Geochemical modeling of CO₂ emissions from volcanic soil microseepage: implications for greenhouse gas budget

Abstract

As the global greenhouse effect intensifies, the emission and balance of greenhouse gases, particularly carbon dioxide (CO₂), have become crucial for achieving global carbon neutrality. Volcanic geothermal regions, as major natural sources of carbon emissions, release substantial volume of greenhouse gases into the atmosphere in various ways including volcanic eruptions, soil microseepages, vents, and hot springs. Among these, soil microseepages are particularly important due to their widespread and persistent nature. However, the geochemical dynamics of CO₂ release from soil microseepage in volcanic regions remain poorly understood. In this study, we propose a novel CO₂ release model employing computational fluid dynamics (CFD) to model CO₂ emissions from soil microseepage in volcanic regions. Our results provide important insights as follows: (1) Low porosity in subsurface strata inhibits CO₂ penetration, while well-developed underground cracks and channels enhance release rates. (2) Favorable gas pathways enable CO₂ to penetrate dense layers, and migrate upward, with migration patterns influenced by gas source pressure, temperature, and soil permeability. Slowing vertical migration increases horizontal diffusion and expands the effective surface release area. (3) Surface release is also influenced by external factors like wind speed, though these do not significantly affect underground seepage. (4) To improve the accuracy of CO₂ flux measurements using the closed chamber method, it is recommended to reverse the initial slope of the CO₂ concentration-time curve. This study provides critical data to enhance global carbon budget assessments and support efforts towards carbon neutrality.

Introduction

As the greenhouse effect intensifies and climate anomalies increase, global focus has shifted to the role of greenhouse gases, particularly carbon dioxide, in climate change [1–7]. The Paris Climate Change Agreement, signed at the 2015 United Nations Climate Conference, emphasized the need for global cooperation to achieve carbon neutrality [3, 4, 6, 8]. This reflects the urgency of addressing emissions and reaching carbon peak.

The implementation of the global carbon plan requires a thorough investigation of carbon sources, as the nature and scale of carbon emissions directly affect the setting and implementation of regional energy conservation and emission reduction policies, as well as national strategic policies. Therefore, carbon emissions from various sources, particularly the scale, patterns, and influencing factors need to be better understood.

Atmospheric carbon dioxide levels are influenced by both natural and anthropogenic factors [3]. Natural sources, including volcanic activity, forest fires, and respiration from plants and animals, contribute significantly to carbon emissions [3, 4, 9, 10]. Volcanic emissions, in particular, are closely related to the nature of volcanic activity and subsurface geological structures [11].

Volcanic soil microseepage, as a significant source of greenhouse gas emissions, particularly carbon dioxide (CO₂), plays a crucial role in the overall release of these gases [12–18]. While it is critical to understand the CO₂ release processes through soil microseepage in volcanic regions for achieving global carbon neutrality, the patterns and mechanisms of CO₂ release remain poorly understood. To better understand the CO₂ release dynamics from soil microseepage in volcanic regions, it is important to gather data on the relevant influencing factors within a specific range. However, field data collection often presents challenges in terms of control and accessibility. As a result, experimental methods may not always be feasible, necessitating the use of alternative approaches such as simulation studies. In this study, we employ computational fluid dynamics (CFD) to model the greenhouse gas release processes in volcanic regions. By establishing a novel release model, we delve into the internal and external factors that may influence CO₂ release in these regions. We then analyze how various factors impact CO₂ emissions. This study offers crucial concepts and methodologies for assessing greenhouse gas emissions, providing theoretical insights and data support for evaluating emissions in volcanic regions and aiding in the implementation of global carbon plans.

Greenhouse gas emission in volcanic regions

Overview of greenhouse gas emission characteristics in volcanic regions

Existing studies suggest that the upper crust of the Earth is a major reservoir of greenhouse gas [4, 11]. Volcanic activity forms networks of cracks and channels beneath the surface, acting as conduits between the Earth’s deep carbon reservoir and the atmosphere, facilitating the transport of carbon to the surface [19]. Large-scale volcanic eruptions release significant volumes of greenhouse gases from deep Earth into the atmosphere, leading to a sharp increase in atmospheric greenhouse gas contents that can even trigger global environmental changes [11, 20–23]. For example, a volcanic eruption in the Tonga region in the South Pacific in 2022, with a plume reaching up to 30 km, injected millions of tons of gases containing CO₂ and SO₂ into the stratosphere, which severely impacted the local climate and environment [24]. Similarly, the 1815 eruption of Mount Tambora volcano in Indonesia, the most powerful eruption of the 19th century, caused the entire Northern Hemisphere to experience an extreme abnormal climate of “Year Without a Summer” [11, 25]. Previous studies have established a prominent coupling between large-scale volcanic activity and global climate change over time [26–29]. These events underscore the significant impact of volcanic activity on global climate and environment, and highlight its role as a key source of global greenhouse gas emissions [30–33]. Thus, understanding greenhouse gas release processes in volcanic regions is crucial for global carbon budget.

There are significant variations in the forms and characteristics of greenhouse gas emissions in volcanic regions depending on the stage of volcanic activity. In volcanology, volcanic activity is typically divided into two phases: eruption and intermittent, based on the level of magmatic activity in the area [34]. The eruption phase refers to the stage when magma erupts to the surface, while the intermittent phase, also known as the dormant period, occurs between eruptions. The frequency of eruptions and the duration of dormancy can vary widely across different volcanic regions. Some active volcanoes can remain dormant for decades or even centuries [35–38].

During a volcanic eruption, large quantities of greenhouse gases (primarily CO₂, CH₄) are expelled into the atmosphere in the form of a jet column [39], leading to a rapid increase in atmospheric greenhouse gas contents in the region and facilitating the migration of deep carbon into the atmosphere. This process is characterized by its short duration, fast speed, and intensity, resulting in a substantial release of greenhouse gases. However, due to the unpredictability of the timing and intensity of volcanic eruptions, the exact scale of greenhouse gas emissions during these events is is difficult to forecast. Consequently, studies on the magnitude of greenhouse gas release during eruptions generally reply on atmospheric measurements taken after the event has ended.

