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Grey Correlation Analysis of Low-Carbon Governance in Yangtze River Delta Cities

The excessive emission of carbon dioxide will bring unpredictable ecological crisis, so it is particularly urgent to study the related factors of carbon emissions. Based on the grey correlation model, 31 factors in 5 aspects are selected as the reference frame for low-carbon governance, and the grey correlation degree of urban carbon emissions is calculated by using the IPCC method to calculate the annual carbon emissions of 9 major cities in the Yangtze River Delta from 2010 to 2019. Through the calculation and analysis of panel data, the following conclusions are drawn: The allocation of urban environmental practitioners is an important factor in carbon governance, and the reasonable and scientific allocation of environmental practitioners can have a significant impact on low-carbon development; the relationship between the amount of industrial power consumption and carbon dioxide emissions is not significant. On the contrary, the power consumption of urban residents can well reflect the level of carbon emissions. High residential power consumption means that the living standard of the people in the region is high, and the corresponding resource and energy consumption is large, so the carbon emissions will increase; the size of population density is particularly important for carbon governance, which is more obvious in economically developed areas. Urban economic development will inevitably lead to the improvement of people's quality of life, a stronger demand for resources, and a significant increase in carbon emissions.


Introduction
With the acceleration of industrialization and urbanization, carbon dioxide emissions have further increased, and the relevant ecological and environmental problems have become increasingly prominent. Since industrialization, carbon dioxide emissions have increased significantly [1]. In 2019 alone, energy-related carbon dioxide emissions reached 33gt [2]. According to the assessment report of the Intergovernmental Panel on Climate Change, more and more evidence show that carbon dioxide emissions are closely related to climate change [3]. The carbon dioxide produced and emitted by human activities exceeds the carrying capacity of nature, causing the earth's greenhouse effect, which in turn leads to the emergence of ecological and environmental problems such as climate warming, sea-level rise, and glacier melting. Industrialization will inevitably bring about the accumulation of labor force, which will lead to the rapid development of urbanization. Today, more than half of the population lives in cities, and this number is expected to be close to 70% by 2050 [4]. The concentration of a large number of people and industrial industries has led to the rapid development of transportation and construction industry, which has increased the demand for energy and produced a large amount of carbon dioxide. In the context of rapid urbanization, cities consume 80% of the total energy, and carbon dioxide emissions exceed 70% of the total [5]. It can be seen that cities play a key role in reducing carbon dioxide emissions and achieving proper governance of carbon dioxide. For the governance of urban carbon dioxide, we need to consider not only the single factor of emissions, but also the comprehensive analysis of pollution factors, governance factors, economic factors, and social factors generated by carbon dioxide, so as to grasp the key to urban low-carbon governance and to realize urban lowcarbon development from the root. China is a big carbon emitter. Facing the ecological problems caused by carbon emissions, China should shoulder the responsibility of reducing carbon emissions and help people and nature coexist harmoniously. As an important regional development center in China, the Yangtze River Delta urban agglomeration has developed rapidly, and its urbanization rate is leading the country. This also leads to a large amount of energy consumption and carbon dioxide emissions, thus facing the dilemma of urban low-carbon development. By studying the correlation between carbon emissions of important cities in this region and other factors, we can point out the direction for urban low-carbon governance, find a way of urban low-carbon, and then provide reference for the low-carbon development of other cities and regions.

