The cooling effect of urban green spaces as a contribution to energy-saving and emission-reduction: A case study in Beijing, China

Building and Environment 76 (2014) 37e43

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The cooling effect of urban green spaces as a contribution to energy-saving and emission-reduction: A case study in Beijing, China Biao Zhang a, Gao-di Xie a, Ji-xi Gao b, *, Yang Yang c a

Institute of Geographical Sciences and Natural Resources Research, Chinese Academy of Science, Beijing 100101, China Nanjing Institute of Environmental Sciences, Ministry of Environmental Protection, Nanjing 210042, China c Beijing Forestry Survey and Design Institute, Beijing 100029, China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 3 December 2013 Received in revised form 17 February 2014 Accepted 3 March 2014

Urban green spaces have been proven to significantly decrease ambient air temperature and mitigate heat islands created by urbanization. However, the environmental benefits of cooling provided by urban green spaces have rarely been measured. In this paper, we estimated the energy-savings and emissionreduction contribution of urban green spaces in Beijing, applying a empirical model. Our calculations suggest urban green spaces play a major role in reducing energy demand and increasing CO2 sequestration. Urbanized Beijing has 16,577 ha of green space which could absorb 3.33
1012 kJ of heat via evapotranspiration during the entire summer. The cooling effect reduced the air conditioning demand by 3.09
108 kWh which amounts to a 60% reduction in net cooling energy usage in Beijing. The annual reduction in CO2 emissions from power plants associated with electricity saving would reach 243 thousand tons with an average of 61 kg/(ha day). Also, the cooling effect and the environmental benefits of urban green space in Beijing largely depend on the green space’s structure and size. Urban managers and landscape planners should take advantage of this research to plan, design and manage green spaces in heat island areas. Ó 2014 Elsevier Ltd. All rights reserved.

Keywords: Cooling effect Energy-saving and emission-reduction Green spaces Urban heat island Beijing

1. Introduction Climate change is the most serious problem people face in the 21st century. Luke Howard first described the concept of an urban heat island (UHI) as early as 1833 [19]; an UHI is the characteristic area of warmth found in urban areas when compared with their non-urbanized surroundings [55]. When non-reflective, waterresistant or impervious surfaces which absorb a high percentage of incoming solar radiation replace naturally vegetated surfaces, an UHI is often created. Taha (1997) and Shahmohamadi et al. (2011), [53,48] investigated the impact of anthropogenic heat on formation of urban heat island, and proposed the three important strategies to minimise the impact of UHI on energy consumption: landscaping, using albedo materials on external surfaces of buildings and urban areas, and promoting natural ventilation. Many scientists agree higher temperatures not only impact the comfort of urban dwellers, but also increase energy use for cooling, ozone

* Corresponding author. Tel.: þ86 25 85287278; fax: þ86 25 85411611. E-mail addresses: [email protected] (B. Zhang), [email protected] (G.-d. Xie), [email protected] (J.-x. Gao), [email protected] (Y. Yang). http://dx.doi.org/10.1016/j.buildenv.2014.03.003 0360-1323/Ó 2014 Elsevier Ltd. All rights reserved.

production, and the risk of death for humans during a heat wave [14,20,41]. Under a scenario of rapid urbanization around the world’s cities, the development of UHIs has recently become a critical environmental issue in many places [30,35,45]. Fortunately, trees and vegetation in an urban environment can greatly improve the microclimates, as well as mitigate UHI development by reducing summer air temperatures [12]. Urban vegetation cools climates through two major processes: shading and evapotranspiration [46,50]. The direct effect of shading refers to the interception of solar radiation by leaves and branches of trees; this reduces the sunlight reaching the ground below the canopy of a tree or plant. An investigation in Australia demonstrated that, tree shade could reduce wall surface temperatures by up to 9  C and external air temperatures by up to 1  C [4]. The indirect effect of evapotranspiration is the sum of evaporation and plant transpiration, which reduces air temperatures because of the use of energy required for transpiration. A systematic study in Chania of Greece showed that, urban green areas could reduce air temperature 3.1  C, mainly through evapotranspiration [16]. Studies in western countries have mainly focused on the treeshading effects which provide cooling energy at the local scale in

