The Negative Influence of Urban Underground Space Development on Urban Microclimate

Abstract

The development of urban underground space can increase the green area of a city and have a positive impact on urban microclimate. However, the negative impacts of urban under-ground space development on the urban microclimate are rarely considered and analyzed. In this study, we focus on analyzing the impact of the development of underground commercial streets under determinant urban form on urban microclimate using outdoor CO concentrations as the evaluation index. In this regard, it was possible to quantitatively evaluate the influences of various development factors (e.g., development intensity of underground commercial streets; location and height of shaft exhaust; and various ground-greening configurations of transverse and vertical trees, large and small shrubs, and grasses) on the outdoor CO concentration. The results showed that higher development intensity increases outdoor CO concentration and its range of effects. Properly increasing the height of shaft exhausts, choosing a dispersed layout for shaft exhausts, and planting large shrubs on the ground in the development area of underground commercial streets can effectively reduce the impact of underground commercial street development on urban air quality.

1. Introduction

Urbanization has promoted social economy and resulted in the deterioration of urban environments in China. Improving the urban environment is an urgent issue to be considered with respect to urban planning and urban residents. To address urban environmental problems, various research and easing measures have been proposed [1,2,3,4,5,6,7]; in particular, landscape greening is accepted as an effective means of easing urban environmental problems [3], which can contribute to reduced air temperature and have a positive effect in terms of decreasing urban air pollution [1,6,8]. In recent years, some studies have suggested that the development of urban underground space is an appropriate measure to solve urban environmental problems [9,10,11]. The development of urban underground space can address the shortage of land resources, alleviate traffic congestion, and protect and improve the urban environment [9,10,11]. Accordingly, protecting and improving the urban environment has become one of the main contemporary topics with respect to the development of urban underground space. The application of this engineering practice to the development of urban underground space has received increasing attention and acceptance from the government, experts, and scholars [12].

Previous studies qualitatively pointed out that underground space development can increase the space requiring landscape greening, and numerous urban functions can be transferred underground, which could effectively improve the urban environment [13]. With the development of CFD technology, the Underground Space Research Center of Army Engineering University carried out interdisciplinary research on urban underground space, urban environment, urban planning, landscape gardening, climatology, etc. The research quantitatively evaluated the influence of different scales of underground space development on urban environment by applying numerical simulations [11]. Improvement of the urban environment was found to be closely related to the overburden thickness of underground space (OTUS) and greening configurations on the ground, suggesting positive guidelines for planning and design of underground spaces [9,10]. However, none of these studies considered and evaluated the negative influences of urban underground space development on the urban environment.

The development of urban rail transit and underground commerce can provide convenient underground travel, shopping, entertainment, and other activities for a large number of people. According to statistics, the maximum daily passenger flow at Nanjing Xinjiekou Subway Station reached 1.6 million, whereas the annual population flow in an underground commerce area was as high as 16.06 million in 2017. Because underground space is relatively confined with poor natural ventilation, its environmental quality is worse than that of outdoor spaces. The humid underground environment and the heat produced by lighting and human metabolism can easily make people uncomfortable. Radon pollution, CO₂, CO, total volatile organic compounds (TVOCs), and microbial bacteria within underground spaces can negatively influence human health [13]. To ensure the safety and comfort of people living and carrying out activities underground, it is necessary to apply mechanical ventilation in order to discharge heat and pollutants from the underground space through shafts. Furthermore, it is necessary to supplement fresh air to people performing activities in underground spaces. When underground air quality is poor, it can have a secondary effect of the quality of outdoor air. Therefore, to decrease the negative influence of underground space development on the urban outdoor environment, is of considerable significance to optimize the planning and design of urban underground space.