Literature Review
In order to study the relevant factors of urban low-carbon governance, scholars have made many useful explorations. Sarwar and Alsaggaf [6] studied the carbon dioxide emission reduction level in Saudi Arabia by using indicators such as GDP, oil consumption, industrialization, urbanization, and education, combined with the effectiveness of government governance and regulatory norms, tested the positive and negative effects of indicators on carbon emissions, and found that the effectiveness of governance and regulatory norms can effectively reduce carbon emissions. Mohareb and Kennedy [7] studied the impact of relevant technological innovation on urban emission reduction with the goal of light passenger vehicles, residential buildings, and commercial buildings and found that technological innovation can greatly reduce carbon emissions, which is beneficial to the realization of emission reduction targets to a certain extent. Friberg [8] studied Brazil's clean development mechanism, clarified the relevant interests, assessed the impact of the mechanism on Brazil's ecological environment, and found that the mechanism was conducive to the development of energy diversification and improved the government's carbon governance capacity. Drożdż et al. [9] investigated the determinants of decarbonization in urban and rural Poland by means of online questionnaires and found that the phenomenon of coal as a direct heat source for urban residents has gradually disappeared, but the coal used for power generation has not been reduced, and efforts need to be made on clean energy to reduce carbon emissions in the future. Lind and Espegren [10] took Oslo as the research object, used the energy system model to analyze the number of low-carbon cities to turn, and found that only taking measures on the energy system is difficult to achieve low-carbon development, but also need to carry out low-carbon development in transportation and other aspects. Falahatkar et al. [11] studied the relationship between urban form and carbon dioxide emissions based on the panel data of 15 important cities in Iran, starting from the compactness, complexity, and centrality of urban form. They found that the faster the city grows, the more complex the form is, and the greater the carbon dioxide emissions are. On the contrary, the more compact the urban form is, the smaller the carbon dioxide emissions are. Azizalrahman and Hasyimi [12] selected Stockholm, Vienna, and Sydney as the entropy model of the characteristics of low-carbon cities and selected 20 indicators including the proportion of gross national product and renewable energy, trying to build a general model to evaluate low-carbon cities. Through calibration and testing, the model can distinguish whether the target city is lowcarbon. Mollaei et al. [13] used the life cycle cost analysis method to build a low-carbon city analysis model based on the United Nations climate data and Iran's national development data. On this basis, the factors affecting economic development and carbon dioxide emissions are discussed. The results show that reducing urban carbon emissions depends on corresponding investment.
The above research provides reference and ideas for analyzing the related factors of urban low-carbon governance. However, due to the late start of the relevant research on urban low-carbon governance factors, the research is relatively insufficient, so it is difficult to grasp the key factors affecting carbon emissions in general, so as to carry out governance activities based on this to achieve urban low-carbon development. In order to fill the research gap, this paper selects 31 factors from the five aspects of urban pollution, environmental governance, energy consumption, social environment, and economic development; analyzes the grey correlation degree with carbon dioxide emissions; and calculates the grey correlation degree of each factor. It finds the factors closely related to carbon emissions, discusses the internal logic behind it, grasps the governance direction as a whole, and helps urban low-carbon development.

Related Factors of Urban Low-Carbon Governance.
In December 2021, the State Administration of market supervision and administration and the National Standardization Administration of China issued the guidelines for the evaluation of low-carbon development level of sustainable development of cities and communities. Carbon emissions, energy, construction and transportation, economy, society, scientific research, policies and regulations, and other indicators are important factors to evaluate the level of low-carbon development of cities. In addition, some scholars have also incorporated indicators such as industrial structure, agricultural production, air quality, water environment, and urbanization rate into the urban low-carbon development evaluation system [14][15][16], which is closely related to urban low-carbon governance. On the basis of combing the relevant concepts of lowcarbon cities, this paper selects reasonable factors closely related to carbon governance for grey correlation analysis and then grasps the key factors of carbon governance.
A low-carbon city is considered to be a city with a lowcarbon development and operation model economically; citizens adhere to low-carbon concepts and behaviors in life, and the government takes a low-carbon society as its development standard and blueprint. Some scholars believe that the construction of low-carbon cities is to take measures such as changing people's ideas, formulating low-carbon policies, and changing people's lifestyle, with the goal of achieving low-carbon emissions. Generally speaking, low-carbon cities are summarized as the following points [17]: (a) the important foundation of low-carbon cities is low-carbon economy, which requires economic development to meet the conditions of low energy consumption, low pollution, low emissions, and high efficiency; (b) the construction of low-carbon cities is no 2 Journal of Environmental and Public Health longer limited to technology and product production, but also involves many social and economic factors such as lifestyle and consumption concept; (c) low-carbon cities should have high economic vitality, meet the requirements of ecological and environmental protection, and improve people's living standards at the same time.
According to these characteristics, combined with the requirements of urban low-carbon governance for lowcarbon emissions, taking urban carbon dioxide emissions as the target, refer to the low-carbon city evaluation system established by Lin et al. [18] and Su et al. [19], to establish low-carbon governance-related factors evaluation system. Through the screening of relevant factors of carbon governance, based on the availability and relevance of data, 31 factors are finally selected to evaluate their grey correlation with carbon governance. As shown in Table 1, the grey correlation evaluation system covers five related factors: urban pollution, environmental governance, power consumption, social environment, and economic development.

