Synergistic Pathways and Potential Assessment for Pollution and Carbon Reduction in Typical Coastal Cities: A Case Study of Haishu District Ningbo

Abstract

Under the “dual carbon” strategy, achieving synergy between pollution reduction and carbon mitigation is crucial for the sustainable development of coastal cities. However, existing studies frequently emphasize single-carbon reduction measures whilst neglecting to acknowledge the cross-sectoral synergistic effects. This study takes the Haishu District of Ningbo in Zhejiang Province as a case study and quantitatively assesses the effectiveness of synergistic governance strategies across four key pathways: photovoltaic power generation, carbon sequestration by green land, new energy transportation, and waste incineration for power generation. The results demonstrate that multi-sectoral coordinated management significantly enhances overall emission reduction efficiency, with the photovoltaic and waste-to-energy pathways showing the highest carbon reduction potential. This study establishes a multi-pathway framework for evaluating the synergistic effects of pollution and carbon reduction. It also provides scientific support for decision-making regarding the transition to a low-carbon economy in coastal cities, and proposes a replicable evaluation methodology that can be used to implement dual carbon strategies in other coastal regions.

1. Introduction

Global climate change has become one of the major challenges facing human society. The global average temperature has already risen by about 1.1 °C above pre-industrial levels, and the concentration of atmospheric CO₂ has exceeded 410 ppm [1]. Excessive emissions of greenhouse gases (GHGs), such as carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O), have led to global warming and frequent extreme weather events, causing imbalances in ecosystems. In response, carbon reduction initiatives are gaining momentum worldwide [2]. Among these efforts, energy and environmental policies centered on the goals of ‘carbon peaking’ and ‘carbon neutrality’ have gradually gained consensus among governments worldwide. For instance, the European Union has committed to becoming climate-neutral by 2050, and China has set an ambitious target to reach carbon peak by 2030 and achieve carbon neutrality by 2060. Under this framework, a series of key measures have been formulated, including adjustments to the energy structure, industrial upgrading, and pollution prevention and control. Excessive emissions of greenhouse gases, such as carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O), have led to global warming and frequent extreme weather events, causing imbalances in ecosystems. In response, carbon reduction initiatives are gaining momentum worldwide. Among these efforts, energy and environmental policies centered on the goals of “carbon peaking” and “carbon neutrality” have gradually gained consensus among governments worldwide. The Chinese government has set an ambitious target to reach carbon peak by 2030 and achieve carbon neutrality by 2060. Under this framework, a series of key measures have been formulated, including adjustments to the energy structure, industrial upgrading, and pollution prevention and control. As one of the important approaches to reducing carbon emissions, coordinated governance of pollutant control and greenhouse gas reduction has attracted widespread attention. Air pollutants (SO₂, NOx, PM_2.5) and greenhouse gases often originate from the same sources, primarily fossil fuel combustion, industrial production, transportation, and waste treatment processes [3]. Therefore, in the process of achieving the “dual carbon” targets, it is not insufficient to focus solely on reducing carbon. A critical scientific question in the current environmental and energy fields is how to maximize the benefits of carbon reduction while implementing pollution control, i.e., how to achieve the synergistic effects of pollution and carbon reduction.

As one of China’s most economically developed coastal provinces, Zhejiang is under significant pressure to reduce its carbon emissions across various sectors, including industry, transportation, energy, and ecology. Coastal cities in particular experience high energy consumption, dense populations, limited land resources, and industrial concentration, all of which present considerable challenges [4]. Against this backdrop, achieving a balance between pollution control and carbon reduction targets while maintaining rapid economic growth has become an urgent issue for policymakers. Previous studies have explored single pathways or sector-specific synergies [5,6,7], a comprehensive quantitative assessment of multiple pathways within a single urban system, particularly for typical coastal cities, remains lacking. In recent years, the Zhejiang provincial government has actively promoted renewable energy, green buildings, low-carbon transport, and the utilization of solid waste as measures to reduce regional carbon emission intensity. However, traditional single-method emission reduction approaches often struggle to align environmental benefits with economic gains [8]. For instance, although photovoltaic (PV) power generation reduces fossil fuel consumption, limited land availability can constrain its implementation. Similarly, promoting new energy vehicles (NEVs) can lower carbon emissions but faces challenges such as insufficient charging infrastructure [9]. Therefore, relying solely on a single technology or policy is insufficient to achieve the desired emission reduction targets [10].

In response to this challenge, an integrated, interdisciplinary approach to coordinated governance is urgently needed. Recent studies have highlighted the importance of multi-sectoral coordination. For example, Fu et al. proposed a multi-channel fusion prediction framework for TBM tunneling thrust based on multimodal decomposition and reconstruction, which demonstrates how integrating heterogeneous data channels can enhance predictive accuracy in complex systems [11]. Inspired by this idea, our study similarly treats four mitigation pathways (photovoltaic power, green land carbon sinks, new energy transportation, and waste-to-energy) as complementary channels, and applies an integrated framework to capture their interactive and synergistic effects. Chen and Yin et al. [12] proposed an integrated modeling approach for sustainable coastal energy development, emphasizing the need for optimal economic and environmental performance through renewable energy sources. Additionally, Mousavinezhad et al. [13] examined the co-benefits of vehicle electrification, demonstrating significant reductions in PM2.5 and ozone levels in major U.S. urban areas, thereby improving air quality and public health. Xiao et al. examine China’s transportation sector, showing that carbon markets and environmental regulations can jointly reduce both CO₂ and air pollutant emissions, though there is regional heterogeneity in effectiveness [14]. In the Yangtze River Economic Belt, a study of 259 prefecture-level cities demonstrates that cross-regional coordination policies—focusing on industrial green transition, clean energy system co-construction, and technology diffusion—significantly enhance pollution–carbon synergy over time [15]. Another international example is the Air Quality Improvement Co-benefits of Low-Carbon Pathways toward Well Below the 2 °C Climate Target in China (2018), which quantifies how low-carbon constraints in power and industry sectors reduce both greenhouse gases and air pollutants under more ambitious climate goals [16]. However, many of these studies focus on one or two pathways (e.g., transportation + industry, or power + policy instruments). This study extends beyond them by integrating four key pathways—green land carbon sinks, PV power generation, new energy transportation, and waste incineration—in a single coastal city context, thereby capturing interactions and trade-offs among multiple mitigation options in a way that captures local socioeconomic and institutional specifics, particularly for similar coastal cities in the Yangtze River Delta and beyond.