In this study, the development of underground commercial streets under the determinant of urban form was selected as the research object, and outdoor CO concentration was chosen as an indicator to evaluate the effects of underground commercial streets on the outdoor microclimate. The aim of this study is to provide insights into the design of underground commercial streets with respect to mitigating negative impacts on the outdoor microclimate. We quantitatively studied the impact of CO concentration discharged from an underground commercial street on the outdoor microclimate under various underground commercial street development intensities, ventilation shaft arrangements, and ground-greening arrangements using ENVI-met CFD simulation software. In contrast to our previous studies [9,10], in this study, we mainly focus on the negative impact of underground space development on outdoor microclimate, which is an important supplement to the interdisciplinary research topic of underground space and urban microclimate. In addition, this study reveals the influence mechanism of underground space development on the urban microclimate and provides several suggestions for the design of underground commercial streets on the basis of the simulation results.

2. The Influence Mechanism of Underground Space Development on Urban Microclimate

The influence of urban underground space development on urban microclimate is essentially based on the theory of urban energy balance. In our previous research, we concluded that the main factors affecting the urban energy balance include three aspects: spatial form, underlying surface, and artificial heat rejection [11,13]. The development of urban underground space affects the urban microclimate by influencing the above three aspects.

2.1. Effects of Underground Space Development on Urban Spatial Form

The large-scale development of urban underground space addresses the limitations of urban above-ground and underground space by placing a large number of urban functions underground, forming a three-dimensional urban space form. Thus, developing underground space can reduce the development of urban above-ground space and change the combination of ground buildings to form novel urban spatial forms. Varying urban spatial forms can not only affect the absorption of solar radiation by structures and heat dissipation of long-wave radiation but also affect the urban wind environment, thereby affecting the urban microclimate.

2.2. Effects of Underground Space Development on Urban Underlying Surfaces

The development of urban underground space can transfer the functions of above-ground buildings with lower environmental requirements to underground space, and more ground area can be reserved for greening; therefore, the attributes of the urban underlying surface can be changed, which could have a positive impact on the urban microclimate. However, when landscape design is applied above underground buildings, the growth environment of plants differs from that of the natural environment. The selection of plant species and the matching of plants mainly depend on the OTUS.

In landscape design above underground buildings, the requirements of the OTUS ascend in the order of grasses, shrubs, and trees. The requirements for different types of plant growth on the OTUS are shown in Figure 1 [14]. The possible effects of different types of plants on the urban microclimate are shown in

Table 1. Possible effects of different types of plants on the urban microclimate.

Plant Type	Urban Microclimate
	Air Temperature	Wind Environment	Radiation	Relative Humidity	Air quality
Large Trees	Reduce solar heat gains by shading; absorb the majority of the heat and reduce the air temperature by photosynthesis and transpiration	Reduce the wind speed at high elevations via the plant canopy and introduce airflow from high elevations to the pedestrian height	Shade, absorb, and reduce long-wave radiation	Increase the level of humidity via plant transpiration	Leaf adsorption and deposition of particulate matter in the air; the plant canopy changes the wind field, hindering the horizontal and vertical transport of pollutants, resulting in increased pollutant concentrations
Small Trees
Large Shrubs		Affect the wind environment at pedestrian height	Partially shade, absorb, and reduce long-wave radiation
Small Shrubs	Reduce land heat storage and strengthen the heat emission of soil; reduce the land surface temperature and air temperature	Typically do not affect the wind environment	Reduce the ground absorption of solar radiation and reduce long-wave radiation from the ground to surroundings
Land Vegetation

.

2.3. Effects on Internal Environmental Control of Urban Underground Space on Urban Microclimate

With the development of rail transit and underground commerce, people’s travel mode and lifestyle have been affected, and underground space has become an important place for urban residents to gather. In order to create an environment suitable for human survival and activities, the urban underground space environment needs to be controlled and improved by ventilation and air conditioning systems, which directly affect the city’s hot and humid environment and air quality indicators.

As shown in Figure 2, ventilation, air conditioning, and environmental control systems in underground spaces are mainly composed of fresh air systems and an exhaust air systems [11]. Exhaust air systems suck polluted air inside underground spaces into exhaust ducts and discharge the polluted air to the outside through ground exhaust shafts. In contrast, fresh air systems suck fresh air from outside into fresh air ducts and release it into underground spaces through air outlets. The combination of exhaust and fresh air systems constitute the internal ventilation systems of underground spaces.

The mechanism of influence of underground space development on urban microclimate is shown in Figure 3.