During the intermittent quiescent period, the volcanic zone remain relatively stable, with no magma erupting to the surface. However, the presence of high-temperature magma chambers underground in these regions causes a continuous heating effect, leading to the formation of high-temperature dry-hot rock systems in the surrounding subterranean rocks [40]. This environment often give rise to high-temperature and high-pressure greenhouse gas chambers, primarily containing CO_2, beneath volcanic areas. These gases migrate and diffuse upwards through pathways such as volcanic channels, cracks, and pores, eventually releasing into the atmosphere. As a result, intermittent volcanic zones continuously emit greenhouse gases within a certain range [17]. The main release forms include soil microseepage, vents, and hot springs [41]. Unlike the rapid emissions during eruption periods, greenhouse gas release from volcanic quiescence is a relatively slow and stable. However, due to its long duration and the extensive release area, the greenhouse gas fluxes from volcanoes during this intermittent period can be substantial [42]. For instance, the Yellowstone volcanic region in the United States releases 400–500 tons of CO₂ into the atmosphere daily, surpassing the emissions during active eruption [43]. Therefore, the study of greenhouse gas emissions during volcanic intermittency has increasingly attracted global attention.

Compared to the eruption period, the greenhouse gas release processes from volcanoes during the intermittent period is more constant. Investigating greenhouse gas emissions from intermittent volcanoes poses a lower risk, with highly representative samples obtained. This enhances the credibility of simulated prediction results in subsequent analyses and calculations. Furthermore, all the Cenozoic volcanoes in China mainland are currently in an intermittent phase [44–48]. Soil microseepage, as one of the most common and significant form of greenhouse gas release in intermittent volcanic regions, plays a vital role in greenhouse gas emissions [3, 18]. The greenhouse gas release process through soil microseepage is generally stable and has significant research potential. Therefore, in this study, we focus on the release process of greenhouse gas from intermittent volcanoes, with an aim to establish a CO₂ release model of soil microseepage in volcanic regions, thereby providing theoretical insights and data support for achieving global carbon budgets.

Methods of investigating greenhouse gas emission in volcanic regions

Greenhouse gas emissions from intermittent volcanoes primarily occur via soil microseepage, vents, and hot springs. The following are the field survey and sampling methods commonly used [1, 49]: (1) For CO₂ released via soil microseepage, field sampling and measurement methods include gas chromatography, open box, and closed chamber techniques, with the closed chamber method being the most prevalent [3, 30, 50]. In this method, the changes in CO₂ accumulation within the chamber is continuously recorded to calculate the CO₂ release flux from soil microseepage at the sampling point. However, a “leveling” effect of CO₂ concentration within the chamber over time can occur, especially during the initial phase of measurement. To address this, a common practice is to discard the first few seconds of data, which may be influenced by perturbation from placing the chamber. A quadratic equation is then fitted to the remaining data, and the first derivative at time “zero” is computed to yield the CO₂ flux rate in units such as grams per square meter per day (g m^− 2 day^− 1). This approach provides more accurate flux measurements, particularly for low-flux scenarios.

(2) For CO₂ released through vents, the aircraft method is often used to measure gas flux. This method involves installing a non-dispersive infrared CO₂ analyzer on a small sampling aircraft. By assessing the difference in CO₂ concentration inside and outside the vent gas column during horizontal movement, and analyzing the trend of CO₂ concentration with height during vertical movement, the CO₂ concentration per unit area can be determined. Combined with measured wind speed, the CO₂ release flux of the vent can be calculated [29].

(3) CO₂ released from hot springs in volcanic regions typically exists in two forms: dissolved and gaseous [31, 48]. The calculation of CO₂ flux involves measuring the concentration of HCO₃⁻ ions in volcanic hot springs for dissolved CO₂, while the gas chemistry method, utilizing a digital soap film flowmeter, is used to measure the gaseous CO₂ flux from hot springs. By incorporating data such as CO₂ content and the number of hot springs, the CO₂ flux released into the atmosphere in the form of bubbles can be determined [32, 33].

Application of CFD in the study of greenhouse gas emission in volcanic regions

Investigations into the forms and scales of the greenhouse gas emissions from volcanoes utilize various techniques that cover most the global volcanic geothermal areas. However, there is a limited number of studies focusing on emission patterns and influencing factors of greenhouse gases in volcanic regions [51, 52].

The underground structure of volcanic areas is complex, with factors, such as gas temperature, pressure, underground cracks, soil permeability, and surface wind speed influencing CO₂ release flux. Traditionally, exploring CO₂ release patterns in volcanic regions necessitates gathering data on CO₂ flux and potential influencing factors. During field work, challenges arise in measuring these factors as some sampling areas may not meet analysis requirements, leading to uncontrollable data variability and hindering the analysis of factor impacts on CO₂ flux. This limitation contributes to the scarcity of research on greenhouse gas release patterns and influencing factors in volcanic regions. Additionally, the complex surface morphology and harsh environments in volcanic regions pose challenges and human risks for large-scale fieldwork.

Another alternative method for studying the CO₂ release patterns in volcanic regions is Computational Fluid Dynamics (CFD). Unlike traditional methods that rely on data, CFD methods begin with the underground characteristics of volcanic areas and the principles of volcanic greenhouse gas release to establish models using simulation methods. By employing CFD method to study the CO₂ release patterns in volcanic regions, researchers can adjust physical and chemical characteristics within the model, control the size and variations of influencing factors, and analyze their effects on CO₂ release from soil microseepage. This can provide a novel approach for feature analysis and understanding mechanisms.

CFD, an evolving discipline facilitated by advancements in computer technology, aims to solve the control equations of fluid mechanics through computational and numerical methods to simulate and analyze fluid mechanics problems [53]. Numerical simulation provides unique advantages in experimental research due to its low cost-effectiveness, short cycle times, ability to capture comprehensive data and simulate different scenarios during actual operations, offering crucial guidance for laboratory applications. Consequently, the application of CFD has expanded beyond traditional fluid mechanics and engineering sectors like aerospace, shipbuilding, power, and water conservancy, into diverse fields such as environment, chemical industry, nuclear energy, and resources [54–56]. In recent years, CFD is being applied in the field of gas release, demonstrating notable analytical advantages [57]. However, studies on the integrating CFD into the analysis of greenhouse gas release dynamics in volcanic areas remain very limited. In this study, we propose a CO₂ release model to simulate the geochemical dynamics of CO₂ emissions using the CFD method, aiming to explore the patterns and mechanisms of CO₂ release from soil microseepage in volcanic regions. Implications for atmospheric greenhouse gas budget are also discussed.