B. Zhang et al. / Building and Environment 76 (2014) 37e43

roads are the second, third, fourth, fifth and sixth ring roads, respectively, in terms of distance from the city center [63]. Urban heat island conditions have been observed for more than a half century in Beijing [66,67]. Based on the climatic data from 11 weather stations in Beijing during the period of 1961e2008, Wang et al. (2009) found the UHI intensity increased slowly from the mid1960s to the late 1970s and subsequently continued to intensify starting in the early 1980s (see Fig. 1). The summertime UHI in Beijing city currently averages 4.5  C [60]. Also, the areas of UHI rapidly expanded from 111 km2 in 1987 to 292 km2 in 2009, and the extent of UHI sharply increased from 11% to 27% of the urbanized area [29] (see Fig. 2). These space-time dynamics of UHI change can be mainly attributed to the rapid urbanization of Beijing [39]. Green spaces are beneficial to UHI mitigation through shading and evapotranspiration [37]. The urban green space in Beijing sprawls across 61,695 ha. We chose 6387 green-space patches totaling 44,356 ha as the study area, all located within the 6th Ring Road (see Fig. 3). Also, we analyzed the structural type of urban green space defined as the combination of different growth forms such as trees, shrubs, or herbs. The statistics revealed seven structural types of urban green spaces in Beijing, which were composed of 21.89% Tree (as a category), 0.09% Shrub, 0.42% Grass, 13.55% Tree-Shrub, 12.52% Shrub-Grass, and 2.38% Shrub-Grass; for a total green space area of 22,556 ha. The remaining 49.15% consists of Tree-Grass-Shrub. Table 1 provides the definition (or description) and percentage of each structural type of urban green space in Beijing; Fig. 3 shows the spatial distribution of these different structural types. 2.2. Research methods A three-step approach was developed. First, an empirical model was designed to represent the heat absorbed by urban green spaces via evapotranspiration, based on a statistical analysis of eight field measurements in Beijing. Second, the energy savings for environmental cooling was calculated by employing the transform coefficient between electrical energy and heat. Finally, an average CO2 emission parameter in Beijing was adopted to estimate the amount of CO2 emission reduction. 2.2.1. Heat absorbed through evapotranspiration Evapotranspiration in green vegetation allows the ambient atmosphere to absorb the latent heat of vaporization during the vaporization of water used by plants [22] and significantly decreases the ambient air temperature. The thermal effects of green

1.4 1.2

0.8 0.6 0.4

2006

2001

1996

1991

1986

1981

0.2

1976

The densely populated northern city of Beijing is the capital of China (39 280 e41 050 N, 115 250 e117 300 E), with a land area of 16,808 million km2 and a population of 19.61 million [3]. Beijing is also a representative metropolis with rapid urbanization and was 86.2% urbanized by the end of 2011. The city is divided into four zones: the city center (Dongcheng and Xicheng districts), the main urban area (Chaoyang, Fengtai, Haidian, and Shijingshan districts), and two suburban areas (Changping, Daxing, Fangshan, Huairou, Mentougou, Pinggu, Shunyi, and Tongzhou districts and Miyun and Yanqing counties). The road network of Beijing City consists of the recently constructed ring roads and radial arteries. The road around the Forbidden City is called be the first ring road, and the outer ring