Technologies such as photovoltaic power generation, green space carbon sinks, new energy transportation and waste incineration for power generation have been gradually introduced. These measures have played a positive role in reducing carbon emissions and improving environmental quality, while also contributing to the sustainable development of cities. Notably, various pollution and carbon reduction pathways have synergistic effects when different technologies are implemented together. For example, photovoltaic power generation reduces the use of fossil fuels and can also help to reduce air pollutant emissions by optimizing the energy structure. Green space carbon sinks provide ecological benefits for improving air quality while enhancing carbon sequestration. Promoting NEVs cuts carbon emissions and significantly reduces pollutant emissions from traditional fuel vehicles. A systematic quantitative assessment of the specific emission reduction contributions of these technologies, the actual effectiveness of coordinated governance and their economic impacts is still lacking. The questions of whether synergistic effects can significantly enhance overall carbon reduction efficiency and how policies can be optimized to maximize these effects under multi-sector integrated governance remain pressing issues requiring in-depth research. Therefore, it is of great significance to develop scientifically sound and reasonable quantitative methodologies for different pollution and carbon reduction pathways, to comprehensively evaluate the synergistic emission reduction effects of various technologies and to provide precise evidence to support policy optimization. This study builds on these efforts by combining multiple technologies, aligning policies, and optimizing industrial structures to improve the overall efficiency of pollution and carbon reduction, providing replicable low-carbon development models for Zhejiang and other coastal cities across the country.

To explore these issues, this study focuses on Zhejiang’s coastal cities, with Haishu District of Ningbo as a case study. Haishu District is a typical coastal urban area characterized by a mix of light industry, commerce, and services, high population density, and significant transportation emissions, making it representative of the challenges and opportunities faced by similar cities in eastern China [17,18]. The district’s carbon emission structure, with a high proportion from the transportation sector and energy activities, provides a compelling case for investigating synergistic governance. Moreover, Haishu District has been actively promoting low-carbon initiatives, such as distributed photovoltaics and new energy transportation, which allows for a practical assessment of multiple pathways. This study, focusing on the period 2021–2022 and using Haishu District of Ningbo as a representative case of an economically developed coastal urban center, quantitatively assesses the effectiveness of synergistic governance strategies across four key pathways: photovoltaic (PV) power generation, carbon sequestration by green land, new energy transportation, and waste incineration for power generation. The study addresses the following research questions: (1) What is the quantitative contribution of key pathways (PV, green space, NEVs, waste-to-energy) to carbon and pollutant reduction in Haishu District? (2) How can synergistic effects between these pathways be maximized through integrated governance? (3) What are the transferable insights for other coastal cities? Based on the above literature review and research questions, we propose the following testable hypotheses to guide the subsequent empirical analysis: Hypothesis 1: The combined implementation of the four pollutant- and carbon-reduction pathways (photovoltaic power generation, green land carbon sinks, new energy transportation, and waste-to-energy) yields synergistic emission reductions, such that the total achieved reduction significantly exceeds the sum of reductions from each path individually. Hypothesis 2: An optimized combination of these pathways improves regional carbon efficiency, i.e., under the same level of resource or cost input, more carbon emissions can be abated. Hypothesis 3: Positive complementary effects exist among the pathways; for example, increasing the share of photovoltaic generation will to some extent enhance the marginal emission-reduction effectiveness of other pathways (such as carbon sinks or new energy transportation). These hypotheses provide the theoretical foundation for the quantitative model and will be examined in the empirical sections of this study. The studies use quantitative methods, such as carbon emission accounting, alongside environmental monitoring data, carbon emission statistics from Zhejiang Province and empirical data from its coastal cities. The study integrates carbon emission accounting (following IPCC guidelines), environmental monitoring data, and spatial analysis. Key indicators include CO₂, NOx, and SO₂ reductions. It then systematically evaluates the synergistic effects and contributions of four key pollution and carbon reduction pathways: photovoltaic power generation, green space carbon sinks, the promotion of new energy vehicles, and waste incineration for power generation. Furthermore, a multi-sector coordinated governance optimization model is constructed to quantitatively analyze optimization strategies for multi-pathway synergistic governance. The study reveals the comprehensive mechanisms of the different technological pathways in terms of carbon reduction, pollution control and economic benefits. The findings illustrate a low-carbon development model for Zhejiang’s coastal cities under the “dual carbon” goals. This supports the construction of more targeted and efficient pollution and carbon reduction policy systems and facilitates the transition to green, low-carbon and sustainable development. The findings also offer replicable policy optimization strategies for other coastal regions.