Method

In this study, the CO₂ release model for volcanic soil microseepage was established using Fluent software, a commercial CFD tool within the ANSYS platform, widely recognized for its robust computational power and versatility in simulating multiphase flow, heat transfer, viscosity, radiation, and various components [58].

When comparing soil microseepage in volcanic regions to other regions, two main characteristics are particularly relevant: (1) The presence of high-temperature and high-pressure gas reservoirs underground, and (2) The development of numerous channels and fractures due to volcanic regions and fault zones, which create rapid pathways for deep CO₂ flow to the surface. These characteristics contribute to higher CO₂ release fluxes from soil microseepage in volcanic regions compared to other regions. To better reflect the underground structural features of volcanic regions, in this study, gas channels were integrated into the simulation.

To streamline the calculation process, the following assumptions are made:

1. The simulation focuses solely on the migration of CO₂ from underground reservoirs to the surface via soil microseepage, omitting the generation of CO₂ gas.

2. It is assumed that during the volcanoes’ intermittent period, the storage and release pathways of CO₂ gas underground remain stable, acting as a constant high- temperature and high-pressure source.

3. The migration of CO₂ upwards through dense and loose soil layers to the surface, where it diffuses into the air, is emphasized. Biochemical and mineral processes generating CO₂ volcanic soils were omitted, considering the continuous nature of this gas source.

4. The effects of water bodies and microorganisms on CO₂ migration were not considered, assuming the soil layers and strata as isotropic porous media.

However, it is important to note that this simplification overlooks several factors that could influence the CO₂ release dynamics. While thermal CO₂ migration is the dominant process, microbial and mineral processes can significantly alter surface fluxes and concentrations, especially at lower temperatures or over longer timescales. This assumption is valid under high-temperature, high-pressure conditions typically found in volcanic regions, but future work should aim to incorporate these biological and mineralogical processes for a more accurate representation.

Additionally, real volcanic terrains often exhibit anisotropy due to variations in lava flows, ash deposits, and fractured zones, which can significantly affect horizontal and vertical gas transport. This simplification assumes isotropic conditions, which may lead to an overestimation of horizontal diffusion. A more detailed model could incorporate these heterogeneities, particularly in future studies.

Therefore, the CO₂ flow model for soil microseepage in volcanic areas is simplified as a constant-pressure gas migration problem in porous media, driven primarily by convection and diffusion based on concentration and pressure gradients. The specific principles underlying this model are detailed as follows.

Basic geometric structure

During the migration of CO₂ gas from a constant pressure gas source from bottom to top, the diffusion characteristics are considered identical in all horizontal directions. Given this uniformity, a two-dimensional model suffices instead of a three-dimensional one, leading to the establishment of a basic two-dimensional geometric model, as shown in Fig. 1. The model dimensions are set at 30 mm in height and 20 mm in width, featuring upper and lower parts with two distinct porous media characteristics. The upper part, 10 mm in height, representing a relatively loose and permeable soil layer, while the lower segment, 20 mm in height, signifies a denser, less permeable formation.

Fig. 1.

Schematic diagram of the basic geometric model

In this simulation setup, a constant pressure gas source is introduced through the model’s bottom inlet. To mitigate edge effects, the inlet width is set slightly smaller than the model’s width. At the top, an outlet positioned where the soil layer meets the surface matches the model’s width, with a monitor placed at this outlet to track the CO₂ molar concentration. This model can capture the fundamental process of CO₂ gas from soil microseepage—from its release from underground reservoirs, passes through dense strata and loose soil layers, to its ultimate migration to the surface.

The complex network of gas channels and fractures underground in volcanic regions serves as pathways for the rapid upward migration of greenhouse gases. To investigate the effect of these gas channels on CO₂ release and simulate a more realistic scenario of CO₂ release from soil microseepage in volcanic areas, gas channels were integrated into the model. These channels and fractures are simplified as vertical gas channels within dense formations, forming a geometric model that includes gas channels (Figs. 2 and 3). The remaining parameters align with the basic geometric model, excluding gas channels. Through simulations, it was found that the CO₂ migration rate in the gas-free channel model was too sluggish to incorporate other influencing factors effectively. Therefore, to streamline calculations and accelerate simulations, the single gas channel model in Fig. 2 was chosen as the foundational geometric model for this model. Further elaboration on this discussion process will be presented in Sect. “The role of gas channel distribution in CO₂ release patterns”.

Fig. 2.

Schematic diagram of a single-gas channel model

Fig. 3.

Schematic diagram of multi-gas channel model

Fundamental laws and governing equations

The migration of components from a constant gas source in porous media pertains to fluid dynamics. The fundamental laws governing fluid dynamics, including mass conservation, momentum conservation, and energy conservation, should be rigorously adhered to during the simulation of CO₂ release from soil microseepage in volcanic regions.

The Law of Mass Conservation is integral to fluid dynamics, ensuring that the component migration in porous media adheres to this law. It stipulates that within a closed system, the total mass of the fluid remains constant, meaning that the mass flowing into a unit volume per unit time equals the mass added within that unit volume. This principle describes the variation in mass within the model over time and at different locations, as expressed by the mass conservation equation (Eq. 1) [59].

1

where ρ is the density of the fluid;∇ represents divergence; u denotes the fluid velocity in multiple directions, encompassing both x and y axes in this simulation context.

The Law of Momentum Conservation serves as a fundamental principle in fluid mechanics, asserting that the total momentum within a fluid system remains conserved in a closed system. This law can be described by the momentum conservation equation, which equates the change in unit momentum to the sum of incoming momentum and the imparted impulse [60]. The momentum conservation equation is shown in Eq. (2).

2

where P denotes the fluid pressure and f represents the applied force on the fluid. In this simulation, where gravity acts as the external force, the Eq. (2) can be changed to the following form:

3

where g is the gravitational acceleration.

Similarly, the Law of Energy Conservation, stemming from the First Law of Thermodynamics and principles of energy transformation, is pivotal in fluid dynamics [61]. It is typically expressed by energy conservation equation (Eq. 4).

4

where E represents the internal energy per unit mass of fluid; K denotes the heat conductivity coefficient; T is the temperature; Qs is the heat source.

Basic model

To ensure the accuracy and validity of calculations, the comprehensive CFD simulation model typically comprises several basic models. In this CO₂ release model for soil microseepage in volcanic regions, the following basic models are involved.

