrial practice is called industrial pollution. Most of the pollution on Earth can be traced back to some kind of industry [1]. Therefore, the reduction of industrial pollution has always been critical in dealing with environmental degradation around the world. Most developing countries facing the rapid growth of industrial pollution found it to be a serious problem that must be brought under control. However, major environmental disasters caused by industrial accidents still occur sometimes in developing countries.

As for China, environmental pollution mainly comes from traditional heavy industries, especially power plants, petrochemical industries, metal smelting, and machinery manufacturing, which produce a large volume of sulfur dioxide, dust, and wastewater. In the face of unprecedented economic and industrial growth levels, China rapidly developed its system of environmental governance [2,3]. It can be used to reform the highly inefficient and strictly regulated energy market [4] and to improve

the regulation of industrial pollution [5]. The reasons may be the two-level pressures including domestic economic growth and international governance status [6]. In recent years, China's governing system resulted in a lot of environmental policy implementations from the central government [7], which have had a noticeable effect on improving the environment [8]. Although great achievements in some fields like solar photo voltaic, wind energy, and nuclear power have been made [9], China still faces many energy-related challenges including air pollution, urbanization, and climate change in the future [10].

The Beijing–Tianjin–Hebei (BTH) region is China's "capital economic circle", including Beijing, Tianjin, and 11 prefecture-level cities of Hebei province. It is one of the three major urban agglomerations and has the strongest industrial base in China, accounting for about 2% of the national land area, 10% of the national GDP, and 8% of the total population. However, there is a huge economic disparity in this region. In terms of per capita GDP in 2019, the per capita GDPs of Beijing, Tianjin, and Hebei were USD 23,600, USD 13,100, and USD 6,800, respectively. In terms of the industrial structure in 2019, Beijing's tertiary industry accounted for 83% and the secondary industry accounted for only 16%, while Tianjin and Hebei's secondary industries accounted for 36% and 41%, respectively. In terms of urbanization rate in 2019, the urbanization rates of Beijing, Tianjin, and Hebei were 86%, 83%, and 56% respectively.

The BTH region in which environmental degradation is prominent in China has been dominated by some energy-intensive industries like mining and steelmaking sectors, especially in Hebei province. The common problem of the three areas is the consumption of fossil energy and the three wastes it brings, including industrial wastewater, sulfur dioxide, and dust emissions, which are the main causes of environmental problems in this region. The three wastes are related to fossil energy because sulfur dioxide and dust emission are mainly produced by fossil energy consumption, while industrial wastewater, although not entirely produced by fossil energy, is overall strongly correlated with energy consumption.

In February 2014, Chinese President Xi Jinping proposed the coordinated development strategy of the BTH region, which is a national strategy in China. The strategy is to build a new capital economic circle and to promote innovation of regional development. First, it is to explore a mode of optimal development of this densely populated and economically intensive area. Second, it will take the lead in making breakthroughs in three key areas: ecological and environmental regulation, industrial upgrading and transfer, and the integration of the BTH transport sector. It is expected to ease the increasing pressure on resources and environment, to accelerate the transformation of economic development patterns, and to promote balanced development for the BTH region.

The Outline of the Coordinated Development Plan in the BTH region approved by the Chinese government in 2015 made a clear definition of the core functions of Beijing: the national political center, the cultural center, the international communication center, and the scientific and technological innovation center. The industries that do not accord with these certain orientations should be gradually relocated to Tianjin and Hebei province. Tianjin is identified as an important city but subordinate to the core city of Beijing, and thus, it is meant to take on complementary functions [11]. By 2019, nearly 10,000 enterprises had been relocated out of Beijing, most of which were high-end manufacturing and high-tech service industries. These industries may not fit Beijing's new orientation, but for Tianjin and Hebei, they can promote local industrial upgrade through the relocation. It is worth mentioning that the "transfer" is not transferring polluting industries. During the transfer process, enterprises should upgrade or switch to sustainable and environmentally friendly businesses. If they do not want to relocate, they can upgrade locally following the new and more stringent environmental protection standards. Therefore, the feature of this strategy is that it seeks to address environmental and social issues in the process of coordinated development while also laying the foundation for sustainable development through environmental constraints.

In recent years, some major projects involving the coordinated development of the BTH region have been launched. For example, Beijing and Hebei will jointly organize the 2022 Beijing Winter Olympics, Beijing and Tianjin are fully supporting the development of the Hebei Xiong'an New Area, and the Chinese government has been supporting the orderly transfer and sharing of Beijing's scientific and technological innovation resources and high-quality public service resources in this region. More importantly, Beijing, Tianjin, and Hebei were together dedicated to deepening joint prevention and control of the increasing regional environmental degradation.

From the perspective of sustainable development, what impacts this strategy has had on fossil energy intensity and related pollution intensities in the BTH region are absent in the literature. To fill this gap, this paper is therefore intended to empirically study the roles of the BTH coordinated development strategy in industrial energy and pollution intensities based on the difference-in-difference (DID) method and the quantile DID method. The results of this study will help clarify the potential problems in the implementation of this strategy and provide some empirical evidence and policy implications for green and sustainable development in this region. The implementation of the BTH coordinated development strategy provides an effective quasi-natural experiment in assessing the impact of specific policy events on industrial energy intensity and related pollution intensities.

The study has made three contributions. First, it investigates the comprehensive impact of the BTH coordinate development strategy on industrial energy and pollution intensities, and the estimation results are robust based on a number of statistical tests. Second, this study applies the quantile DID method to evaluate the heterogeneous impact of the strategy on industrial energy intensity in the BTH region at different quantile levels of industrial energy intensity. Third, this study uses the data of industrial energy pollutions to estimate the different impacts of the strategy on the environment among different energy pollution sources. Such heterogeneous effects can help make policy implications based on different pollution sources under the context of the strategy.

There are three findings from this study. First, based on the DID method results, China's BTH coordinated development strategy, on average, tends to restrain regional industrial energy intensity in this region. Second, based on the quantile DID method results, the BTH coordinated development strategy tends to restrain the industrial energy intensity in lower quantile level cities but tends to increase the industrial energy intensity in higher quantile level cities. Third, the impacts of the BTH coordinated development strategy on the environment vary among three industrial energy-related pollution sources including wastewater, sulfur dioxide, and dust emissions.

In Section 2, we introduce the study areas and provide some basic statistics about this region. In Section 3, we conduct a literature review and analyze the mechanism between the BTH coordinated development strategy and industrial energy consumption. Section 4 estimates a regression model for industrial energy intensity based on the DID method. Section 5 assesses the impact of the BTH coordinated development strategy on industrial energy and pollution intensity empirically. Finally, Section 6 provides the conclusion, policy implications, and discussions.