3. Research Scheme

3.1. Basic Urban Form Unit

In this study, the typical determinant urban form model of urban morphology was used to analyze the influence of underground commercial street development on the out-door air quality. In the field of urban morphology, 200 m × 200 m is generally selected as the appropriate unit grid and fractal hierarchy of urban texture [15]. Therefore, in this re-search, a basic urban block scale of 200 m × 200 m was selected as a standard to establish an urban form model. The layout design of the model was based on the format of a squared figure, and it was assumed that the underground commercial streets were developed as four streets (Figure 4). Detailed model design parameters are shown in

Table 2. Model design parameters of determinant urban form.

Land Use Attribute	Block Size (m)	Building Height (m)	Single Building Size (m)	Plot Ratio	Ground Building Area (m2)	Building Floor Height	Number of Building Storeys	Height–Width Ratio of Street Canyon
Commercial building	200 × 200	36	13.3 × 46.6	2.5	100,404.36	4	9	36/20

.

3.2. Case Setting

3.2.1. Evaluation Index and Evaluation Criterion

In the present study, the pollutants discharged through the exhaust shaft from an underground commercial street were selected as the research object. According to [16], the main gas pollutants in underground commercial streets are CO₂ and CO. CO₂ can be easily diluted outdoors and increase in its concentration with a limited impact on the health and safety of residents [17]. Accordingly, the CO concentration at a 1.5 m height was selected as an index to evaluate outdoor air quality in this study.

According to the “ambient air quality standard” (GB 3095-2012) [18], ambient air quality functions are divided into three categories with defined limits of CO concentration in different functional areas (

Table 3. Limits of outdoor CO concentration.

Pollutant	Sampling Time	Concentration Limit			Concentration Unit
		Grade I Standard	Grade II Standard	Grade III Standard
CO	1 h	10	10	20	mg/m3

). Class I areas refer to the natural reserves and other areas that require special protection with implemented grade I standards. Class II areas refer to residential, mixed commercial, and transportation areas with implemented grade II standards. Class III areas refer to specific industrial areas with implemented grade III standards. Urban underground commercial streets are class II functional areas, and grade II standards should be implemented, with an ambient CO concentration in the air not exceeding 10 mg/m³. Therefore, 10 mg/m³ was taken as the standard to evaluate the influence of shaft air exhaust from underground commercial streets on outdoor air quality. A CO concentration in excess of 10 mg/m³ indicates that the development of underground commercial streets causes air pollution in the outdoor urban environment. On the contrary, a CO concentration of less than 10 mg/m³ indicates that the impact of underground commercial street development on air quality is acceptable.

In this study, the spatial distribution of outdoor CO concentration was qualitatively analyzed under different scenarios. With 10 mg/m³ as the threshold value, we extracted data on the quantity (N) of CO concentrations exceeding 10 mg/m³ at each time point in different scenarios. Because the grid size in the horizontal direction is 2 m × 2 m and the area occupied by each grid is 4 m², the pollution area exceeding 10 mg/m³ in different schemes can be calculated as 4 N m².

3.2.2. Naming and Summary of Cases

In this study, we chose underground commercial street development intensity, the layout of exhaust shafts, and various ground-greening strategies above underground buildings as the research variables, as changes in outdoor pollutant concentrations are related to these variables. The development intensity of underground commercial streets is related to the calculation of the intensity of pollutant emissions. In this study, three cases of underground commercial street development were considered, all of which included a first floor, second floor, and third floor. The layout of exhaust shafts influences the outdoor diffusion of pollutants. Accordingly, we considered the influence of the location of exhaust outlets and the height of exhaust shafts on the diffusion of CO. We considered the location of exhaust outlets in two cases with concentrated or dispersed emissions, respectively. Regarding the height of exhaust shaft, it was divided into emissions from the zone near the ground and from the higher-altitude zone. The type of ground greening mainly depends on the OTUS. In this study, only single-plant configurations were considered for ground greening, with five types considered: transverse tree, vertical tree, large shrub, small shrub, and grass.