2. Methods

2.1. Model Design and Computation Method

In line with the hypotheses formulated in the introduction, the model is designed to quantify both the individual and joint contributions of the four pathways, thereby enabling the verification of synergistic effects (Hypothesis 1), efficiency improvements from optimized combinations (Hypothesis 2), and potential complementary interactions (Hypothesis 3). Quantitative analysis methods include carbon emission accounting models, which utilize the carbon emission calculation method of the IPCC (Intergovernmental Panel on Climate Change). Total carbon emissions are calculated based on data on energy consumption, industrial production, and transportation. Carbon emissions across different sectors in the coastal cities of Zhejiang are determined using energy consumption volumes and carbon emission factors. The calculation formula is as follows:

$${\mathrm{CO}}_2=\sum (E_i\times {EF}_i)$$

(1)

where $E_i$ represents the amount of energy consumed, and ${EF}_i$ denotes the carbon emission factor. To improve the accuracy of carbon emission estimation, the accounting model was refined by incorporating sector- and energy-specific emission factors. Emission factors for different fuels and industrial sectors were obtained from the Guidelines for Provincial Greenhouse Gas Inventories (NDRC) and the China Energy Statistical Yearbook. These authoritative sources provide detailed data on energy production, consumption, and emission intensities, allowing the model to account for variations across industries, transportation, and energy use. Integrating these factors ensures a more precise representation of carbon emissions in Haishu District and enhances the robustness of subsequent pathway analysis. The system boundaries for life-cycle assessment (LCA) for PV, NEVs, and waste incineration include production, operation, and end-of-life phases where relevant data were accessible. For PV modules, emissions from manufacturing were estimated based on literature values from the Chinese Life Cycle Database (CLCD), assuming a lifespan of 25 years. For NEVs, emissions from battery production were included based on published LCA studies for electric vehicles in China [19]. For waste incineration, emissions from construction and decommissioning of the plant were considered negligible compared to operational emissions, consistent with common LCA practice for waste management infrastructure [20].

This study calculates the carbon emission reduction amounts separately for four key areas: photovoltaic power generation, green space carbon sequestration, new energy transportation and waste incineration power generation. A sectoral accounting method is then used to disaggregate the energy consumption figures into various sectors, such as industry, transport, construction and waste treatment, to analyze the contribution of each sector to carbon emission reduction in more detail. During the analysis, the carbon emissions involved in manufacturing photovoltaic modules, constructing green spaces, producing and operating vehicles, and generating power from waste incineration are calculated, taking into account energy inputs and waste emissions within system boundaries. The study calculates the carbon reduction contributions during the production, operation, and disposal phases for each pathway. It also calculates the unit carbon emission reduction (kg CO₂-eq) and pollutant reduction (e.g., kg NOx, kg SO₂), in order to compare the environmental benefits of the different pathways.

2.2. Data Sources and Processing

This project uses environmental and carbon emission data, as well as socioeconomic data. The environmental and carbon emission data for the period 2021–2022 were sourced from the Haishu district environmental monitoring station, enterprise carbon emission reports, the transportation department’s statistics, and other relevant sources. Socioeconomic data includes indicators such as GDP, energy consumption, transport volume and waste treatment volume, primarily obtained from the Ningbo Statistical Yearbook 2021–2022 (China Statistical Database) and government bulletins. Remote sensing and geographic information system (GIS) data (using ArcGIS 10.8 software) are employed to evaluate the capacity of urban green spaces to sequester carbon. To ensure the scientific rigor of the analysis and the accuracy of the data, this study adopts a multi-source data fusion approach to collect environmental, energy and carbon emission data from coastal cities in Zhejiang Province. Specifically, carbon emission data are mainly derived from the annual carbon emission reports of the Zhejiang environmental monitoring station and the local government’s energy consumption statistics, while pollutant emission data are provided by air quality monitoring data released by the Zhejiang provincial department of ecology and environment. Additionally, socioeconomic data such as the GDP growth rate, changes in energy structure and the penetration rate of new energy vehicles are collected to support the analysis of the impact of different pollution and carbon reduction pathways on the regional economy. The carbon emission factors were sourced from the China Energy Statistical Yearbook and the Provincial Greenhouse Gas Inventory Compilation Guide, considering specific factors for different energy types (e.g., coal, petrol, natural gas) and sectors where available.

2.3. Uncertainty and Limitations

The carbon accounting methodology relies on standard emission factors and reported energy data, introducing uncertainties from (1) potential inaccuracies in energy consumption statistics, (2) variability between actual and default emission factors, and (3) assumptions in the LCA system boundaries. To examine the sensitivity of results, we performed a preliminary sensitivity analysis by varying key parameters (e.g., grid emission factor, vehicle mileage) by ±10%, consistent with common practices in urban carbon accounting [21]. The analysis showed that the ranking of pathway effectiveness remained stable, although absolute values could vary up to ±8~12%. Carbon sequestration estimates for green spaces are based on average photosynthetic rates and may not fully capture seasonal or species-specific variations. Future studies could incorporate dynamic modeling and primary measures to reduce uncertainties.

3. Results and Discussion

3.1. Characteristics and Challenges of Carbon Emission in Coastal Cities in Zhejiang

As global climate change intensifies, urban carbon emissions have become a central issue in global carbon reduction efforts. Under the ongoing advancement of China’s “Dual Carbon” strategy, the carbon emission characteristics, structural profiles, and evolving trends of cities, serving as crucial spaces for diversified industrial development and population agglomeration, which directly affect the pathway to achieving national carbon peaking goals and impose higher demands on local urban green transformation. Haishu District in Ningbo City, a representative coastal city, exhibits distinct and complex carbon emission characteristics shaped by its unique regional geography, industrial structure, and energy consumption patterns. Zhejiang’s coastal cities, as one of the economically developed coastal regions in China, face unique challenges in pursuing low-carbon development under the “Dual Carbon” goals (Figure 1a). In recent years, industrial restructuring and technological innovation in this region have driven shifts in energy consumption patterns; however, issues such as high total carbon emissions and insufficient synergy between pollutants and carbon reduction governance persist. According to data from the Zhejiang provincial government and relevant departments, the main sources of carbon emission in this region are four major sectors: industry, transportation, buildings and waste treatment. The industrial and transportation sectors account for the largest proportions of emissions (Figure 1b).