1.0

1961

2.1. The study area

Heat island intensity/ 0C

2. Materials and methods

1971

low-density residential neighborhoods. Oliveira et al. (2011) [37] observed that a small green space (0.24 ha) was cooler than the surrounding areas. The greatest observed temperature difference was 6.9  C, which occurred between a shaded site in the garden and a sunny during the hottest month of the year in Lisbon, Portugal. Correa et al. (2012) [10] measured the thermal comfort in forested urban canyons of low building density in the city of Mendoza, Argentin. The evaluation showed that the road channels forested with Platanus acerifolia had the best behavior. There also have been many similar studies in Brazil [51], Mexico [21], Switzerland [34], USA [2,49], England [17], and Germany [56]. Since 1970, many new megacities have arisen in Asia because of rapid urbanization [54], and the interactions between urban green spaces and microclimate conditions in high-density cities have been paid much attention [9,52,57,62]. Mahmoud (2011) [33] measured user’s thermal comfort in an urban park in Cairo, Egypt, and revealed an alteration in human comfort sensation between different landscape zones. A preliminary study in Taipei, randomly surveyed the cool-island intensity of 61 city parks, and found that during summer the cooling effect of parks was stronger than in winter [8]. The study in high-rise high-density residential developments of coastal Hong Kong, suggested that increasing the tree cover from 25% to 40% in the pocket parks could reduce daytime urban heat island intensity (UHI) by further 0.5  C [18]. Another experience from Hong Kong revealed that roof greening was ineffective for human thermal comfort near the ground, and trees were more effective than grass surfaces in cooling pedestrian areas [43]. However, studies in Chinese mainland cities usually centered on the effect of evapotranspiration in urban green space on microclimate, because almost all residents live in multi-story buildings (e.g. Refs. [24,64,65]. Because of the lack of the information related to the direct cooling effect, public and city managers in China often ignore or underestimate the environmental contribution of urban green spaces to the cooling effect. More importantly, the influences of species composition, size, growth, crown density, and the spatial arrangement of urban green spaces in large and dense cities on cooling potential have been recognized [22,42,47], but few studies paid attention to the cooling effect and the environmental benefits of urban green spaces with various structural characteristics. The objective of this research is to measure the ecological benefits of the cooling effect associated with the use of green spaces where heat islands are more likely to occur. We specifically focus on the environmental contribution of the cooling effect on energy-savings and carbon-emission-reduction, which generated from seven types of urban green spaces in Beijing. After this introduction related to current research we next elaborate on the methods and data used in this study followed by findings and concluding remarks.

1966

38

Year Fig. 1. Dynamic change in the intensity of Beijing’s heat island from 1961 to 2008.

B. Zhang et al. / Building and Environment 76 (2014) 37e43

Percentage of urban heat island to built-up area

300

30

240

24

180

18

120

12

60

6

Percentage of heat island to built-up area (%)

Urban heat island area (km2)

Urban heat island area

0

0 1987

1994

1998

2000

2001

2007

2009

Year Fig. 2. Ratio of the urban heat island area to the urbanized area in Beijing from 1987 to 2009.

areas in Beijing have been investigated in five case studies using eight field instruments [5,26,31,40,58]. For example [58], compared the cooling effects of three types of green space (square with a tree layer, open urban space, and lawn), and found the greening square could decrease temperature by 0.3e3.3  C (average of 1.9  C) compared with a none-greening square. Ma & Li (2007) [40] investigated the different influences of habitats such as an avenue, lawn, and piazza on the urban microclimates using a HOBO portable meteorological station. Their results show avenues with trees could reduce the temperature by a maximum of 2.76  C. Also, a test related to the environmental effects of three typical urban green spaces (tree-shrub-herbage, tree-herbage and lawn) in Yuan Da-Du Park of Beijing shows the air temperature above green spaces decreased with increasing vegetation coverage and areas with a higher tree-shrub-herbage experienced a wider range of