In the case study, the development area of each underground commercial street is 20 m × 200 m = 4000 m². According to the Code for Fire Protection Design of Buildings [19] and the standard defining one fire protection unit for every 2000 m² and requiring an exhaust system, two sets of exhaust systems (i.e., exhaust shafts) are required for the development of each underground commercial street. The outdoor air pollution caused by air from the shaft exhaust in underground commercial streets can be regarded as point-source pollution, whereas the area of the exhaust outlet can be used as an indicator of the size of the pollution source.

To analyze the influence of changes in the above factors on urban air quality, each model was numbered. The conventions related to naming and numbering are described in

Table 4. Case numbering conventions.

First Number	Second Number	Third Number	Fourth Number	Fifth Number
Urban form	Underground commercial street development intensity	H: Exhaust air shaft height	L: Exhaust air shaft arrangement	Ground greening
Determinant urban form	I1: Development layer 1 I2: Development layer 2 I3: Development layer 3	H2: 2 m H5: 5 m H8: 8 m	L1: Centralized arrangement L2: Dispersed arrangement	TT: Transverse tree VT: Vertical tree BS: Big shrub SS: Small shrub G: Grass

. For example, B-I₁-H₂-L₁ denotes that in the determined urban form, in the first layer of underground commercial street, the height of the exhaust air shafts is 2 m, and the exhaust outlets are arranged in a centralized manner; B-I₂-H₂-L₂-BS denotes that in the determined urban form, in the second layer of underground commercial street, the height of the exhaust air shafts is 2 m, the exhaust outlets are arranged in a dispersed manner, and ground greening is achieved with large shrubs. The floor plan of the case models is shown in Figure 5.

3.2.3. Simulation Settings

(1)

Simulation software

In this study, ENVI-met V4.0, a commonly used microclimate simulation software, was applied to simulate the influence of shaft exhaust in underground commercial streets on outdoor CO concentrations. In ENVI-met, it is possible to define the pollution source intensity, emission mode, pollutant category, weather conditions, greening conditions, etc., based on the user requirements. ENVI-met has been successfully applied to model outdoor pollutant diffusion, and its applicability was validated by field measurements in our previous study [9]. The mechanisms of ENVI-met with respect to dispersion and deposition of particulate matter are shown in

Table 5. The mechanisms of ENVI-met with respect to dispersion and deposition of particulate matter.

Model	Equation	Description
Atmospheric model	Navier–Stokes equations: $\frac{\partial u}{\partial t}+u_i\frac{\partial u}{\partial x_i}=-\frac{\partial p^{\prime }}{\partial x}+K_m\left(\frac{{\partial }^{2}u}{\partial x_i{}^2}\right)+f\left(v-v_g\right)-S_u\frac{\partial v}{\partial t}+u_i\frac{\partial v}{\partial x_i}$ $=-\frac{\partial p^{\prime }}{\partial y}+K_m\left(\frac{{\partial }^{2}v}{\partial x_i{}^2}\right)+f\left(u-u_g\right)-S_v\frac{\partial w}{\partial t}+u_i\frac{\partial w}{\partial x_i}$ $=-\frac{\partial p^{\prime }}{\partial z}+K_m\left(\frac{{\partial }^{2}w}{\partial x_i{}^2}\right)$ Continuity equations: $\frac{\partial u}{\partial x}+\frac{\partial v}{\partial y}+\frac{\partial w}{\partial z}=0$ $\frac{\partial E}{\partial t}+u_i\frac{\partial E}{\partial x_i}=K_E\left(\frac{{\partial }^{2}E}{\partial x_i{}^2}\right)+\mathrm{Pr}-Th+Q_E-\epsilon \frac{\partial E}{\partial t}+u_i\frac{\partial \epsilon }{\partial x_i}$ $=K_{\epsilon }\left(\frac{{\partial }^2\epsilon }{\partial x_i{}^2}\right)+c_1\frac{\epsilon }{E}\mathrm{Pr}-c_3\frac{\epsilon }{E}Th-c_2\frac{{\epsilon }^2}{E}+Q_{\epsilon }$	For outdoor airflow, non-hydrostatic, incompressible equations are applied. The trajectory simulation of pollutants is described by the Lagrangian equation, which can simulate gas and particle diffusion. Temperature and relative humidity are calculated based on the advection–diffusion equation. The turbulence and airflow exchange processes are based on two additional k-ε governing equations proposed by Mellor and Yamada [20].
Pollutant deposition model	Deposition caused by gravity: $X\downarrow \left(z\right)=-vs/d\frac{X\left(z\right)}{\epsilon z}$ Leaf adsorption and deposition: $\frac{m_{plant}}{\partial t}Xplant\left(z\right)·\frac{1}{LAD\left(x,y,z\right)}·\rho$	The leaf surface is the main attachment site for particle settling. The equation governing particle deposition in ENVI-met consists of two parts: gravity-induced deposition and leaf-surface adsorption deposition. ENVI-met does not take into account the secondary suspension of particles after a brief stay on the surface.
Vegetation model	$J_{f,h}=1.1\left(T_f-T_a\right)$ $J_{f,evap}={\gamma }_a{}^{-1}\Delta q\delta f_w+{\gamma }_a{}^{-1}\left(1-{\delta }_c\right)\Delta q$ $J_{f,trans}={\delta }_c{\left({\gamma }_a+{\gamma }_s\right)}^{-1}\left(1-f_w\right)\Delta q$	The effect of vegetation on turbulence is simulated by introducing additional governing equations related to leaf area density (LAD).