Constant gas source model

The constant gas source model is a type of fluid constant flow model. In this simulation framework, the model’s inlet is set as the CO₂ pressure inlet, where the parameters such as CO₂ concentration, surface pressure, and gas temperature remain constant over time to emulate an underground CO₂ chamber in volcanic settings. To facilitate calculations, the gas source is idealized as an ideal gas, following the ideal equation of gas state (Eq. 5) [62].

5

where V is the gas volume, n is the gas number of moles, and R is the molar gas constant.

It is important to note that while the ideal gas law is a reasonable approximation for the low-pressure, low-temperature conditions typically encountered near the surface in our model, its applicability may be limited in high-pressure geothermal environments. In such conditions, real gas behavior, including compressibility effects, may significantly influence flow dynamics. For future studies involving deeper or higher-pressure underground systems, a more complex real gas model may be necessary to accurately capture the fluid dynamics.

Porous media model

The porous media model characterizes a substance composed of dense small voids separated by a solid skeleton. Generally, substances with internal and surface pores, like sponges or soils, can be effectively described using porous media models. In this study, the focus lies on the overall impact of porous media on the fluid flow of rather than specific fluid behavior within the media. The porous media are viewed as a resistance source, influencing fluid motion through a resistance source term introduced in the momentum equation [63]. This leads to the supplementation of Eq. (3) as follows:

6

where S_u represents the resistance source term of the momentum equation, consisting of the viscosity loss and inertia loss terms (Eq. 7).

7

where µ is the dynamic viscosity, D_u is the viscosity coefficient, and C_u is the inertia resistance.

Due to the isotropy of the porous medium region in this model, the viscous and inertial resistance coefficients are uniform in all directions the same porous medium. The viscous resistance coefficient is inversely related to the permeability of the soil layer or formation, with the coefficient being the reciprocal of the permeability in this model. As the CO₂ flow rate is slow, the inertial resistance coefficient can be ignored.

Gas convection model

Convection occurs when a fluid is subjected to pressure gradients or external forces. In this model, post-release from a constant pressure gas source, CO₂ ascends due to pressure gradient, leading to convection. For slow fluid flow rate, laminar convection predominates, following Darcy’s law. This phenomenon is typically represented by the following equation (Eq. 8):

8

where k represents the permeability of the porous medium.

When fluid flow rate increases, turbulence should be taken into account. In this case, fluid movement involves irregular motion, resulting in partial velocity perpendicular to the primary direction. Turbulent motion is commonly represented by the equation of turbulent motion (Eq. 9) [64]:

9

Gas diffusion model

CO₂ migration in this model is affected by diffusion due to concentration gradients. The diffusion of fluids in porous media mainly involves the following three types [65]: (1) Fickian diffusion. It occurs when the pore size of the porous medium is significantly larger than the mean free path of CO₂ molecules, allowing the gas to diffuse with interaction with the pore walls. (2) Kundsen diffusion. It happens when the pore size is smaller than the mean free path of CO₂ molecules, leading to frequent collisions with the pore walls during diffusion. (3) Surface Diffusion. It involves CO₂ being adsorbed onto the pore walls and diffusing along the surface.

This model does not consider the surface diffusion of CO₂ in porous media. The fundamental equation for gas diffusion is shown in Eq. (10):

10

where J represents the fluid flow per unit time, D is the fluid diffusion coefficient, and ∂c/∂x denotes the spatial changes in fluid concentration.

These basic models collectively contribute to a holistic understanding of CO₂ release dynamics in volcanic regions through soil microseepage, ensuring a robust and accurate simulation approach.

Model setup

On the basis of setting up a foundational geometric model and incorporating control equations and basic models, a fundamental framework for analyzing CO₂ release from soil microseepage in volcanic areas is constructed. In this model, CO₂ gas starts from the base gas source, traverses through the porous medium region, and ultimately reaches the surface before diffusing into the atmosphere.

To analyze the release patterns of CO₂ stemming from soil microseepage in volcanic regions, it becomes imperative to investigate potential influencing factors and simulate their variations. For streamlined factor selection and adjustment, the basic model is divided into three parts: constant gas source, porous medium, and surface. These divisions allow for the classification of influencing factors into the gas-related, medium-related, and surface-related categories. By defining the magnitude and evolution of these diverse factors, simulations are conducted on the corresponding CO₂ release processes of soil microseepage. This enables the monitoring of fluctuations in CO₂ release rates and concentrations, facilitating an in-depth analysis of the effects of various factors on soil microseepage dynamics. The subsequent delineation outlines the specific factors considered in the model setup.

Gas factor modeling

In volcanic regions, the presence of high-temperature and high-pressure gas reservoirs underground significantly affects greenhouse gas emissions. This study focuses on simulating the effects of gas pressure and temperature conditions on the release of CO₂ from soil microseepage in these areas. The model adjusts gas pressure or temperature, monitoring the resulting variations in CO₂ molar concentration at the outlet.

Initially, a gas reservoir is simulated by setting a gas source with constant temperature and pressure. The gas source pressure is varied between 1.1 bar and 8.0 bar, while keeping other conditions constant, to observe CO₂ release under different pressures (Table 1).

Table 1.

Pressure condition settings for modeling

Initial conditions	Value
Pressure	1.0 ~ 8.0 bar
Temperature	330 K
Number of gas channels	1
Soil layer permeability	1e−8 m2
Soil layer porosity	0.55
Formation permeability	1e−17 m2
Formation porosity	0.15
Surface wind speed	0 m/s

Similarly, CO₂ release is simulated under varying different temperature conditions by adjusting the gas source temperature between 280 K and 370 K, with all other conditions kept constant (Table 2).

Table 2.

Temperature condition settings for modeling

Initial conditions	Value
Pressure	1.5 bar
Temperature	280 ~ 370 K
Number of gas channels	1
Soil layer permeability	1e−8 m2
Soil layer porosity	0.55
Formation permeability	1e−17 m2
Formation porosity	0.15
Surface wind speed	0 m/s

Medium factor modeling

Greenhouse gases enter the model space through a constant-pressure gas source and migrate upwards through porous media. The formation of gas channels within the porous medium region and its permeability significantly influences the migration process of these gases.

Another critical aspect of soil microseepage in volcanic regions is the presence of numerous underground channels and cracks, which serve as favorable pathways for CO₂ to escape to the surface. In this study, three models are developed: non-gas channel model, single-gas channel model, and multi-gas channel mode. These models are used to study the impact of gas channels passing through geological layers on CO₂ release from soil microseepage. Schematic diagrams of each model are shown in Figs. 1, 2 and 3, and the specific settings are detailed in Table 3.