Grey Relational Model of Urban Low-Carbon Governance.
Grey correlation theory began in the late 1970s. The interval analysis method founded by Moore is considered to be the initial form of grey system [20]. Deng made great contributions to the development of grey correlation theory in the 1980s, and the classical grey system he founded was widely recognized by people [21]. The main function of the classical grey system is to deal with the discrete series dimensionless and get the correlation degree of each factor. As the grey relational model gradually matures, the model is used in various scenarios, such as industrial innovation evaluation, ecological environment impact evaluation, and economic growth factor analysis [22,23]. In the face of the rapid increase of carbon dioxide emissions, resulting in increasingly prominent ecological and environmental problems, the research on carbon governance is particularly important and urgent. Many scholars have made in-depth analysis of carbon governance factors with the help of various theoretical tools, but the application of grey correlation system model is less. This paper uses the classical grey correlation system model to study the carbon governance factors of important cities in the Yangtze River Delta and analyzes the correlation between economic, social, environmental, and other factors and carbon governance.
Step 1. Matrix processing of index data The panel data is presented in the form of a matrix, in which the first row of the matrix is the reference sequence x 0 of dependent variables and the comparison column X i of other independent variables. where i = 1, 2, 3, ⋯, s; m = 1, 2, 3, ⋯, T; I is the index to be investigated, s is the total number of indicators, and N is the total number of observation points.
Step 2. Calculation of index relevance (1) Dimensionless processing of index data The various indicators of panel data are different in dimension, which is difficult to compare and easy to make mistakes in the calculation of results. In order to eliminate the obstacles with different dimensions, the panel is treated dimensionless.
(2) Calculation of correlation coefficient: (3) Correlation calculation: Step 3. Sort the correlation degree and analyze the reasons for the difference of correlation degree.

Empirical Analysis
4.1. Data Source. Aiming at the Yangtze River Delta region, this paper selects nine major cities such as Nanjing, Changzhou, Wuxi, Suzhou, Hangzhou, Ningbo, Wenzhou, and Huzhou for research and analyzes the factors related to carbon governance in the development process of these major cities. There are many factors involved in carbon dioxide emissions. Of course, its governance should also be comprehensively considered from many aspects. In order to be close to reality and conform to the internal logic, mechanism, and operation mechanism of carbon governance, this paper selects 31 closely related factors for grey correlation analysis from five aspects: urban pollution, environmental governance, power consumption, social environment, and economic development.
CO 2 refers to the total amount of carbon dioxide emitted by the consumption of various fossil fuels in the region; i refers to coal, oil, and natural gas; E i is the consumption of respective energy; L i and N i are the carbon emission and coal conversion coefficient of energy; 44/12 is the ratio of carbon dioxide to carbon relative molecular weight.

