**2. The Study Areas**

As shown in Figure 1, Beijing is located in the north of China and north of the BTH region. It borders Tianjin in the east and some cities of Hebei in the rest. It is the capital of the People's Republic of China, a municipality directly under China's central government, a national central city, and a megacity in the world. The total area of Beijing is 16,410 square kilometers, and it has 16 districts under its jurisdiction. By the end of 2019, there were 21.536 million permanent residents and 18.5 million urban residents, representing an urbanization rate of 86.6 percent. The GDP of Beijing in 2019 was RMB 3537.13 billion yuan, and the per capita GDP was RMB 164,000 yuan. The proportions of primary, secondary, and tertiary industries are 0.3%, 16.2%, and 83.5%, respectively. Besides, the energy consumption per unit of GDP in 2019 was about 0.25 tons of standard coal per RMB 10,000 yuan, which was the lowest in China.

**Figure 1.** Locations of the Beijing–Tianjin–Hebei (BTH) region and Henan Province.

Tianjin, a municipality directly under the central government of China, is a national central city, a megacity, and the largest port city in north China. It borders Beijing in the west and some cities of Hebei in the rest. The total area of Tianjin is 11,966 square kilometers with 16 districts under its jurisdiction. By the end of 2019, there were 15.618 million permanent residents and 13.038 million urban residents, representing an urbanization rate of 83.5 percent. The GDP of Tianjin in 2019 was RMB 1410.43 billion yuan, and the per capita GDP was RMB 90,000 yuan. The proportions of primary, secondary, and tertiary industries were 1.3%, 35.2%, and 63.5%, respectively. Besides, the energy consumption per unit of GDP in 2019 was about 0.41 tons of standard coal per RMB 10,000 yuan. Now, the ecological civilization has also become an important element for the urban development of Tianjin under the coordinated development strategy of the BTH region [11].

Hebei province is located in the north of China, bordering the Bohai Sea in the east and Beijing and Tianjin in the inner ring. The total area of Hebei province is 188,800 km2, and it has 11 prefecture-level cities under its jurisdiction, including Shijiazhuang, Tangshan, Qinhuangdao, Baoding, Handan, Xingtai, Zhangjiakou, Chengde, Cangzhou, Hengshui, and Langfang, with Shijiazhuang as its provincial capital. By the end of 2019, there were 75.920 million permanent residents and the permanent resident urbanization rate was 57.6 percent. The GDP of Hebei in 2019 was RMB 3510.45 billion yuan, and the per capita GDP was RMB 46,000 yuan. The proportions of primary, secondary, and tertiary industries were 10.3%, 39.7%, and 50.0%, respectively. Besides, the energy consumption per unit of GDP in 2019 was about 0.84 tons of standard coal per RMB 10,000 yuan, which was much higher than the national average of 0.51 tons of standard coal equivalent per RMB 10,000 yuan. Hebei province is rich in mineral resources because 156 kinds of mineral resources have been discovered, 39 of which are among the top 5 in China. It has formed a mining economic system with metallurgy, coal, building materials, and petrochemicals. In April 2017, the central government decided to establish Xiong'an New Area in Hebei province. In August 2019, the state council established a new China (Hebei) Pilot Free Trade Zone.

The control group, Henan Province, is located in central China, south of Hebei province, with a total area of 167,000 km2. It has jurisdiction over 17 prefecture-level cities, including Zhengzhou, Kaifeng, Luoyang, Pingdingshan, Anyang, Hebi, Xinxiang, Jiaozuo, Puyang, Xuchang, Luohe, Sanmenxia, Shangqiu, Zhoukou, Zhumadian, Nanyang, and Xinyang, with Zhengzhou as the provincial capital and Jiyuan as a county-level city directly under the jurisdiction of Henan Province. By the end of 2019, there were 109.52 million permanent residents and the permanent resident urbanization rate was 53.2 percent. The GDP of Henan in 2019 was RMB 5425.92 billion yuan, and the per capita GDP was RMB 56,000 yuan. The proportions of primary, secondary, and tertiary industries were 8.5%, 43.5%, and 48.0%, respectively. Energy consumption per unit of GDP in 2019 was about 0.48 tons of standard coal per RMB 10,000 yuan. Henan province is also rich in mineral resources because 142 kinds of mineral resources have been discovered, 58 of which are among the top 5 in China.

