**3. Literature Review and the Mechanism**

As mentioned above, the BTH region is facing great environmental pressure in the process of rapid urbanization [12]. Pollution in this area is closely related to energy consumption [13], especially for energy consumption in industries and transportation [14]. Besides, though some of the cities in the BTH region have entered the postindustrial stage, none of them has crossed the turning point of the environmental Kuznets curve [15]. Because the BTH coordinated development strategy concerned in this paper focuses on environmental regulation, industrial upgrading, and transportation, a literature review is conducted accordingly.

First, for environmental regulation in the BTH region, recently, many studies made quantitative or qualitative analyses on its impact on energy consumption and gave some suggestions for further improvement. For example, by using a computable general equilibrium (CGE), Li et al. [16] found that, over the entire BTH area, the environmental policies could generate an average annual loss of 1.4% of gross regional product growth in the action plan scenario and of 2.3% in the enhanced action plan scenario. These results suggest that more joint measures are needed to promote energy conservation and emission reduction in the BTH region. For determining how policies and regulations support energy efficiency measure proliferation, Wang et al. [17] conducted a questionnaire survey of the enterprises in the BTH industrial transfer and found that the importance of awareness and investment priorities is great, so policy makers need to pay more attention to economic and legal tools and to appropriately increase supervision and punishment. By quantitatively analyzing the environmental policies of the BTH region, Zhang et al. [18] found that environmental regulations have direct and indirect spatial effects on industrial structures across regions and that environmental regulations have a long-term promotion effect on industrial structure upgrade and energy conservation.

Second, for industrial upgrades in the BTH region, the literature has made analyses about the effects of industrial upgrading on energy consumption for different industries or different regions. For example, by calculating the total factor energy efficiency (TFEE) of 27 industries in the BTH region, Li et al. [19] found that, because of the technological spillover effect from Beijing enterprise, Hebei has the highest total factor average energy efficiency in the production and supply of electric power and heat power industry, and Tianjin has the highest total factor average energy efficiency in the manufacture of raw chemical materials and chemical products and in the smelting and processing of ferrous metals. By measuring the energy rebound effect of industrial enterprises in the BTH region from 1996 to 2015, Li et al. [20] found that Hebei faces greater pressure to attain high energy conservation and emission reduction goals in the future than the other regions. By quantitatively analyzing the delinking indicators on industry growth and environmental pressures in the BTH region from 1996 to 2010, Wang and Yang [21] found that the carbon emissions in the BTH region were dominated by the secondary industry, which accounted for about 80% of total carbon emissions, and that the energy structure and energy intensity made significant contributions to the industrial decoupling progress. By quantitatively measuring the total factor carbon emission performance (TFCP) and the carbon emission mitigation potential (CMP) of 39 industrial sectors in the BTH region, Wang et al. [22] found that the manufacture of the nonmetallic mineral product sector and the production and distribution of electric power and heat power sector belong to the low TFCP–high CMP quadrant.

Third, for transportation in the BTH region, some studies analyzed the energy consumption and related pollutions brought by transportation development in the BTH region. For example, by using the panel data from 1995 to 2016, Guo and Meng [23] found that transportation energy intensity and the economic effect is the main factor increasing carbon dioxide emissions. They also found that the contributing factors to the carbon dioxide emission reduction in the transport sector are the energy structure effect, the freight turnover of unit industrial output effect, and the industrialization effect. By analyzing vehicular emission trends from road vehicles of the BTH region in the period 1999–2010, Lang et al. [24] found that, due to the rapid development of freight traffic, emissions of NOX and PM10 kept increasing in Tianjin and Hebei. By analyzing the driving forces behind carbon emission of the BTH region from 2005 to 2013, Zhu and Li [25] found that the effect of energy intensity from the transportation sector always plays a negative role in Tianjin and Hebei but a positive one in Beijing.

How did the BTH coordinated development strategy affect industrial energy consumption in this region? The above studies focused on the environment and on sustainable development in this region. However, there is no direct literature on the comprehensive policy influence of the BTH coordinated development strategy on industrial energy intensity and related pollution emission intensities. As mentioned above, the strategy focuses on environmental regulation, industrial upgrade, and transportation; thus, we will analyze the relations between energy consumption and environmental regulations, transportation, and industrial upgrade combined with the existing literature.

The first one is environmental regulation. Environmental regulation will bring lower energy consumption for enterprises, and it can be divided into two situations based on the policy effects. One is that simple environmental regulations force a reduction of energy consumption, which may not be conducive to economic development. In the short term, it may harm the interests of enterprises, which will resist regulations, leading to weak policy effects. In the long term, it may force enterprises to upgrade their technology, but economic loss cannot be recovered [26]. The second one is to encourage industrial upgrading and technological progress while implementing environmental regulation, thus reducing energy consumption while maintaining corporate interests. Under the BTH coordinated development strategy, governments will break regional administrative restrictions; promote a revolution in energy production and consumption; promote green, circular, and low-carbon development; strengthen environmental protection and governance; and expand regional ecological space. The focus will be on the joint prevention and control of environmental pollution, the strengthening of environmental pollution control, the implementation of clean water action, the development of a circular economy and ecological protection, and plans to build some national parks and forest parks around the capital to actively tackle climate change. At present, Beijing, Tianjin, and Hebei have in-depth cooperation in various aspects such as improving coordination mechanisms, unified planning, unified legislation, unified standards, and joint law enforcement, and the results of collaborative environmental governance have been remarkable. For example, air quality in the three places has further improved, and the annual average concentration of PM2.5 has shown a downward trend. The PM2.5 average concentration in the BTH region has dropped by 46% compared with 2014, of which 85.9 μg/m<sup>3</sup> in Beijing dropped to 42 μg/m<sup>3</sup> in 2019, a decrease of 51%. Besides, in terms of ecological and environmental protection, China has supported ecological restoration in Zhangjiakou and Chengde and formulated an implementation plan for afforestation from 2015 to 2017 in the northwest ecological conservation area.