39

temperatures [31]. The locations of five measurement stations have been marked in Fig. 3. In conclusion, these research studies demonstrate the presence of a plant community with a reasonable structure and composition could improve the effect of urban green spaces in decreasing summer temperatures (see Table 2). How to calculate the heat absorbed by the green space from the surrounding air through plant evapotranspiration? In 1994, Yang [61] proposed an empirical method to convert between temperature and heat. He hypothesized the mean height of city buildings was 100 m, and the area of vegetation in urban microclimate study was 10 m2. Then a theoretical air column having the 10 m2 base and a height of 100 m could be considered as a computational unit. When the plants in this 1000 m3 air column consume the heat DQ through evapotranspiration, the surrounding air temperature reduce DT. Therefore, the heat consumed (DQ) from evapotranspiration could be determined by temperature reduction (DT) and the volume heat capacity of the air (rc) in Equation (1). The empirical model has been usually adopted to measure the latent heat consumption of urban forests through evapotranspiration in China (e.g. Refs. [27,32,36].

DQ ¼ DT
rc

(1)

where DQ represents the heat absorbed by the green space from the surrounding air through plant evapotranspiration (J/(m3
h)), rc is the volume heat capacity of the air (1256 J/(m3
C)), and DT is the decreased air temperature ( C), which can be acquired from Table 2. So we take advantage of Equation (1) to calculate the heat absorbed via evapotranspiration by Beijing’s urban green spaces. However, the above-mentioned case studies in Beijing neglected to measure the cooling effects of Shrub and Tree-Grass structure types, so we calculated the heat absorption capacity of the Shrub type based on the leaf area ratio of Shrub to Grass [7], and assumed

Fig. 3. Spatial distribution of seven structural types of urban green space within the 6th Ring Road of Beijing.

40

B. Zhang et al. / Building and Environment 76 (2014) 37e43

Table 1 Urban green space statistics for seven structural types in Beijing. Structure type

Definition/description

Area (ha)

Percentage (%)

Tree

Tree canopy density 0.2; total cover of shrubs and herbs <20%. Total cover of shrubs 20%; tree canopy density <0.2; total cover of herbs <20%. Total cover of herbs 20%; tree canopy density <0.2, total cover of shrubs <20%. Total cover of herbs <20%; tree canopy density <0.2, total cover of shrubs 20%. Tree canopy density <0.2; total cover of shrubs and herbs 20%. Total cover of shrubs <20%; tree canopy density is < 0.2; total cover of herbs 20%. Tree canopy density 0.2; total cover of shrubs and herbs 20%.

9708

21.89

Shrub

Grass

Tree-Shrub

Shrub-Grass

Tree-Grass

Tree-Grass-Shrub

40

0.09

188

0.42

6012

13.55

5553

12.52

1055

2.38

21,800

ES ¼

n X

(2)

j ¼ 1 i ¼ 1 where j represents the jth structure type, Ai is the area of ith green space area (ha). 2.2.2. Energy savings from the cooling effect of green space Exposure to elevated air temperatures can result in a spectrum of outcomes on human health and well-being ranging from mild discomfort to life-threatening medical conditions [11,23,25]; hence the intensification of heat stress episodes is limiting the quality of life in many cities [44]. To mitigate heat stress in urban environments, the inhabitants often consume more energy using air conditioners and increasing peak electrical demand. Urban green space helps cool urban environments lowering summertime energy demands. So, the environmental benefits of urban green space in the form of air temperature regulation can be related to energy savings from the cooling effect of green space. In view of the typical high temperatures and electrical loads during the Beijing summer [68], we choose 90 summer days as the study period; the hottest days of summer are usually June to August based on the cooling degree

Table 2 Structural type of urban green spaces in Beijing and their documented effects in reducing ambient temperature. Structural type

Green space type

Measurement station

Max air temperature reduction ( C)

Tree Grass

Avenue Residential green space Park green space Greenway Square green area Residential green space Residential green space Roadside green space

A B C D C B B E

2.34 [40] 0.9 [26] 0.8 [58] 1.3 [5] 1.9 [58] 1.3 [26] 4.8 [26] 2.6 [70]

Tree-Shrub Shrub-Grass Tree-Grass-Shrub

a
COP
Qji

(3)

where ES is the amount of energy savings through reducing air temperature by urban green space, COP (Coefficient Of Performance) is the efficiency of artificial cooling (air conditioner), and a represents the transformation coefficient for changing heat into electrical energy (1 J heat ¼ 0.278
106 kWh electrical energy).