Results and Discussion
4.3.1. Grey Correlation Coefficient. The article first initializes the 31 index data of urban pollution, environmental governance, power consumption, social environment, and economic development and initializes the carbon emission data. See Table 2 for details. Secondly, the correlation coefficient between 31 indicators and carbon emissions is calculated, in which the grey resolution is set as ρ = 0:5, taking the added value of the secondary industry as an example; see Table 3 for details. It can be seen from the above figures that, the grey correlation between various factors and carbon governance has not changed significantly. The reason is that the Chinese government has long insisted on strengthening the protection of the ecological environment while developing the economy, trying to get rid of the extensive development mode of high energy consumption and high carbon emissions, and taking the road of sustainable green development. However, in 2014, the grey correlation between various factors and carbon governance fluctuated upwards, which led to the rise of the subsequent grey correlation. The reason behind this is that the Chinese government made a clear commitment to "reaching the peak of carbon" for the first time at the UN climate summit in 2014 and launched practical actions to this end, formulating a series of emission reduction policies. Of course, these policies are multifaceted and comprehensive, thus resulting in an overall upward fluctuation.
Through Figure 1, it can be seen that the grey correlation between the number of employees in the water conservancy, environment and public facilities management industries,  and carbon governance has been maintained at a high level. The number of environmental management personnel is positively related to carbon emissions. Only scientific and reasonable allocation of environmental personnel can have a positive impact on carbon governance. Secondly, the grey correlation between the standard discharge of industrial wastewater and carbon treatment fluctuates greatly and continues to maintain a downward trend. As an important industrial base, the Yangtze River Delta region has its own environmental pollution, and the problem of water pollution is particularly serious. With the adjustment of industrial structure, the pollution of high-tech industries on water resources has been greatly reduced, resulting in a decline in the correlation between the standard discharge of wastewater and carbon treatment.
According to Figure 2, it is not difficult to find that the correlation between industrial soot emissions, industrial sulfur dioxide, and carbon governance has dropped significantly since 2016 and has remained at a low level. It was the most serious year of China's smog problem. Smog has become the top priority of the country, and the control of smog has risen from a local requirement to a national will. As an important component of smog, sulfur dioxide and industrial soot have become the focus of governance, while the related carbon emissions are not important enough. From Figure 3, the correlation between urban domestic electricity and carbon governance remains at a high level, because in areas with high urbanization, the substitution of domestic electricity for fossil energy is significant and irreversible. The more widely     Journal of Environmental and Public Health electricity is used, the less carbon emissions will be. However, the correlation between industrial power consumption and carbon governance remains at a low level. It is difficult for industrial power to comprehensively replace fossil energy, and the cost of industrial power consumption is much higher than that of fossil energy. In the Yangtze River Delta, where the market economy is highly developed, the use of fossil energy will not be reduced because of electricity, and the use of fossil energy will lead to an increase in industrial power consumption. According to Figure 4, we can see that many factors of the social environment are closely related to carbon governance. Carbon dioxide emissions come from people's production and life, and the root of carbon governance lies in people's governance, low-carbon life, and low-carbon consumption. In addition, the grey correlation between the total amount of water resources and carbon governance shows an upward trend. Water resources are the source of human life. The protection of water resources is related to the normal development of people's production and life. With the reduction of water resources pollution by industry and life, the carbon dioxide emitted by normal production and life is also closely related to the total amount of water resources. From Figure 5, the grey correlation between the added value of the secondary industry in GDP, product sales revenue, and carbon governance shows a downward trend. The secondary industry is usually dominated by industry, and the early products are also dominated by industrial products. For a long time, the Chinese government has no longer satisfied with the extensive, high-energy consumption and high emission industrial production mode, emphasizing the transformation from made in China to intelligent manufacturing in China. The original traditional industry is gradually replaced by modern industry with low energy consumption and high efficiency. Therefore, there is a decline in the correlation between two factors and carbon governance.

Correlation Degree of Carbon Governance Section.
According to the grey correlation degree calculation model, the grey correlation degree between 31 factors in 5 aspects and carbon governance in major cities in the Yangtze River Delta can be calculated (Table 4), which shows the correlation between 31 different factors and carbon emissions in a decade.
The grey correlation between various factors in urban governance and carbon dioxide emissions shows that the grey correlation between the domestic sewage treatment rate and the comprehensive utilization rate of general industrial solid waste is high and balanced, and the impact is relatively significant. First of all, because the Yangtze River Delta has a dense population, residents' living carbon emissions are also very high. Secondly, the scale of industrial production is huge. Generally, industrial solid waste is often accompanied by carbon dioxide emissions. The higher the utilization rate, the more industrial waste gas will be treated. The grey   7 Journal of Environmental and Public Health correlation degree of industrial dust removal is uneven. The correlation degrees of Shanghai and Suzhou are higher, which are 0.9553 and 0.9393, respectively, and the correlation degrees of Hangzhou and Huzhou are lower, which are. 4672 and 0.4802, respectively. The reason behind this is that Shanghai and Suzhou have complementary industrial structures, resulting in highly similar energy structures, and the use of fossil energy is relatively common. In terms of industrial structure, Hangzhou and Huzhou are dominated by modern high-tech industries such as e-commerce, tourism, and electronic technology. Their demand for fossil energy is low, and the removal of industrial dust has little impact on carbon dioxide emissions.
The grey correlation degree of various factors in environmental pollution and carbon dioxide emissions shows that Shanghai, Wuxi, and Wenzhou have a high grey correlation degree of the annual average concentration of inhalable fine particles, which are 0.9312, 0.9496, and 0.9591, respectively. These three cities have developed rapidly in the past ten years. Both industrial emissions, domestic emissions, and construction waste gas are closely related to carbon emissions. However, from the perspective of sulfur dioxide emission factors, the grey correlation degree of this factor is obviously low. As an important pollution gas, sulfur dioxide is under strict monitoring and treatment, and its emission is not necessarily related to carbon dioxide emission.
The grey correlation degree of various factors in economic development and carbon dioxide emissions shows that Suzhou, Hangzhou, and Huzhou have higher grey correlation degree of the number of employees in the tertiary industry, which are 0.9665, 0.9767, and 0.9718, respectively. In the process of industrial city transformation, the booming of the tertiary industry in these cities has reduced carbon dioxide emissions, while Shanghai has a low grey correlation degree, only 0.6668. This is due to the early development of industry and the tertiary industry in Shanghai and its dense population. The original industrial structure is relatively reasonable and fixed, and the carbon emissions are relatively stable, which leads to the development of the tertiary industry having little impact on carbon emissions. The grey correlation degree of various factors in social development and carbon dioxide emissions shows that Shanghai has the highest grey correlation degree of population density, which is 0.9786. Shanghai has a developed economy, complete industrial departments, high people's living standards, and high consumption will inevitably lead to high emissions. Population density has a significant impact on carbon emissions. From the perspective of product sales income, Wenzhou has the lowest grey correlation degree, only 0.6502. The industrial structure of Wenzhou determines that the demand for industrial power is far greater than that of fossil energy. Most small commodities are processed with supplied materials, which belongs to low emission industry. Therefore, the impact of product sales income on carbon dioxide emissions is not significant enough.