The second one is industrial transfer and upgrade. Under the BTH coordinated development strategy, the industries to be relocated and transferred from Beijing are mainly energy-intensive industries; tertiary industries like logistics bases and wholesale markets; public service sectors such as education, medical care, or training institutions; as well as some company headquarters. The transfer principle is a combination of the role of government and the role of market. In addition to Beijing and Tianjin, the central core function area of the "four areas" also includes Baoding and Langfang in Hebei province. These two cities will focus on the relocation and transfer of noncapital functions of Beijing and will take the lead in realizing interconnected development. It is worth noting that many regional cooperation projects have been completed, such as Beijing Automotive Industry Corporation Huanghua plant project, Beijing Hyundai Motor Cangzhou plant project, Sinopec Beijing Yanshan Branch Caofeidian ten-million-ton refining project, and Tianjin Binhai–Zhongguancun

science park, etc. Industrial transfer and upgrade can have an impact on energy consumption through several channels. First, enterprises transferred from Beijing, such as high-end manufacturing and the internet industry, will promote industrial upgrading and have a strong technology spillover effect, thus reducing energy consumption in surrounding cities. Second, the transfer of industries such as higher education and science and technology R&D sectors, as well as preferential policies to attract talents in Xiong'an New Area, can improve the level of human capital and can reduce energy consumption. Third, the establishment of the Hebei Free Trade Zone in 2019, the first large-scale free trade zone in China covering the three provincial-level administrative regions of Beijing, Tianjin, and Hebei, will drive the transformation and upgrade of Hebei's manufacturing industry and will reduce energy consumption by participating in international competition. Fourth, taking advantage of the industrial advantages of Beijing and Tianjin, Hebei province accelerated the formation of many industrial clusters, such as Zhangjiakou renewable energy demonstration area, Beidaihe life health industry innovation demonstration zone, Shijiazhuang high-end biomedical industry base, and Baoding and Cangzhou auto equipment manufacturing industry. Until the end of 2019, Hebei province altogether accepted 9773 enterprises transferring from Beijing and Tianjin. Actually, quite a few studies have quantitatively measured the reduction of energy consumption and emissions caused by industrial transfer and upgrade. For example, Li et al. [27] found that rationalization and upgrade of manufacturing structures mitigate CO2 emissions during the period of 2003–2014; Wang at al. [28] demonstrated that advancement of the industrial structure has increased carbon emission efficiency in China between 2003 and 2016; Zhu et al. [29] found that the increase in the proportion of secondary industries would increase energy-related smog pollutions in 73 key cities of China during 2013–2017; and Zhang et al. [30] found that the industrial structure can reduce energy-related haze pollution through the path of rationalization in China from 2006 to 2016, etc.

The third one is transportation. The BTH region will construct the Beijing–Tianjin–Hebei intercity transportation network based on rail transit and will build the world-class aviation hub as well as the port group of Tianjin–Hebei, thus improving regional integrated transportation. First, traffic construction and development in the short term will undoubtedly increase travel demand and energy consumption, but in the long term, the effects of transportation on economic growth are pronounced. Transportation integration can lead to talent flow and economic exchange, can promote industrial-technological progress and industrial upgrade, and then can significantly improve GDP, thus reducing energy consumption per unit GDP. Second, transportation integration can shorten logistics distance and can reduce energy consumption. For example, parcels before, especially air express parcels, generally arrived in Beijing first and then transferred to Tianjin and Hebei by truck. High-speed highway construction shortens logistics distance and reduces energy consumption. Third, the green transportation system in the BTH region, including promotion of new energy public transportation and encouragement of the purchase of new energy vehicles, will also reduce fossil energy consumption. Fourth, the integrated transportation planning of the BTH region is more efficient than decentralized planning and the energy efficiency of the transportation sector is also higher. Until 2020, the BTH region initially built a one-hour economic circle connected by high-speed rail and highways, which is about an hour from Beijing to surrounding cities. By 2030, an intercity railway network with four verticals, four horizontals, and one ring will be formed. The "four verticals" include Beijing–Baoding–Shijiazhuang–Xingtai–Handan, Beijing–New Airport–Hengshui, Cangzhou–Tianji–Chengde, and Qinhuangdao–Caofeidian–Binhai–Huanghua Port. The "four horizontals" include Beijing–Tianjin–Yujiabao, Beijing–Tongzhou–Tangshan–Caofeidian, Tianjin–Bazhou–Baoding, and Shijiazhuang–Cangzhou–Huanghua Port. One ring is the intercity transportation ring of Beijing.

Based on the above analyses, the BTH coordinated development strategy may have a big impact on industrial energy and pollution intensities, but the direction is uncertain and needs to be empirically tested. This paper is different from previous studies: first, this study 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, it 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; and third, it 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 to make policy implications based on different pollution sources under the context of the BTH coordinated development strategy.