49.15

DQji
Ai

n X

j ¼ 1 i ¼ 1

the absorbed heat capacity of the Tree structure type was the same as that of Tree-Grass. Thus, the total amount of heat absorbed (TQ) via evapotranspiration by urban green space in Beijing can be shown as follows:

TQ ¼

days measured in Beijing [59]. Based on the transformation coefficient between electrical energy and heat, the energy saved from the cooling effect of urban green spaces in Beijing can be determined by equation (3):

2.2.3. Emission reductions from the cooling effect of green space Under a scenario of climate change, the relationships between greenhouse gases (GHG), global warming and energy have attracted considerable public attention. Greenhouse gases, such as water vapor, carbon dioxide, methane, nitrous oxide, and fluorinated gases, absorb and emit radiation at specific wavelengths. The rising concentrations of GHG generally produce an increase in the average temperature of the Earth’s atmosphere. Most greenhouse gas emissions come from energy use around the world [2]. In urban environments, increasing atmospheric temperatures result in increased use of electrical energy and the production additional GHG emissions caused by the increased combustion of fossil fuel. Urban green spaces can reduce both air temperatures and energy needed to cool buildings, thereby lowering atmospheric CO2 emissions around the world. Estimates suggest generating 1 MWH of electrical energy is equivalent to producing about 1.1208 tons of CO2 emissions in Beijing, China [13]. Greenpeace’s annual report in 2009 revealed that the carbon intensity was 785 g/kWh from large electric power group in China. Based on this assumption, converted to kWh, the reduction in carbon emissions via temperature reduction by urban green spaces can be estimated as:

CE ¼

n X

b
ESji

(4)

j ¼ 1 i ¼ 1 where CE is the amount of CO2 emission reduction resulting from the electrical energy savings resulting from lowering air temperatures and b represents the CO2 emission factor of electrical energy (0.785 kg CO2 ¼ 1 kWh electric energy). 3. Results and discussion 3.1. Heat absorbed via evapotranspiration As China’s capital, Beijing has undergone rapid urbanization [69]. Accompanied by rapid urbanization, the creation of an UHI has become a serious environmental problem. Can the city mitigate this heat island effect and enhance its well-being by constructing more green spaces to help the city endure periods of intense heat? The estimate calculated here shows the urban green spaces in Beijing could absorb about 3.33
1012 kJ of heat through evapotranspiration on hot summer days. The urban green space with TreeShrub-Grass structure could absorb heat of 21.89
1011 kJ, which accounting for 65.7% of total absorbed heat. The next was Tree, Tree-Shrub, and Shrub-Grass, which absorbed about 6.15
1011 kJ, 2.61
1011 kJ and 1.96
1011 kJ, respectively. And the urban green spaces of Tree-Grass, Grass, and Shrub were relatively minor contributors to summertime heat absorption (see Fig. 4). The quantity

B. Zhang et al. / Building and Environment 76 (2014) 37e43

Average absorbed heat by per ha green space 8 15000 6 10000 4 5000

2

0

0 Tree

Shrub

Grass

Tree-Shrub

Tree-Grass

Shrub-Grass Tree-ShrubGrass

Structure type of urban green sapces

Fig. 4. Spatial extent, levels of heat absorption and heat absorption capacities of seven structure types of urban green spaces in Beijing. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