According to the 2021 greenhouse gas (GHG) emissions inventory of Haishu District, the total GHG emissions that year amounted to approximately 1.7313 million tons of CO₂ equivalent. Energy-related activities were the main contributor, accounting for 97.9% of total emissions. The transportation sector, being a major energy consumer, was responsible for 59.47% of CO₂ emissions from energy activities, making it a key area for carbon reduction in the Haishu District. Further analysis reveals that CO₂ emissions in the district’s GHG structure far exceed those of other gases, such as methane (CH₄) and nitrous oxide (N₂O). Additionally, sectors such as industry, services, residential life and construction are significant sources of carbon emissions. Although their emission intensity per unit is lower than that of the transport sector, their overall contribution remains substantial due to the large scale and high density of their activities. Figure 1c–f show the composition of GHG emissions from different sources in Haishu District. As shown, energy activities and waste treatment are the two primary sources. Within energy activities, fossil fuel combustion (including coal, petroleum, and natural gas) dominates, characterized by high carbon intensity and difficulty in controlling it. GHG emissions from waste treatment mainly arise from CH₄ and N₂O released during the landfill of municipal solid waste and wastewater treatment processes. Haishu District is primarily composed of the light industry and service sectors. Although the district’s overall industrial carbon emission intensity is relatively low, certain industries with high energy consumption (such as papermaking, building materials, and electricity generation) still contribute to regional emission peaks. Spatially, carbon emissions show a pronounced pattern of “higher in urban centers, lower in suburban areas.” This is especially evident in central urban areas where there is high traffic density, as a dense population and concentrated energy consumption render these zones carbon emission hotspots. For instance, major transportation corridors, industrial parks and large mixed-use commercial and residential areas typically form clusters of carbon emission. These areas not only consume large amounts of energy but also exhibit low energy use efficiency, leading to the simultaneous emission of pollutants and carbon—demonstrating a clear synergistic relationship between carbon and pollutant emissions. This characteristic provides technical and managerial entry points for subsequent synergistic control measures.

Studies on emission source synergies show that emissions of SO₂, NOx, and VOCs from stationary combustion sources are highly correlated with CO₂ (Figure 1c–f), exhibiting the typical characteristics of “same source, same pathway, synchronized” emissions. For instance, the energy sector (especially power generation and industrial combustion) is a dominant source for both CO₂ and SO₂/NOx, while transportation is a major contributor to CO₂ and NOx. This overlap provides a clear scientific basis for targeting synergistic governance measures, suggesting that policies aimed at decarbonizing the energy and transport sectors will yield the greatest co-reductions in conventional air pollutants. Notably, due to fossil fuel combustion, the transportation and electricity production sectors simultaneously emit various pollutants and greenhouse gases, making them primary targets for coordinated governance. However, control is also more difficult to achieve. On the one hand, these sectors often involve cross-departmental management (e.g., transport, housing and urban development, energy), and lack a unified emission reduction platform. On the other hand, current carbon accounting systems still suffer from limited coverage and inaccurate data, which restricts the scientific and targeted implementation of policies. In summary, carbon emissions in Haishu District are characterized by a high proportion of energy-related emissions, significant transportation emissions, a concentrated industrial structure and pronounced spatial differentiation. These features lay the foundation for coordinated governance of pollutant and carbon reduction while also presenting significant challenges.

3.2. Carbon Sequestration Effect of Park Green Areas

The assessment of urban sustainability must also consider ecological resilience, particularly the vulnerability of green spaces, which are crucial for carbon sequestration and climate adaptation [22]. Against the backdrop of the “Dual Carbon” targets, urban green space, as vital natural carbon sinks, are becoming increasingly important for urban low-carbon transformation and the construction of ecological civilization. Studies on urban green space vulnerability, such as those conducted in metropolitan areas like Kolkata, highlight the importance of integrated assessment frameworks that couple environmental pressures with ecosystem responses [23]. This study incorporates this perspective by evaluating green spaces not only as carbon sinks but also as integral components of urban ecological resilience. Urban green spaces fulfill their carbon sink function primarily through two pathways: photosynthesis by plants and the accumulation of soil organic carbon. During growth, plants absorb atmospheric CO₂ and convert it into organic matter that is stored within the biomass. Simultaneously, some of the carbon is transferred to the soil through litterfall, roots, and other means, forming a stable soil carbon pool. In urban environments, arboreal species generally exhibit higher carbon sequestration efficiency than shrubs and herbaceous plants due to their longer growth cycles and developed woody structures. Furthermore, multi-layered plant communities (e.g., tree–shrub–grass composite structures) positively impact carbon storage per unit area [24]. The carbon sequestration capacity of urban green spaces is influenced by various factors, including plant species, tree age, community structure, management practices, and habitat conditions. The rational configuration of plant communities and the optimization of green space spatial structure are key strategies for enhancing the capacity of urban areas to sequester carbon [25]. Haishu District, as a typical coastal city, has abundant green space resources and diverse ecosystems, providing significant potential for carbon sequestration. Especially in newly built or upgraded urban parks, introducing and scientifically arranging high carbon-sequestration plant communities has significantly improved the carbon sink function of green spaces.