. Detailed information about the software can be found in [20].

(2)

Source intensity setting

According to design standards [21,22,23,24,25], the limit of CO concentration in underground commercial streets is 5 mg/m³. Considering the most unfavorable conditions, 5 mg/m³ was set as the initial concentration of CO discharged from the underground commercial streets to the outside. It was assumed that the depth of each layer of the underground commercial street was 5.1 m, with the exhaust air volume of each exhaust shaft set to 2000 m² × 5.1 m × 6 ACH = 61,200 m³/h on the first developed layer and a source intensity of E = 5 mg/m³ × 61,200 m³/h = 306,000 mg/h, i.e., 85,000 μg/s. In the scenario with two layers, the source intensity was E = 170,000 μg/s, whereas in the case with three layers, the source intensity was E = 255,000 μg/s.

(3)

Simulation parameters

In this study, meteorological data for a typical summer day in Nanjing was selected for simulation input, with the focus on business hours, i.e., from 10:00 AM to 10:00 PM. The outdoor CO concentration was analyzed throughout a 24 h period. The specific simulation parameters are shown in

Table 6. Simulation settings.

Parameter	Definition	Parameter Value
Meteorological conditions (typical meteorological conditions on a summer day in Nanjing)	Wind speed (m/s)	2.4
	Wind direction (°)	157.5
	Original atmospheric temperature (K)	294.95
	Outdoor atmospheric pressure (Pa)	100,250
	Relative humidity (%)	80
Model settings	Building material	Concrete
	Building color	Gray
	Number of embedded grids	10
	Grid dimensions (X × Y × Z)	100 × 100 × 30
	Grid step (X × Y × Z)	2 × 2 × 7.5
Plant settings	Small shrub	1 m × 1 m × 1 m (L × W × H)
	Big shrub	3 m × 3 m × 2 m (L × W × H)
	Transverse tree	7 m × 7 m × 6 m (L × W × H)
	Vertical tree	5 m × 5 m × 10 m (L × W × H)
	Grass	H: 0.2 m
Pollution source	Pollutant	Carbonic oxide (CO)
	Area	2 m × 2 m
	Height	2 m/5 m/8 m
	Location	centralized /dispersed
	Source intensity	85,000 μgs−1 170,000 μgs−1 255,000 μgs−1

0°, 90° 180°, and 270° denote the wind directions of north, east, south, and west, respectively; L, W, and H denote length, width, and height, respectively.

.

4. Analysis and Discussion

We analyzed the influence of the development intensity of the underground commercial streets, the height of exhaust shafts, and the location of exhaust shafts on outdoor urban air quality. Then, we considered and analyzed the influence of ground greening over various underground spaces on outdoor urban air quality. The cases selected for detailed analysis and their basic information are presented in

Table 7. Case summary.