variances of absorbed heat by seven types of Beijing’s green spaces showed closely relationship with their areas. On average, per hectare urban green area in Beijing could absorb heat of 8.35
108 J/day via evapotranspiration. A previous study conducted by Chen et al. (1998) [7] estimated per hectare urban greening land in Beijing could consume heat energy of 4.48
108 J/ day. Therefore, our estimated result is consistent with this previous study. Also, the average amounts of heat absorbed by urban green spaces showed distinct differences in seven structure types (see Fig. 4 and Table 2). The Tree-Grass-Shrub green space in Beijing possessed the highest heat absorbing capacity of 11.16
108 J/ (ha
d). The next was Tree-Grass and Tree, with an average of 7.04
108 J/(ha
d) during summer. The heat absorbing capacities of Tree-Shrub, Shrub-Grass, and Shrub reached 4.83
108 J/ (ha
d), 3.92
108 J/(ha
d) and 3.23
108 J/(ha
d), respectively. For the Grass, the heat consumption per hectare of urban green space was 2.56
108 J/day through evapotranspiration. Fig. 4 presents the simulated heat absorbing capacities and amounts for seven structural types of urban green space in Beijing. The low values show the space potentially available for additional green space community structures. 3.2. Energy saving from cooling effect In the summer, elevated temperatures make people consume more energy to produce a more comfortable environment, especially in cities suffering from the development of heat islands. Estimates show the net cooling energy usage in residential areas of Beijing reached 4.87
108 kWh in 2005 and 5.70
108 kWh in 2006 [28]. When the urban environment temperature is higher than 26  C, the electrical burden will increase by 3.97
108 W for each 1  C of increased temperature in Beijing [68]. Evapotranspiration from urban trees, alone or in combination with shading, can produce an “oasis effect” during summer in which urban ambient temperatures are lowered significantly, so buildings enclosed in such cooler environments will consume less energy for air conditioning [2]. At present, the common COP of household air conditioner is 2.6e3.2 in Beijing. Based on our estimate, the electrical demand for air-conditioning in buildings in summer is being reduced by 3.09
108 kWh as a result of the cooling effect of urban green spaces in Beijing. Also, the cooling-energy savings could potentially account for a 60% reduction in net cooling energy usage in Beijing’s urban landscape. Western studies have documented similar results of energy savings from the presence of urban forests.

For example [1], found the seasonal cooling-energy savings of shade trees on two houses in Sacramento reached about 30%. Properly located trees and shrubs around a mobile trailer in Florida reduced the daily air-conditioning electricity use by as much as 50% [38]. Also, the energy savings associated with cooling effects varied with the locations of urban green spaces in Beijing. There were 2.57
104 ha of urban green spaces between the 5th and 6th ring roads, and they could annually save 1.66
108 kWh with the average of 0.65 kWh/(m2 a). The next was the urban green space located between the 4 and 5th ring roads with the area of 1.15
104 ha, which reduce the summertime energy demand by 8.29
107 kWh annually, and the average capacity was 0.72 kWh/ (m2 a). The energy savings of urban green spaces located between the 3 and 4th ring roads and the 2 and 3rd ring roads reached 2.84
107 kWh and 1.94
107 kWh annually, on the average of 0.82 kWh/(m2 a) and 0.84 kWh/(m2 a), respectively. However, the urban green space within the 2nd Ring Road, which covers 1465 ha, lowered the energy usage by 1.24
107 kWh, with the average capacity of 0.84 kWh/(m2 a). Thus, the total amount of energy savings increased from the inner to the outer areas of Beijing city, with a decreased energy saving capacities of per unit green space in the outer areas (Fig. 5). 3.3. Carbon emission reduction from cooling effect World energy use is the main contributor to atmospheric CO2 [1], so a reduction in energy consumption can decrease CO2 emissions from the fossil fuel burned in power plants. In recent years, the total amount of energy consumed and CO2 emitted annually increased by 5.1% and 4.0%, respectively, while the average amount of CO2 emissions caused by energy usage reached 9.78 t/person [6]. Since the cooling effects of urban green spaces in Beijing can lower energy demand, an increased use of urban green spaces can lower CO2 emissions from power plants. Our simulations show the emission of about 243 thousand tons of CO2 per summer could be avoided from power plants, because of the cooling effect of urban green spaces in Beijing. Analysis of the spatial variations of CO2 emission reduction in different areas is also necessary. Fig. 6 shows the amounts of CO2 emission reduction caused by green spaces in different districts or counties. The Chaoyang District green space had the highest contribution, accounting for 30% of the total amount of green space CO2 emission reduction. Next were the Haidian and Fengtai green spaces with 24% and 10% reductions in total emissions, respectively.