This study evaluates the carbon sequestration potential of urban green spaces, taking Tongpenpu Mountain Sports Park in Haishu District, Ningbo City, as an example. The total area of the park is approximately 353,333 m², 80.92% of which the green area accounts for 285,928 m². This includes a variety of vegetation types, such as trees, shrubs and lawns, which form a typical multi-layered plant community structure. Through on-site surveys and CAD mapping (Figure 2), the distribution and planting area of various plant types within the park were calculated in detail. The results show that the area covered by trees is 64,463.49 m², the area covered by shrubs is 193,385.05 m², and lawn area is 28,079.46 m². Based on the assignment of unit area carbon sink values to different vegetation types and estimation via the photosynthetic rate method [26], the park’s average annual carbon sequestration was calculated to be 91,750.05 kg CO₂, or approximately 91.75 metric tons. This indicates the substantial carbon sink potential and notable demonstrative value of the park. While the absolute carbon reduction (91.75 tCO₂/year) is modest compared to energy-related pathways, the co-benefits for urban livability, air quality improvement, and biodiversity are significant. The investment and maintenance costs for urban green spaces are relatively low and provide long-term ecological service value that is not easily monetized, representing a high return on investment for urban sustainability.

Due to their large photosynthetic surface area and long growth cycles, arboraceous plants sequester significantly more carbon than other plant types. Common tree species in the park, such as Sophora japonica, Robinia pseudoacacia, Fraxinus chinensis, and Cherry trees, have carbon sequestration capacities of 8–12 g/(m²·d) per unit leaf area. Shrubs such as the purple-leaf plum (Prunus cerasifera), wild apricot (Prunus armeniaca) and the Malus micromalus also demonstrate relatively strong carbon sequestration capacity per unit area, although their overall contribution is limited by lower carbon storage per unit volume. Although lawns have a lower carbon sequestration capacity, their extensive coverage and rapid growth cycles contribute positively to the dynamic carbon cycle. Additionally, the structure of plant communities significantly affects carbon sequestration capacity. Studies show that composite communities consisting of trees, shrubs, and grasses can achieve carbon sink efficiencies 1.5 to 3 times higher than those of single-layer grasslands or shrub lands. This reflects the synergistic effects of biodiversity and ecological stability in enhancing carbon sink capacity.

Therefore, prioritizing the use of locally adaptable, high carbon-sequestering tree and shrub species and promoting the configuration of multilayered tree–shrub–grass structures can enhance the carbon storage capacity of green space. Simultaneously, guiding the development of urban “pocket parks,” community green spaces, and small-scale greening projects can improve the integration of urban green areas with street spaces, building edges, and public nodes. This can achieve a synergistic enhancement of carbon sink systems and municipal spatial planning.

3.3. Effects of Photovoltaic Power Generation on Pollution and Carbon Reduction

For a long time, the energy consumption in Haishu District has been dominated by electricity and natural gas, with a significant proportion of electricity still derived from fossil fuels. According to 2021 data, carbon emissions from energy-related activities in the district totaled approximately 1.435 million tons of CO₂ equivalent, accounting for 82.9% of the district’s total emissions. Carbon emissions from the power system remain high due to the predominance of thermal power in the purchased electricity mix, resulting in elevated carbon intensity. Against the backdrop of the transition to a low-carbon energy consumption structure, solar photovoltaic (PV) power generation is emerging as a vital pathway for urban energy greening. PV power offers notable advantages in terms of carbon reduction and provides additional benefits in terms of pollution reduction, air quality improvement and optimization of the energy structure.

As a coastal city, Haishu District leverages its favorable solar radiation conditions and abundant rooftop space to actively promote distributed PV deployment. The district is progressively developing a diversified clean energy system characterized by initiatives such as “PV + Rural Areas,” “PV + Industry,” and “PV + Public Services.” In response to the national energy transition initiative, Haishu District launched a pilot program for distributed PV applications, particularly promoting the installation of rooftop PV power stations on rural and industrial/commercial buildings. This study uses the Longguan Township PV project as a case study to evaluate its pollution and carbon reduction effects systematically, and to explore its potential for replication under the regional “Dual Carbon” strategy.

As a demonstration zone for “Photovoltaic Villages,” Longguan Township piloted distributed PV projects in 2014, gradually expanding them across the entire township. By 2021, the township had completed PV installations with a total capacity of over 13.3 MW, generating 48,387 MWh of electricity each year (see Figure 3). This study uses the accounting model for PV power generation set out in the China Certified Emission Reduction (CCER) methodology, using the 2021 baseline emission factor of the Zhejiang power grid (0.7921 tCO₂/MWh) to calculate the annual reduction in carbon emissions from the PV project, and evaluate its benefits in terms of reducing pollution and carbon emissions. The calculation formula is as follows:

CO₂ emissions reduction = annual power generation × baseline emission factor

Additionally, to evaluate the co-benefits of reduced pollution, we refer to data from the China Electricity Council indicating that NOx emissions from thermal power generation are 152 mg/kWh. This value is used to calculate the corresponding NOx emission reductions. The accounting boundary encompasses all operational PV projects in Longguan Township, including those in the six villages of Longxi, Shanxia, Hengcun, Houlong, Jinxi, and Longfeng, as well as rooftop PV installations on industrial and commercial buildings (see

Table 1. Summary of emission reduction data of photovoltaic power generation projects in Longguan Township.

Project Location	Annual Electricity Generation (MWh)	CO2 Emission Reduction (tons)	NOx Emission Reduction (tons)
Longxi Village	19,355	15,331.1	2.95
Shanxia Village	7742	6132.43	1.18
Heng Village	2903	2299.47	0.44
Houlong Village	4839	3832.97	0.74
Jinxi Village	1935	1532.71	0.29
Longfeng Village	4839	3832.97	0.74
Industrial and Commercial Roofs	48,387	38,327.34	7.35
Total	91,860	71,288.99	13.68

). Based on our calculations, the annual power generation from Longguan’s PV stations is 48,387 MWh, resulting in a CO₂ emission being reduced by 71,288.99 tons and NOx emissions by 13.68 tons. These data demonstrate that replacing thermal power with PV generation reduces CO₂ emissions by over 71,000 tons annually, making it an effective technological solution for the green transformation of the regional power system. The levelized cost of electricity (LCOE) for distributed PV in China has decreased to a range of 0.2–0.4 CNY/kWh, making it increasingly competitive with retail grid electricity. The associated carbon reduction cost is estimated at 200–400 CNY/tCO₂, which is economically viable, especially when considering future carbon pricing and pollution-related health cost savings.