Case Number	Development Intensity	Shaft Height	Shaft Location	Ground Greening
(a) Development Intensity
B-I1-H2-L1	I1—development layer 1	H2—2 m	L1—concentration	None
B-I2-H2-L1	I2—development layer 2
B-I3-H2-L1	I3—development layer 3
(b) Shaft Height
B-I2-H2-L2	I2—development layer 2	H2—2 m	L2—dispersion	None
B-I2-H5-L2		H5—5 m
B-I2-H8-L2		H8—8 m
(c) Shaft Location
B-I2-H2-L1	I2—development layer 2	H2—2 m	L1—concentration	None
B-I2-H2-L2			L2—dispersion
(d) Ground Greening
B-I2-H2-L1-TT	I2—development layer 2	H2—2 m	L1—concentration	TT-Transverse tree
B-I2-H2-L1-VT				VT—Vertical tree
B-I2-H2-L1-BS				BS—Big shrub
B-I2-H2-L1-SS				SS—Small shrub
B-I2-H2-L1-G				G—grass

. Furthermore, the selected models are shown in Figure 6. The analysis methods and concepts of other cases are similar to those selected for detailed analysis.

4.1. Influence of Development Intensity on Air Quality

Figure 7 shows the influence of different development intensities of underground commercial streets with a shaft exhaust height of 2 m and a centralized emission system on the outdoor CO concentration at a height of 1.5 m above the ground at 12:00 noon. The results indicate that the outdoor CO concentration is always less than 10 mg/m³ with one layer of underground commercial streets, which indicates that the influence of CO emissions from exhaust shafts on the outdoor air quality is acceptable. However, the outdoor CO concentration increases with an increase in development intensity. With three layers of underground streets, the outdoor CO concentration exceeds the allowable concentration limit of 10 mg/m³ in most areas. This situation indicates that an increase in development intensity of underground commercial streets and an increase in pollution source intensity can cause severe air pollution.

With 10 mg/m³ taken as the threshold value, the outdoor CO concentration above the threshold was analyzed at 1.5 m above the ground under different development intensities (Figure 8). As a result of changes in meteorological conditions, the regional variation in outdoor CO concentration exceeding 10 mg/m³ shows an initial decreasing trend, followed by an increase. With one layer of underground development, the outdoor CO concentration is less than 10 mg/m³ at various time periods. With two layers of underground development, the area in which the CO concentration exceeds 10 mg/m³ increases to as much as 1368 m². With three layers of underground development, this value can reach 6220 m², indicating that the outdoor CO concentration and the development intensity of underground commercial streets are positively correlated.

4.2. Influence of Shaft Location on Air Quality

Figure 9 shows the outdoor CO concentration with two layers of underground development with a 2 m shaft exhaust height and both centralized and dispersed emission scenarios. The outdoor CO concentration is close to or exceeds the 10 mg/m³ threshold near some of the exhaust outlets under both emission scenarios. On the contrary, when the exhaust shafts are arranged in a dispersed manner, then the CO concentration is lower than with a centralized arrangement.

With 10 mg/m³ taken as the threshold, the influence range of outdoor CO concentra-tion exceeding 10 mg/m³ with varying shaft locations was analyzed, and results are presented in Figure 10. The area in which the outdoor CO concentration exceeds 10 mg/m³ is similar under both exhaust scenario, with an initial decreasing trend, followed by an increase.

The area of outdoor CO concentration exceeding 10 mg/m³ is slightly smaller in the dispersed exhaust scenario, indicating that in the determinant urban form, a dispersed arrangement of exhaust shafts is preferrable with respect to reducing the outdoor CO concentration.

4.3. Influence of Shaft Height on Air Quality

Figure 11 shows the influence of different shaft heights on the outdoor CO concentration with two layers of underground development and with a dispersed exhaust shaft arrangement. In general, when the shaft height increases from 2 m to 5 m and to 8 m, the outdoor CO concentration decreases, indicating that an increase of the height of the exhaust shafts is conducive to reducing the CO concentration at the pedestrian level.