Table 3.

Gas channel condition settings for modeling

Initial conditions	Value
Pressure	1.1 bar
temperature	330 K
Number of gas channels	0, 1, 3
Soil layer permeability	1e−8 m2
Soil layer porosity	0.55
Formation permeability	1e−17 m2
Formation porosity	0.15
Surface wind speed	0 m/s

Soil permeability is also a key factor affecting the rate of CO₂ release from soil microseepage. Typically, soil permeability ranges from 1e-6 to 1e-8 m². In this simulation, the permeability is varied between 1e-6 and 1e-9 m² to examine its on CO₂ release under different gas source pressure conditions. Three pressure conditions are set—1.01 bar, 1.1 bar, and 2.0 bar—while keeping other conditions consistent. The specific settings are outlined in Table 4.

Table 4.

Settings of soil permeability conditions for modeling

Initial conditions	Value
Pressure	1.01 bar, 1.1 bar, 2.0 bar
Temperature	330 K
Number of gas channels	1
Soil layer permeability	1e-6 ~ 1e-9 m2
Soil layer porosity	0.55
Formation permeability	1e−17 m2
Formation porosity	0.15
Surface wind speed	0 m/s

Surface factor modeling

After gas migrates through porous media areas and reaches the surface, environmental factors at the surface can influence the release process of greenhouse gases. This simulation mainly focuses on the effects of surface wind speed and narrow surface space on the CO₂ release from soil microseepage in volcanic regions.

An initial simulation was conducted to study the effect of surface wind speed by introducing a horizontal airflow above the original model’s outlet. By varying the wind speed, the resulting changes in CO₂ molar concentration at the outlet are monitored while keeping all other conditions constant. The specific settings are presented in Table 5.

Table 5.

Setting of surface wind speed conditions for modeling

Initial conditions	Value
Pressure	1.1 bar
Temperature	330 K
Number of gas channels	1
Soil layer permeability	1e-8 m2
Soil layer porosity	0.55
Formation permeability	1e−17 m2
Formation porosity	0.15
Surface wind speed	0 ~ 20 m/s

Additionally, the complex terrain of volcanic regions, including areas with narrow surface spaces due to the factors like volcanic rock stacking and seismic deformation, can impede the rapid diffusion of CO₂ from porous media into the upper atmosphere This may affect the CO₂ release rate from soil microseepage. To simulate this scenario, a narrow area above the surface was added to the original model (Fig. 4). In this setup, CO₂ enters the narrow area after emerging from the surface and can only escape through the narrow area above. For comparison, all other conditions are kept consistent with those in the pressure simulation described in Sect. 4.1. The specific settings are shown in Table 1.

Fig. 4.

Schematic diagram of the narrow space model of the surface

Results

Gas factors

Table 6; Fig. 5a show the simulated CO₂ molar concentrations (ranging from 6.70 mol/m³ to 58.09 mol/m³) at the surface outlet under different pressures (P = 1.1-8.0 bar) by adjusting the gas source pressure, indicating that pressure can significantly increase CO₂ molar concentration. However, as pressure increases, the rate of increase in CO₂ molar concentration gradually diminishes.

Table 6.

Pressure modeling results

Gas pressure (bar)	Molar concentration of CO2 (mol/m3)
1.1	6.70
1.2	9.40
1.5	15.19
2.0	21.58
2.5	26.83
3.0	31.47
4.0	39.85
5.0	45.52
6.0	50.14
7.0	54.49
8.0	58.07

Fig. 5.

Factors influencing CO₂ molar concentration. (a) Relationship between molar concentration of CO₂ and gas source pressure; (b) Molar concentration of CO₂ in relation to temperature; (c) Relation between CO₂ molar concentration and surface wind speed; (d) Comparison between CO₂ molar concentrations with and without addition of narrow space

Table 7; Fig. 5b illustrate the simulated CO₂ molar concentrations (ranging from 1.14 mol/m³ to 73.72 mol/m³) at the surface outlet at different temperatures (T = 280–370 K) by changing the gas source temperature, indicating a gradual and then rapid increase in the CO₂ migration rate with rising temperature.

Table 7.

Temperature modeling results

Gas temperature (K)	Molar concentration of CO2 (mol/m3)
280	1.14
290	2.43
300	6.17
310	8.80
320	12.50
330	15.19
340	23.51
350	35.52
360	53.14
370	73.72

Surface factors

In the simulation of surface wind speed, CO₂ molar concentrations at the surface outlet under different wind speeds were determined by setting the gas flow velocity horizontally on the surface (Table 8). The relationship between CO₂ molar concentration at the surface outlet and surface wind speed is shown in Fig. 5c. Table 8; Fig. 5d demonstrate that transitioning from no wind to wind on the surface accelerates the CO₂ migration rate at the outlet, but a continuous increase in wind speed does not sustain a significant effect on the CO₂ release from soil microseepage.

Table 8.

Modeling results of surface wind speed

Surface wind speed (m/s)	Molar concentration of CO2 (mol/m3)
0	6.70
4	7.52
8	7.79
12	7.76
16	7.82
20	7.82

Additionally, the simulation involving a narrow space on the surface showcases how this alteration affects the molar concentration of CO₂ at the surface outlet. A comparison of these values with the initial CO₂ molar concentrations (before the narrow space was introduced) is shown in Fig. 5d, indicating that as the gas source pressure increases, the discrepancy between CO₂ release rates in the narrow space model and the initial model gradually increases. This suggests that the presence of a narrow surface space affects the mixing of CO₂ from soil microseepage with the atmosphere, causing CO₂ to accumulate within the confined area, which subsequently reduces the overall CO₂ release rate.

Medium factors

In the gas channel simulation, three models (i.e. non-gas channel model, single-gas channel model, and multiple-gas channel model) are established to analyze CO₂ release under identical conditions. The time required for a single-gas channel to reach steady-state release is set as the reference duration T₀. Table 9 shows the outlet CO₂ molar concentrations of the three models at T₀, reflecting that an increase in gas channels can significantly accelerate the CO₂ migration rate, leading to a notable increase in the outlet CO₂ molar concentration.

Table 9.

Channels modeling results

Number of gas channels	Molar concentration of CO2 (mol/m3)
0	0
1	6.70
3	42.51

For non-gas channel model, the simulated CO₂ molar concentration at the outlet is 0 at T₀. For this model, the simulation time was extended to obtain the CO₂ molar concentration at the outlet at different time intervals, as shown in Table 10.