The above analysis shows that photovoltaic (PV) power generation has become a key technology in helping cities achieve their carbon peaking and carbon neutrality goals. Its low-carbon, distributed, and scalable characteristics provide inherent advantages in the construction of “zero-carbon communities” and “zero-carbon industrial parks.” The synergistic benefits of PV power are reflected in several ways: it can replace high-carbon coal-fired power systems directly, reducing the carbon intensity of electricity production. Simultaneously, it reduces traditional air pollutants such as SO₂ and Nox, improving air quality, lowering air-conditioning energy consumption and facilitating the development of a more diverse energy structure. In this context, Haishu District should use Longguan Township as a demonstration site to develop an integrated model combing rural energy transition, ecological revitalization, pollution and carbon reduction. This involves exploring various photovoltaic application scenarios and designs, encouraging the unified management and integration of rooftop resources, promoting integrated rooftop PV retrofits in industrial parks, and setting up a “PV + energy storage + intelligent energy management “ system. These measures aim to enhance the utilization efficiency and flexibility of new energy, thereby further unleashing its comprehensive value in green development.

3.4. Carbon Emission Reduction Contributions of New Energy Transportation and Green Mobility

Within the strategic pathway towards urban carbon peaking and carbon neutrality, reducing emission in the transport sector is critical. Transportation accounts for significant energy consumption and concentrated carbon emissions, and also contributes substantially to the emission of conventional pollutants. Carbon emissions from transport systems primarily originate from the consumption of fossil fuels, especially the widespread use of gasoline and diesel in motor vehicles, which makes urban road traffic a major source of greenhouse gas emissions. According to available data, traditional fuel-powered vehicles emit an average of 2.2–2.5 kg of CO₂ per 100 km. In 2021, the transport sector in Haishu District was responsible for 59.47% of total CO₂ emissions from energy-related activities, ranking first among all sectors and highlighting the pivotal role of transport in regional greenhouse gas management. In addition to greenhouse gases, the transport sector generates substantial NOx, CO, and PM_2.5 emissions, which put sustained pressure on urban air quality. This issue is particularly prevalent along main urban roads with high vehicle density and low traffic efficiency. To address these challenges, Haishu District has actively promoted the development of a new energy transportation system and the transition to green mobility in recent years. A preliminary multi-path emission reduction framework centered on the use of electric vehicles, the enhanced public transport and optimized non-motorized travel systems has taken shape, with encouraging initial results.

In order to address the ongoing issue of carbon emission from conventional transportation, Haishu District has leveraged Ningbo City’s broader strategic layout to systematically promote the adoption of new energy vehicles and the development of a green transportation system. This approach opens up pathways for reducing carbon emissions through new energy transportation. This includes replacing traditional fuel-powered vehicles with new energy vehicles, actively promoting the transition from fuel to electric power for buses, taxis, and government fleets, accelerating the construction of charging infrastructure with public charging stations deployed across major commercial zones, residential areas and highway entrances and exits, and fostering the development of green public transport with an increased electrification rate for public buses and a steady expansion of rail transit coverage to major functional zones.

To evaluate and analyze the carbon reduction benefits of new energy transportation and green mobility, estimates were made based on the annual mileage, fuel types, and fuel consumption of buses, private cars, and taxis in Haishu District in 2021 (see

Table 2. Driving characteristics of new energy vehicles in Haishu District.

Vehicle Type	Annual Mileage (km)	Fuel Type	Fuel Consumption (L/100 km)	Emission Factor (Kg CO2/L)	Electricity Consumption (KWh/100 km)	Annual Carbon Emissions (t)
Conventional Bus	38,188,212.5	Diesel	25	2.60	/	24,822.34
Conventional Car	2067,612	Gasoline	8	2.37	/	392
Conventional Taxi	111,187,839	Gasoline	7	2.37	/	18,446.06
Battery Electric Bus	38,188,212.5	Electric	0.74	/	50	12,172.5
Battery Electric Car	2067,612	Electric	0.29	/	10	137.08
Battery Electric Taxi	111,187,839	Electric	0.74	/	15	11,057.63

). The data in Table 2 is sourced from the information announcement of the Haishu District People’s Government of Ningbo City (https://www.haishu.gov.cn/). The results indicate that the total anticipated carbon emissions from travel activities involving traditional fuel vehicles amounted to 43,660 tCO₂ per year. Taking the carbon emissions from these conventional vehicles as the baseline scenario and those from electric vehicles as the project scenario, the carbon reduction achieved by new energy vehicles was primarily calculated using the CDM methodology “AMS-III.C: Small-scale Methodology for Emission Reductions through Use of Hybrid Vehicles.” Substituting transportation modes with pure electric buses, taxis, and private cars and using an average grid emission factor of 0.78 tCO₂/MWh for the East China power grid, with an assumed average power transmission and distribution loss of 15% for vehicle charging, resulting in calculated transportation sector carbon emissions of 23,367.21 tCO₂ in Haishu District. This demonstrates a reduction of over 20,000 tCO₂ per year (a 46% decrease from the baseline) in the transport sector and effectively alleviates air pollution in urban areas. Although the upfront purchase cost of NEVs and charging infrastructure investment is high, the lower operational and maintenance costs compared to conventional vehicles, coupled with government subsidies, are improving their economic viability. The societal benefit from reduced air pollution further enhances the cost–benefit profile of this transition.