20

1

16

0.8 Electric energy saving

12 8

0.6

Energy saving of per unit green space

0.4

4

0.2

0

Energy saving of per unit green space (KWh/m2·a)

Absored heat (10 11J)/Green space (ha)

10

Amount of absorbed heat

20000

Average absorbing heat capacity of per hectare urban green space (10 8J/ha∙d)

12 Area of urban green space

Electric energy saving (107KWh)

25000

41

0 Within the 2nd ring roads

Between 2nd Between 3rd Between 4th Between 5th and 3rd ring and 4th ring and 5th ring and 6th ring roads roads roads roads

LocaƟon Fig. 5. Energy savings from the cooling effects provided by urban green spaces located between different ring roads in Beijing.

80000

80

60000

60

Total amount of CO2 emission reducƟon

40000

40

CO2 emission reducƟon per ha green area

20000

20

Haidian

Chaoyang

Fengtai

Shijingshan

Tongzhou

Changping

Daxing

Fangshan

Xicheng

Dongcheng

0

Shunyi

0

Average amount of CO2 emiision reducƟon per ha green area (t/ha·day)

B. Zhang et al. / Building and Environment 76 (2014) 37e43

Mentougou

Total amount of CO 2 emission reducƟon(t)

42

County or district Fig. 6. CO2 emission reduction from the urban green spaces in different counties or districts within 6th Ring Road of Beijing.

The green spaces in other districts and counties made smaller contributions to CO2 emission reductions from theirs cooling effects. So, the green spaces of Chaoyang, Haidian, Fengtai, Shijingshan, and Changping were the main contributors to CO2 emission reduction, and their cumulative ratio reached 80%. The saving associated with an annual reduction in CO2 emissions from power plants was 61 kg/(ha day). However, the greatest emission reduction capacity was the Dongcheng district green space, followed by Xicheng, Chaoyang, Shijingshan, Haidian, and Mentougou; the lowest was the Shunyi green space (see Fig. 6). The emission reduction capacities of different regions were affected by the spatial extent and structural types of urban green spaces.

4. Conclusions and discussions In the context of the emerging issues related to climate change and heat stress episodes, a collaborative research study related to the relationship between UHIs and green areas should be conducted. Many empirical studies indicate urban green spaces are particularly beneficial to improving urban microclimates, thereby mitigating the effects of UHIs. Few investigations measured the environmental benefits of cooling effects provided by urban green spaces. The present study quantified the energy-saving and emission-reduction ecological benefits provided by seven types of urban green spaces in Beijing. Results show the current urban green spaces in Beijing should be absorbing 3.33
1012 kJ of heat via evapotranspiration during summer, which reduces of 3.09
108 kWh of electrical power demand for air conditioning and prevents 243 thousand CO2 emissions from power plants. The cooling effect of urban green spaces in Beijing contributed a 60% energy savings in net cooling energy usage. Also, the cooling effects and associated energy-savings and emission reductions of urban green spaces showed relatively strong correlation with the structural types in each area. So, this paper described the impacts of summertime UHIs and evaluated the benefits of using urban green spaces to lower ambient temperatures in Beijing, and contributes to the theoretical and empirical understanding of the human community’s well-being in relation to urban green spaces. Also, such evidence conveys the importance of the structural characteristics of green land. Urban foresters, ecologists, and landscape planners should take advantage of this type of data to plan, design and manage green spaces in heat island areas. In this way, communities can realize energy savings, and cities and urban neighborhoods can be made more livable.