Figure 12 shows that with a shaft height of 2 m, the area with an outdoor CO concentration exceeding 10 mg/m³ reaches 1032 m². With a shaft height of 5 m, the area of outdoor CO concentration exceeding 10 mg/m³ is only 120 m², whereas with a shaft height of 8 m, the outdoor CO concentration is less than 10 mg/m³ at all times. This indicates that the taller the exhaust shaft, the smaller the area where the outdoor CO concentration exceeds 10 mg/m³. Accordingly, appropriately increasing the height of exhaust shafts is conducive to improving the outdoor air quality.

4.4. Influence of Ground Greening on Air Quality

Figure 13 shows the influence of ground greening on outdoor CO concentration with varying soil depths, two layers of underground development, a shaft exhaust height of 2 m, and a centralized emission arrangement. The outdoor CO concentration varies depending on the greening type. When grass, transverse trees, or vertical trees are used, relatively high CO concentration values close to or even exceeding 10 mg/m³ are obtained. When small shrubs or big shrubs are used, the outdoor CO concentration is significantly reduced, with similar values for the two scenarios. This indicates that the outdoor CO concentration can be effectively reduced by implementing ground greening with small or big shrubs.

The influence range of outdoor CO concentration exceeding 10 mg/m³ under different ground greening conditions is analyzed and shown in Figure 14. When transverse trees or vertical trees are applied, the obstruction of airflow due to tree canopies can decrease the diffusion of CO, resulting in an increased CO concentration. Under configurations with transverse trees and vertical trees, the area where outdoor CO concentration exceeds 10 mg/m³ reaches 1280 m² and 512 m², respectively. When grass is used for ground greening, the area with an outdoor CO concentration exceeding 10 mg/m³ reaches 836 m². Finally, when small shrubs and big shrubs are applied, the area with an outdoor CO concentration exceeding 10 mg/m³ is substantially decreased to 96 m² and 32 m², respectively. In terms of the determinant urban form, the effects of different types of greening on outdoor CO concentration can be ranked from highest to lowest as follows: big shrub > small shrub > vertical tree > grass > transverse tree.

5. Conclusions

In order study the negative impact of underground space development on outdoor microclimate, we analyzed the development of underground commercial streets under determinant urban form and the associated outdoor concentration of discharged CO. Furthermore, we evaluated the influence of development intensity of underground commercial streets, the location and height of exhaust shafts, and ground greening on outdoor CO concentration.

(1)

The development intensity of underground commercial streets and outdoor CO concentration are positively correlated, whereas the height of exhaust shaft and the outdoor CO concentration are negatively correlated. Accordingly, the higher the development intensity, the higher the outdoor CO concentration and the larger the influence range. Furthermore, the higher the exhaust shaft height, the lower the outdoor CO concentration and the smaller the influence range.

(2)

A dispersed arrangement of exhaust shafts is more conducive to reducing the outdoor CO concentration when compared to a centralized arrangement. Therefore, to reduce the outdoor CO concentration, it is appropriate to select a dispersed arrangement for exhaust shafts.

(3)

Planted trees in the outdoor area of an underground commercial street development are not conducive to reducing the outdoor CO concentration. However, when big shrubs are planted, the area with an outdoor CO concentration exceeding 10 mg/m³ is the smallest. Accordingly, it is appropriate to plant big shrubs in the outdoor area of underground commercial street developments with the aim of reducing the outdoor CO concentration.

6. Limitations and Future Directions

This study is subject some limitations. First, we only included summertime environmental parameters in our numerical simulation and did not consider other seasonal climatic factors; therefore, our conclusions may not be easily generalized. Secondly, according to the relevant design criteria, we chose 5 mg/m³ as the initial concentration of CO discharged from the underground commercial streets to the outside, which is theoretically feasible. However, the CO concentration discharged from underground commercial streets varies constantly in reality. Finally, we only discussed the impact of CO concentration discharged from underground commercial streets on the outdoor microclimate and did not consider the impact of other discharged pollutants, such as PM₁₀, CO₂, NO, etc. Future studies should address the above limitations and focus on the positive and negative effects of different types of underground space development projects on the outdoor microclimate so as to optimize development strategies.