As can be seen above, new energy vehicles have carbon reduction capabilities and demonstrate synergistic advantages in urban pollution control. Electric vehicles have no tailpipe emissions, which significantly reduces NOx and PM_2.5 pollution levels on urban roads. Promoting electric buses and rail transit can improve travel efficiency, ease traffic congestion, and reduce idling pollution. New energy vehicles also produce low operating noise, which enhances the urban acoustic environment. However, it is important to note that the carbon reduction effect of new energy vehicles depends heavily on the upstream power structure. In the current context of predominant coal power, achieving full ‘net-zero’ emissions still requires the simultaneous development of clean energy grid integration and energy storage systems. Haishu District is promoting the “green travel + digital transportation + clean energy” development model to shift from “emission-reduction vehicles” to a “carbon-reducing travel” system and ultimately transform urban transportation from a source of carbon emission to a carbon-neutral element.

3.5. Carbon Reduction Potential and Pollution Control of Municipal Solid Waste Incineration Power Generation

Urban waste management is fundamental to the operation of cities and an important part of the governance of pollution control and greenhouse gas reduction. Compared to traditional landfill methods, waste incineration for power generation effectively reduces waste volume, extends landfill life and recovers energy through waste heat, partially replacing fossil energy and achieving the “win–win” effects of carbon reduction and pollution control. Haishu District in Ningbo City has actively promoted the resourceful and environmentally friendly treatment of municipal solid waste, establishing a local waste incineration treatment center with a matching power generation system. This makes it an important practical field for reducing urban pollution and carbon emissions. According to statistics, Ningbo City’s municipal solid waste incineration power generation project (this project) has a design capacity of 821,250 tons of waste per year, with the main incineration power plant located in Yinzhou. The plant uses advanced grate furnace incineration technology and flue gas purification systems with strong carbon reduction and pollutant control capabilities (Figure 4). It is estimated that implementing the municipal waste incineration power generation project could reduce 181,083 tCO₂ equivalents per year, making it one of the most significant carbon-reducing technologies in Haishu District’s efforts to reduce pollution and carbon emissions. At the same time, the resource utilization of slag and fly ash should be promoted to enhance the resource value at the incineration end. The use of slag and fly ash in construction materials, for example in geopolymer production, can further enhance sustainability, establishing a link with research on material science synergies [27,28,29]. The waste incineration project’s economic feasibility is supported by municipal waste disposal fees (tipping fees) and revenue from electricity sales. While the capital investment is high, it offers a stable long-term return and avoids the future environmental costs associated with landfill management, such as methane leakage and groundwater contamination.

Waste incineration is a necessary aspect of urban clean governance and an important part of the “dual carbon” strategy for low-carbon energy. When combined with renewable energy and smart sanitation systems, waste incineration can transition from an “end-of-pipe disposal method” to a comprehensive platform for “resource recovery, energy generation and carbon reduction.” To further leverage the synergistic benefits of waste incineration, we recommend establishing a waste sorting and incineration coordination mechanism to ensure incineration heating value and efficiency. We also recommend introducing smart monitoring systems to improve the accuracy and transparency of pollutant emission control. At the same time, the resource utilization of slag and fly ash should be promoted to enhance the resource value at the incineration end [30]. A replicable and scalable waste-to-energy management model can be provided for typical cities be establishing a closed-loop low carbon solid waste management system of “sorting–transportation–incineration–utilization”.

3.6. Policy Optimization Suggestions for Multi-Path Synergistic Governance

Driven by the “dual carbon” goals, achieving synergistic reductions in pollutants and greenhouse gas emissions through multi-path coordination has become a core issue in the modernization urban governance systems. A systematic evaluation of key paths, such as the carbon emission structure in Haishu District, carbon sequestration in park green spaces, photovoltaic power generation, new energy transportation and waste incineration power generation, shows that these paths control pollutant emissions and have significant carbon reduction potential (

Table 3. Annual synergistic emission reduction benefits of typical paths in Haishu District.

Path Type	Synergistic Pollution Reduction Targets	Annual CO2 Emission Reduction (tons)	Normalization Unit	Normalized Reduction	Synergistic Pollution Reduction Benefits	Key Comparative Insight
Park Green Space Carbon Sequestration	PM2.5, O3	91.75	per hectare (ha)	~0.32 tCO2/ha/year	Microclimate Regulation, Dust Adsorption	Low carbon efficiency but high ecological co-benefits.
Photovoltaic Power Generation	NOx, SO2	71,288.99	per MW installed	~5360 tCO2/MW/year	Pollution-Free Emission, Energy Substitution	Very high carbon efficiency and pollution reduction per unit capacity.
New Energy Transportation	NOx, CO2, PM2.5	23,367.21	per 1000 vehicles replaced	~1450 tCO2/1000 vehicles/year	Zero Tailpipe Emission, Noise Reduction	Significant for targeted emission hotspots (urban roads).
Waste Incineration for Power Generation	CH4, NOx, SO2	181,083	per 10,000 tons waste treated	~2200 tCO2/10,000 tons/year	Volume and Carbon Reduction, Pollutant Removal	Highest absolute output, efficient waste-to-energy conversion.

). The design of scientific policy mechanisms can facilitate multi-path coordination to maximize the potential of synergistic governance.