Table 10.

Non-gas channel modeling results

Simulation duration	Molar concentration of CO2 (mol/m3)
T0	Approximately zero
5T0	Approximately zero
10T0	Approximately zero
15T0	Approximately zero
20T0	Approximately zero

Regarding the simulation of soil layer permeability, the CO₂ molar concentration at the surface outlet was determined under various gas source pressure conditions by adjusting the soil layer permeability (Table 11). Figure 6 illustrates the correlation between the CO₂ molar concentration at the surface outlet and soil layer permeability To enhance clarity, the horizontal axis is represented by the viscous resistance coefficient, which is the reciprocal of soil layer permeability. As the viscous resistance coefficient increases, the CO₂ molar concentration at the outlet exhibits a pattern of initial slow increase followed by a sharp decrease under different gas source pressure conditions.

Table 11.

Permeability modeling results of soil layer

Gas pressure (bar)	Viscous resistance coefficient (log)	Molar concentration of CO2 (mol/m3)
1.01	6.0	2.04
1.01	6.5	2.06
1.01	7.0	2.07
1.01	7.5	2.10
1.01	8.0	2.15
1.01	8.5	2.23
1.01	9.0	1.87
1.01	9.5	0.56
1.01	10.0	0.29
1.1	6.0	6.63
1.1	6.5	6.74
1.1	7.0	6.54
1.1	7.5	6.62
1.1	8.0	6.78
1.1	8.5	7.18
1.1	9.0	7.40
1.1	9.5	4.69
1.1	10.0	2.61
2.0	6.0	21.31
2.0	6.5	22.15
2.0	7.0	21.12
2.0	7.5	22.07
2.0	8.0	22.02
2.0	8.5	22.87
2.0	9.0	23.13
2.0	9.5	22.44
2.0	10.0	18.81

Fig. 6.

Relation of CO₂ molar concentration and viscous resistance coefficient under different gas source pressures. (a)1.01 bar; (b)1.1 bar; (c) 2.0 bar

Discussion

Model verification

The reliability of the CFD model proposed in this study can be validated by comparing it with the experimental data on CO₂ release from volcanic soil microseepage reported by Camarda et al. [66]. In their study, Camarda et al. designed a cylindrical metallic container with a bottom-up CO₂ supply in the laboratory to simulate CO₂ transport through a soil layer of known permeability. The CO₂ release rate was adjusted from slow to fast by varying the gas flux using an air pump. They inserted a gas probe connected to an infrared gas analyzer (IRGA) at different depths within the container and measured the CO₂ flux using the dynamic concentration method. The principle of the dynamic concentration method can be summarized by the following equations:

11

12

where J_CO2 is the soil CO₂ flux; C_d is the dynamic concentration of CO₂; Ø_S is the volumetric flux entering the probe, controlled by a pump; C_S is the CO₂ concentration in the soil gas entering the probe, measured by the IRGA; K₁ and K₂ are constants.

The variation curves of CO₂ volume fraction from the top to the bottom of the container were plotted under different gas fluxes (0.1–22.0 kg m^− 2 d^− 1), as shown in Fig. 7a. For our model, the CO₂ release rate was similarly adjusted from slow to fast by changing the gas source pressure. The corresponding CO₂ volume fraction curves from the top to bottom of the model were plotted under different pressures (50–3000 Pa), as shown in Fig. 7b.

Fig. 7.

Experimental vs. simulated results. (a) Experimental variation curve of CO₂ volume fraction from the top to the bottom of the experimental device in Camarda et al. [66]; (b) Modeling variation curve of CO₂ volume fraction the top to the bottom of the model in this study

As can be seen from Fig. 7, the modeling results are in good agreement with the experimental data, indicating that when CO₂ release is slow, the variation curve of CO₂ volume fraction from the top to the bottom of the experimental device is approximately linear while the curve gradually shows a a more pronounced curvature as CO₂ rate increases. This behavior is attributed to the fact that slow CO₂ release in volcanic soil microseepage is primarily governed by diffusion, whereas rapid CO₂ release is predominantly influenced by convection. Therefore, this model is effective in accurately analyzing the CO₂ release characteristics in volcanic soil microseepage scenarios.

Pressure-driven changes in CO₂ molar concentration

As shown in Fig. 5a, the molar concentration of CO₂ significantly increases with the increase of gas source pressure in the range of 1.1-8.0 bar. At lower gas source pressures, the molar concentration of CO₂ exhibits an approximate linear relationship with the change of gas source pressure, indicating a slow CO₂ flow rate, and predominantly laminar convection, which conforms to Darcy’s law [42]. However, as the gas source pressure increases, the rate of change in CO₂ molar concentration with pressure gradually decreases, and turbulence occurs during convection. As the pressure further increases, the molar concentration of CO₂ tends to stabilize until saturation. This suggests the presence of an upper limit for CO₂ transport in the soil layer under specific temperature and permeability conditions [67].

To further analyze the effect of gas source pressure on CO₂ release, the overall CO₂ modeled molar concentration distribution is plotted (Fig. 8). Figure 8a and b show the CO₂ distribution at gas source pressures of 1.1 bar and 2.0 bar, respectively. From Fig. 8, it can be seen that at the bottom of the model, an increase in gas source pressure accelerates the migration rate of CO₂ from bottom to top accelerates, resulting in a higher overall molar concentration of CO₂ at the bottom. However, at the top of the model, the horizontal migration of CO₂ does not experience the same acceleration, and the overall molar concentration at the top does not increase significantly. These results are consistent with those reported by Marín-Moreno et al. [68], who found that vertical CO₂ migration is more sensitive to pressure variations than horizontal diffusion due to the inherent characteristics of soil permeability.

Fig. 8.

Overall CO₂ molar concentration distribution of the model under different air source pressures. (a) 1.1 bar; (b) 2.0 bar

As the gas source pressure increases further, although the molar concentration of CO₂ in the central area directly above the gas channel increases, the effective release area (see the green and above areas at the outlet in Fig. 8) at the model outlet slightly decreases. This decrease may be due to a weakened horizontal diffusion effect, as the vertical flow rate of CO₂ increases, and the resistance exerted by the porous medium perpendicular to the main flow direction also increases, thereby reducing the horizontal diffusion of CO₂ [69] .