In general, Beijing is dominated by the service industry, Tianjin is dominated by the processing and manufacturing industry and by the port service industry, and Hebei is dominated by the resource-intensive industry and agriculture because of the rich mineral resources and rural labor force. Therefore, we can also conclude the following socioeconomic characteristics in the BTH region. First, the issues of industrial pollution are very serious. The heavy industries in Hebei and Tianjin lead to great pressure on the environment, especially serious air pollution. Second, economic development is very unbalanced. Hebei is geographically advantaged with three port cities including Tangshan, Cangzhou, and Qinhuangdao, and most of the cities are very close to Beijing and Tianjin. However, there is a huge gap in economic development as the above statistics show. Third, as the capital city, Beijing has strong political, economic, cultural, scientific, and technological strength. However, there has always been a "siphon effect" of talents, funds, and other resources transferring from Hebei and even Tianjin to Beijing. Therefore, Beijing has a very weak radiation effect on the surrounding cities. Fourth, under the BTH coordinated development strategy, Tianjin has changed to a more sustainable urban development model. For example, the Sino-Singaporean Eco-city replaced Hanggu to become one subcenter of the Binhai New Area of Tianjin, as it is a national project that retains a high level of support from the central government for environment protection in the BTH region.

Based on the outline of the coordinated development plan, Beijing is the national center for political, cultural, and international exchanges and for scientific and technological innovation; Tianjin is the national advanced manufacturing research and development base, the northern international shipping core zone, and the financial innovation operation and reform pilot demonstration zone, and Hebei is an important base for modern trade and logistics in China, a pilot area for industrial transformation and upgrading, a demonstration area for new urbanization and urban-rural integration, and a supporting area for the BTH ecological environment. The overall orientation of the BTH region is "a world-class city cluster with the capital as the core, a leading area for coordinated regional development and reform, a new national engine for innovation-driven economic growth, and a demonstration area for ecological restoration and environmental improvement". The overall development plan can be concluded as "one core, two cities, three axes, four areas, and multiple nodes". "One core" means Beijing, and "two cities" refer to Beijing and Tianjin. They are the main engines for the coordinated development of the BTH region so the linkage between Beijing and Tianjin needs to be further strengthened. "Three axes" refer to the three industrial development and urban agglomeration axes, including Beijing–Tianjin, Beijing–Baoding–Shijiazhuang, and Beijing–Tangshan–Qinhuangdao. "Four areas" refer to the central core functional area, the eastern coastal development area, the southern functional extension area, and the northwest ecological conservation area. "Multiple nodes" include regional central cities such as Shijiazhuang, Tangshan, Baoding, and Handan and node cities such as Zhangjiakou, Chengde, Langfang, Qinhuangdao, Cangzhou, Xingtai, and Hengshui, with the focus on improving these cities' comprehensive carrying capacity and service capacity and on promoting industry and population aggregation in an orderly manner.

There are some reasons why the Chinese government made great effort to develop this region. First, the coordinated development of this region is conducive to solving Beijing's "big-city diseases" such as population expansion, traffic congestion, housing difficulties, environmental degradation, and resource shortage. Second, the BTH region lags behind China's Yangtze River Delta and Pearl River Delta but it has a huge development potential. Third, the economic capabilities are extremely uneven among cities. Beijing and Tianjin lead the country in urbanization rate and per capita GDP, while those of Hebei cities are below the national averages. For example, Beijing's pillar industries are finance, information technology, and science and technology research, but Tianjin and Hebei are dominated by medium- and low-end manufacturing industries with lots of pollution. The milestones are as follows. By 2017, remarkable progress was made in the orderly relocation of noncapital functions of Beijing and some breakthroughs occurred in key areas such as transportation integration, environmental protection, and industrial upgrading and relocation. By 2020, the population of Beijing should be controlled within 23 million, an integrated regional transport network will be shaped, the environment

will effectively improved, major progress will be made in the coordinated development of industries, and the development gaps within the region should be narrowed. By 2030, the core functions of Beijing will be improved, the regional economic structure will become more reasonable, the environmental quality will improve, and the levels of public services should be balanced. In the end, the BTH region will become a region with strong international competitiveness and influence, thus playing a greater role in guiding and supporting China's economic and social sustainable development.