To effectively compare the efficiency and relative strengths of the four key pathways, their annual carbon reduction benefits are normalized against relevant operational scales in Table 3. This analysis reveals that while waste incineration and photovoltaics dominate in absolute annual reduction, their efficiency per unit area differs significantly. Photovoltaic power generation demonstrates the highest carbon reduction efficiency per unit of installed capacity and land use. In contrast, park green spaces, while contributing modestly in absolute terms, provide invaluable, non-monetized co-benefits for urban livability and ecological resilience. New energy transportation offers a balanced profile, providing substantial absolute reductions and critical air quality improvements. This normalized comparison provides crucial insights for policymakers to make informed decisions based on land constraints, investment priorities, and desired co-benefits. In summary, waste incineration power generation and photovoltaic power generation currently offer the greatest potential for synergistic benefits. Meanwhile, green transportation and green spaces for carbon sequestration play important roles in enhancing urban resilience and ecosystem services. There multi-paths exhibit significant complementarity in terms of spatial distribution, energy efficiency, and resource utilization. This requires urgent institutional integration and policy coordination to form a ‘unified’ governance system. The strengths of our method include the integration of multiple data sources and a multi-pathway framework that allows for a holistic assessment. However, the limitations include the reliance on regional average emission factors. The advantage of our approach over alternative single-pathway studies is the ability to capture synergies and trade-offs between different mitigation options.

Although Haishu District has already implemented measures for synergistic governance in several areas, many constraints remain at the institutional level. These include fragmented policy goals, the independent setting of pollution control and carbon reduction within the policy system, a lack of coordination mechanisms for paths, fragmented information platforms, inconsistent data standards and an over-reliance on administrative measures to reduce pollution and carbon emissions, resulting in low public initiative and participation. Solving these problems requires advancing a governance system reform that integrates ‘synergistic governance, system thinking, and mechanism integration’, thereby improving synergy and agility at the urban system level (Figure 5). Concrete policy instruments could include: (1) green fiscal policies: subsidies for PV installation and NEV purchases, carbon trading market participation for waste-to-energy plants; (2) technical standards: mandatory green building codes incorporating PV, emission standards for incineration plants; (3) spatial planning: integrated zoning for green corridors, EV charging hubs, and distributed energy systems. To optimize synergistic policies, long-term regional governance plans should be developed that clearly outline the list of synergistic paths, phased goals, and responsibility allocation. These plans should also promote the joint formulation of the ‘Synergistic Emission Reduction Action Plan’ by multiple departments. At the same time, the integration of multiple paths should be demonstrated, supporting the construction of integrated demonstration zones such as ‘photovoltaic + transportation + green buildings’, and the implementation of projects such as ‘zero-carbon communities’, ‘green parks’, and ‘multi-energy complementary demonstration blocks’. These efforts aim to promote the coordinated layout of photovoltaic rooftops and electric vehicle charging facilities to achieve spatial reuse and system integration. To contextualize Haishu District’s collaborative governance model, we compare it with Shenzhen and Xiamen, a leading coastal megacity in low-carbon transformation. In Low-Carbon Transformation in Megacities: Benefits for Climate Change Mitigation and Socioeconomic Development (2024), Shenzhen’s energy-related carbon emissions in 2019 reached approximately 50 million tonnes of CO₂, with its industrial sector contributing ~21.3%, and transportation about ~16.8% of emissions. Under scenarios incorporating strong low-carbon measures, Shenzhen’s carbon intensity (per unit GDP) was projected to drop ~23.8% by 2030 from 2020 levels. These data demonstrate that while large cities like Shenzhen benefit from scale and infrastructure, Haishu District’s integration of multiple pathways (PV, green land, NEVs, waste incineration) in a medium-sized context offers a model with potentially faster per capita or per area gains, especially when local policy, land constraints, and governance coordination are effectively managed [31]. Xiamen has implemented multi-stakeholder collaborative strategies involving government, enterprises, and local communities to enhance energy efficiency and promote low-carbon development [32]. Compared with these cities, Haishu District demonstrates a unique model by integrating multiple pathways—green land carbon sinks, photovoltaic power generation, new energy transportation, and waste incineration—within a medium-sized coastal city context. This approach not only highlights the complementarity of different pathways in spatial distribution, energy efficiency, and resource utilization, but also provides a practical reference for other similar cities in the Yangtze River Delta and beyond. The integrated governance framework developed in this study, while applied to Haishu District, can be adapted to other coastal cities with similar socioeconomic and geographical characteristics. For non-coastal cities, the pathway priorities might shift (e.g., less emphasis on coastal-specific issues), but the methodological approach of multi-pathway synergy assessment and integrated policy design remains applicable.

4. Conclusions

This paper systematically evaluates the current status of carbon emissions in Haishu District, a representative coastal city in Zhejiang province. This study provides a quantitative assessment of multiple synergistic pathways for pollution and carbon reduction in a typical coastal city. The main achievements are: (1) establishing a multi-pathway framework; (2) quantifying the emission reduction potential of each pathway; and (3) proposing an integrated governance model. We recommend that policymakers: (a) develop integrated plans that combine these pathways; (b) implement targeted financial incentives; and (c) establish cross-departmental coordination mechanisms. The novel contribution of this study lies in the development and application of a quantitative, multi-pathway assessment framework for a typical coastal city, providing a replicable model for integrated urban climate action planning. This study contributes to interdisciplinary fields by integrating methods from environmental science (carbon accounting), economics (cost–benefit analysis), and policy analysis (governance framework), thereby offering a holistic approach to urban sustainability. Haishu District has a strong technical foundation and institutional reserves in terms of multi-source synergy, path integration, and urban low-carbon planning. Its experience in synergistic governance can be replicated and promoted in other coastal cities in the Yangtze River Delta region. This would provide data support and policy recommendations for the ‘dual carbon’ strategy in Zhejiang’s coastal cities, as well as offering replicable low-carbon transformation paths. Future research should focus on dynamic modeling of pathway interactions, more detailed life-cycle assessments, socio-economic impact analysis, and the evaluation of policy implementation effectiveness.