Temperature-driven enhancements in CO₂ migration

As can be seen from Fig. 5b, the molar concentration of CO₂ increases with increasing temperature within the simulated temperature range. This is mainly because higher temperature enhances the kinetic energy of gas molecules, accelerating CO₂ transfer within the model [70]. At lower temperatures, the effect of temperature on CO₂ molar concentration is relatively small, indicating that the resistance of porous media is the dominant factor under these temperature and fixed porosity conditions. The increased molecular dynamics due to temperature are insufficient to overcome the resistance of the porous media region, resulting in slow diffusion being the primary mode of gas migration [70].

However, as the temperature continues to rise, the greater increase in CO₂ molar concentration indicates that the gas energy progressively overcomes the resistance of soil’s porous media, enhancing convection and allowing the gas to be released more rapidly into the surface air. This aligns with Venturi et al. [71], who observed that at higher temperatures, gas convection becomes more pronounced, leading to faster CO₂ release from the soil to the atmosphere in volcanic regions.

It is worth noting that the pore permeability properties of the porous media do not change with temperature during the simulation process. In fact, the soil layer in volcanic areas is not a stationary porous medium. An increase in temperature could potentially alter the internal structure and properties of the soil layer, thereby affecting its ventilation capacity [72]. Thus, the effect of temperature on the CO₂ release from soil microseepage in volcanic areas is complex and multifaceted.

The role of gas channel distribution in CO₂ release patterns

The modeling results of gas channel show that increasing the number of gas channels can significantly accelerate the CO₂ migration rate, leading to a substantial increase in the molar concentration of CO₂ at the model outlet (Table 8). To further analyze the effect of gas channels on CO₂ release, we plot the overall CO₂ molar concentration distribution maps of different models at T₀ (Fig. 9), and for non-gas channel models at various time intervals (Fig. 10).

Fig. 9.

Cloud map of CO₂ molar concentration distribution in different models. (a) Non-gas channel; (b) single-gas channel; (c) multi-gas channel

Fig. 10.

Cloud map of CO₂ molal concentration distribution in non-channel model at different duration. (a) T₀; (b) 5T₀; (c) 10T₀

From Figs. 9 and 10, it can be seen that in the non-gas channel model, the CO₂ molar concentration distribution does not significantly change with time, indicating that low permeability formations pose significant barriers to the upward release of CO₂. Without gas channels, it is difficult for CO₂ deep underground volcanic areas to penetrate the dense strata, resulting in a slow release rate [73].

In contrast, in the presence of a single-gas channel, CO₂ is released into the atmosphere through relatively loose soil layers, but this accelerated release occurs only directly above the gas channel. When multiple-gas channels are present, the synergistic effect between channels enhances CO₂ release rate even in the areas between the channels, expanding the surface CO₂ release area, as shown in the area between the three channels in Fig. 9c. This suggests that the distribution of gas channels is a decisive factor influencing CO₂ release from soil microseepage in volcanic regions. The areas with high gas release typically coincide with those with highly developed underground fractures and channels. This is supported by Cappelli et al. [74], which found that regions with elevated CO₂ release rates were linked to well-developed underground fractures and gas migration pathways. In field exploration, high CO₂ release areas should be prioritized to infer the distribution of underground fractures and further identify potential hazardous areas in volcanic regions.

In addition, in CO₂ release modeling, gas channel is also the primary factor that should be taken into account. In the non-gas channel models, the gas migration rate is slow, making such models time-consuming and computationally expensive for analyzing CO₂ release patterns, and thus unsuitable for this purpose [75]. Conversely, the complexity of multi-gas channel models, due to the interactions between channels, make them difficult to simulate. The single-gas channel model, however, allows for rapid attainment of a steady state in the CO₂ release process, unaffected by other channels. This model offers fast simulation speeds and effectively simulate the underground structure of volcanic areas. Therefore, the single-gas channel model is the most appropriate for the simulating the CO₂ release process and its influencing factors in the soil microseepage of volcanic areas.

Influence of surface conditions on CO₂ release dynamics

As shown in Fig. 5c, surface air flow accelerates the CO₂ release rate from soil microseepage in volcanic areas. This is mainly because air flow promotes the mixing of CO₂ with the atmosphere, preventing the accumulation of CO₂ released by soil microseepage at the surface. However, surface air flow does not affect the rate of soil microseepage itself, thus the molar concentration of CO₂ does not continue to increase with rising surface wind speed.

In Fig. 5d, the measured CO₂ molar concentration decreases after adding a narrow space, with the deviation gradually increasing as gas pressure rises. This indicates that in a confined space, the inability of CO₂ gas to diffuse and mix with the atmosphere leads to a continuous increase in CO₂ concentration. This accumulation may hinder the ongoing entry of CO₂ from soil microseepage into the narrow space, resulting in a gradual decrease in the increase rate of CO₂ concentration within the space [76].

Similarly, when using the closed chamber method for field sampling of CO₂ release flux, the CO₂ concentration inside the chamber gradually increases over time [42, 50]. Although a fan device in the chamber accelerates gas flow, the increase in CO₂ concentration still affects the entry of additional CO₂ from soil microseepage into the chamber. Consequently, the rate of CO₂ entering the chamber from soil microseepage is not constant during sampling. The change in CO₂ concentration at the initial moment of measurement represents the actual CO₂ release rate from soil microseepage at that location. Over time, the rate of change in CO₂ concentration within the chamber tends to stabilize. Relying on the average rate of change in CO₂ concentration to calculate the release flux may lead to an underestimation of the actual CO₂ release rate [17, 42]. To obtain more accurate data on CO₂ release from soil microseepage, it is suggested to plot the CO₂ concentration-time curve after measurement and use the initial slope of this curve, which reflects the initial CO₂ release rate, as the most accurate value. This method, supported by practices in flux chamber studies [42], can overcome the limitations of relying on average concentration changes.

Correlation between soil layer permeability and CO₂ molar concentration distribution

Under the conditions of a gas source pressure of 1.1 bar and viscous resistance coefficients (reciprocal of the permeability) with logarithmic values of 8.0 and 9.0, the overall CO₂ molar concentration distribution maps (Figs. 11a-b) reveal that the horizontal migration of CO₂ actually accelerates as the viscous resistance coefficient increases. This results in an overall increase in the molar concentration of CO₂ and an expanded CO₂ release area at the surface [77].

Fig. 11.