Conclusion
Based on the above analysis, this paper studies the low-carbon governance of major cities in the Yangtze River Delta, analyzes the grey correlation between 5 aspects and 31 factors and carbon emissions, and draws the following conclusions: (1) From the perspective of urban governance, first of all, the allocation of urban environmental practitioners is an important factor in carbon governance. The reasonable and scientific allocation of environmental practitioners can have a significant impact on low-carbon development. Secondly, water resources should be taken as an important indicator factor for the carbon governance. Water resources should be taken as a necessary basis for production and life. The governance of water resources can drive the realization of multiple environmental governance goals, including carbon governance. Finally, the removal of industrial smoke and dust is generally carried out in areas with high fossil energy consumption. The amount of industrial smoke and dust removal has a significant impact on carbon emissions, which should be given necessary attention (2) From the perspective of environmental pollution, we should focus on industrial wastewater discharge and industrial ammonia nitrogen discharge, because these factors do not reduce the grey correlation degree by controlling haze, and its grey correlation degree is still significant. Through the monitoring of the above two factors, we can clearly understand the regional carbon emission level (3) In terms of power consumption, the relationship between industrial power consumption and carbon dioxide emissions is not significant. On the contrary, the power consumption of urban residents can well reflect the level of carbon emissions. High residential power consumption means a high living standard of the people in the region, and the corresponding large resource and energy consumption will increase carbon emissions (4) From the perspective of economic development, the correlation between industrial development and carbon emissions should be analyzed according to the actual situation of each region. The basis of urban development is different, and the grey correlation degree of industry is also different. In cities with rapid industrial development, but mainly heavy industry, attention is paid to the number of manufacturing employees. In cities with a large scale of light industry development, we should pay attention to the total output value of industries above scale (5) From the perspective of social environment, the size of population density is particularly important for carbon governance, which is more obvious in economically developed areas. Urban economic development will inevitably lead to the improvement of people's quality of life, a stronger demand for resources, and a significant increase in carbon emissions (6) On the whole, there is a high degree of grey correlation between various factors and low-carbon governance, which is closely related to the national policy of environmental governance that the Chinese government has long adhered to. In 2014, the grey correlation degree of various factors and low-carbon governance has obviously increased and fluctuated, and the subsequent grey correlation degree has been driven higher. The reason behind this is that in 2014, the Chinese government explicitly made the commitment of "carbon peak" at the UN climate summit for the first time and took practical actions to develop a series of emission reduction policies. It can be seen that relevant national policies have played a key role in urban low-carbon governance

Data Availability
The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
The author declares that there are no conflicts of interest.