Cloud map of the overall CO₂ molar concentration distribution under different viscous resistance coefficients. (a) 8.0; (b) 9.0

From Fig. 6, it is evident that the convective effect of porous media on CO₂ is almost negligible when the viscous resistance coefficient is small. As the coefficient increases, the vertical convection velocity of CO₂ decreases, which to some extent reduces the resistance exerted by the porous medium perpendicular to the main flow direction. This reduction enhances CO₂ diffusion horizontally, increasing the effective release area and slightly raising the overall CO₂ molar concentration at the outlet [78]. This is also supported by the CO₂ molar concentration distribution maps (Fig. 11).

However, as the viscous resistance coefficient further increases, convection is significantly hindered, leading to a notable decrease in overall CO₂ molar concentration. This suggests that under constant pressure conditions, CO₂ microseepage in the soil layer is mainly affected by convection. As confirmed by Wang et al. [79], soil permeability plays a key role in regulating the balance between convection and diffusion, with increased resistance hindering convective flow and limiting CO₂ migration pathways. Additionally, Fig. 6 demonstrates that an increase in gas source pressure can mitigate the effect of the viscous resistance coefficient on CO₂ release rate.

Implications for atmospheric greenhouse gas budget

The findings of this study have significant implications for understanding the global atmospheric greenhouse gas budget, particularly concerning the role of volcanic soil microseepage as a significant source of CO₂ emissions. Volcanic regions, characterized by persistent and widespread CO₂ emissions through soil microseepage, represent a critical yet often underappreciated component of the global carbon cycle. Previous studies [12, 50] emphasized the contribution of diffuse volcanic CO₂ emissions to regional carbon budgets. Our geochemical modeling provides deeper insights into the complexity and variability of CO₂ release mechanisms, reinforcing the importance of considering volcanic soil microseepage in global greenhouse gas inventories [80].

Our results highlight the importance of subsurface geological characteristics, such as the soil layer permeability and the presence of gas channels, in accurately determining both the rate and extent of CO₂ emissions. These factors not only influence the amount of CO₂ that migrate to the surface but also affect the spatial distribution of emissions across volcanic regions (Figs. 6, 9 and 11). By demonstrating that areas with well-developed underground cracks and channels correlate with higher CO₂ release, this study highlights the necessity of accurately mapping and modeling these subsurface features to better estimate their impact on the global carbon budget [76].

Traditional methods of measuring CO₂ fluxes, such as the closed chamber method, may underestimate emissions, particularly in areas with complex subsurface structures (Fig. 5d) [17, 42, 81]. The closed chamber method is limited by its inability to capture the full variability of CO₂ fluxes, particularly in regions with highly heterogeneous subsurface features. This study emphasizes the need for a more accurate approach, such as using the initial slope of the CO₂ concentration curve, to obtain more accurate measurements. This recommendation has significant implications for improving the precision of greenhouse gas inventories, particularly in volcanic regions where traditional methods may fail to account for the full extent of CO₂ emissions.

Furthermore, the effect of surface conditions, such as wind speed, on the dispersion and mixing of CO₂ once it reaches the atmosphere, also has broader implications for atmospheric greenhouse gas dynamics. Surface wind can significantly affect the local concentration of greenhouse gases and potentially influence regional climate patterns (Fig. 5c) [76]. These interactions between surface and subsurface processes underscore the importance of a holistic approach to understanding CO₂ emissions. Our study contributes to this broader understanding by linking subsurface emission mechanisms with surface dispersion dynamics, offering a more comprehensive view of natural CO₂ emissions and their role in the atmospheric greenhouse gas budget.

Volcanic soil microseepage is thus a vital contributor to natural CO₂ emissions, and understanding the factors that control its variability is crucial for improving global carbon budget models. By integrating insights from geochemical modeling with surface and subsurface dynamics, this study provides a more accurate framework for assessing the role of volcanic regions in the global greenhouse gas budget.

Conclusion

Soil microseepage in volcanic regions is characterized by two key features: the presence of high-temperature, high-pressure gas reservoirs, and the widespread development of underground gas channels and cracks. In this study, a novel CO₂ release model is proposed for soil microseepage in volcanic regions, incorporating these characteristics, and examines the effects of various influencing factors on CO₂ release. The following conclusions can be drawn from this study:

1. In volcanic regions, the lower porosity of the underground strata makes it challenging for CO₂ to penetrate the dense overlying layers and reach the surface. The distribution of underground gas channels is the decisive factor affecting the CO₂ release rate of soil microseepage. Areas with high gas release typically align with those with well-developed underground cracks and channels. During field exploration, the surface gas release patterns can be used to infer the distribution of underground fractures, aiding in the identification of potentially hazardous areas.

2. In the presence of advantageous gas channels, CO₂ can penetrate dense layers reach the soil layer, and migrate upward to the surface. The flow of CO₂ within the soil layer involves both diffusion and convection, with convection being the dominant process. This process is influenced by various factors such as gas source pressure, temperature, and soil porosity. Reducing the vertical convective velocity of CO₂ can promote its horizontal diffusion, thereby expanding the effective release area on the surface.

3. Once CO₂ migrates to the surface, factors such as wind speed and confined surface spaces can influence the release process, but they do not affect the underground soil microseepage itself.

4. While using the closed chamber method to measure CO₂ release flux, the accumulation of CO₂ within the chamber can slow the rate at which CO₂ enters, leading to an underestimated final flux. This problem can be mitigated by the initial slope of the CO₂ concentration curve in the chamber, allowing for more accurate CO₂ release data from soil microseepage.

5. This study provides valuable insights into the mechanisms of CO₂ release from volcanic soil microseepage and offers practical recommendations for improving the accuracy of greenhouse gas inventories.

Author contributions

Xianzhe Duan: Conceptualization, Methodology, Project administration, Writing-Original Draft, Writing-Review&Editing. Haoran Sun: Data curation, Software, Validation, Writing-Original Draft. Nan Li: Investigation, Data curation, Project administration, Writing-Original Draft, Writing-review&editing. Jiale Dou: Software, Data curation, Writing-Original Draft.

Funding

This work was financially supported by the National Foreign Expert Project (G2022029012L), Natural Science Foundation of Hunan Province of China (2023JJ30505), Key Scientific Research Fund of Hunan Provincial Education Department (23A0327), Scientific Research Project of University of South China(20224130214), and Hengyang City Guidance Plan Project (2021jh013).

Data availability

No datasets were generated or analysed during the current study.

Declarations

Ethics approval and consent to participate

Not applicable.

Consent to publish

Not applicable.

Competing interests

The authors declare no competing interests.