However, a number of limitations of the present study should be acknowledged. For example, local meteorological condition, the characteristics of urban green spaces (such as species composition, plant age, microsite locations in relation to buildings), the albedo of city surface, and quantification method (shading, evapotranspiration, or both) have been ignored. Also, trees and other large vegetation can serve as windbreaks or wind shields to reduce wind velocity in the vicinity of buildings. In certain climates, tree shelterbelts are used to block hot and dust-laden winds. In the summertime, the impacts can be positive and negative [15]. So, the effects of local weather conditions and wind-driven transport of pollutants should be taken into account. Air temperature reduction during the day, caused by evapotranspiration, may lead to higher temperatures during the night. The data provided by authors just describe the effects of urban green spaces on outdoor temperatures, such as avenues or Yuan Da-Du Park, and the assessments were based on these data. However, in summer, inhabitants use air conditioners in buildings. So, the assessment for the energy savings from the cooling effect of green space is perhaps inaccurate. The real effect of “green cooling” on the building near green zones and other zones may also need to be further investigated. Therefore, we will conduct more specific and in-depth studies to minimize the uncertainty of quantification and valuation of this ecosystem service. Acknowledgments This work was supported by a grant from the National Natural Science Foundation of China (No. 31200531) and the National Science and Technology Support Program (No. 2012BAC01B08). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.buildenv.2014.03.003. References [1] Akbari H, Kurn DM, Bertz SE, Hanford JW. Peak power and cooling energy savings of shade tree. Energ Build 1997;25:139e48. [2] Akbari H. Shade trees reduce building energy use and CO2 emissions from power plants. Environ Pollut 2002;116:119e26. [3] Beijing Statistical Bureau. Beijing statistical yearbook 2010. Beijing: China Statistics Press; 2010 [in Chinese]. [4] Berry R, Livesley SJ, Aye L. Tree canopy shade impacts on solar irradiance received by building walls and their surface temperature. Build Environ 2013;69:91e100. [5] Chen J, Cui S, Li ZY. Summer effect of street trees and lawns on microclimate in Beijing. J Beijing Univ 1983;1:15e25 [in Chinese]. [6] Cai SJ, Chen HY. Research on the relationship between energy consumption and CO2 emission reduction in Beijing. Beijing: Metallurgical Industry Press; 2010 [in Chinese]. [7] Chen ZX, Su XH, Liu SZ, Gu RZ, Li YM. A study of ecological benefits of urban forests in Beijing (2). J Chin Landsc Archit 1998;14(2):51e3 [in Chinese]. [8] Chang CR, Li MH, Chang SD. A preliminary study on the local sool-island intensity of Taipei city parks. Landsc Urban Plan 2007;80:386e95. [9] Cohen P, Potchter O, Matzarakis A. Daily and seasonal climatic conditions of green urban open spaces in the Mediterranean climate and their impact on human comfort. Build Environ 2012;51:285e95. [10] Correa E, Ruiz MA, Canton A, Lesino G. Thermal comfort in forest urban canyons of low building density. An assessment for the city of Mendoza, Argentina. Build Environ 2012;58:219e30. [11] Dessai S. Heat stress and mortality in Lisbon, Part Ⅰ. Model construction and validation. Int J Biometeorol 2002;47:6e12. [12] Dimoudi A, Nikolopoulou M. Vegetation in the urban environment: microclimatic analysis and benefits. Energy Build 2003;5:69e76. [13] Dong HJ, Geng Y. Study on carbon footprint of the household consumption in Beijing based on input-output analysis. Resour Sci 2012;34(3):494e501 [in Chinese]. [14] European Environment Agency (EEA). Environment and health. EEA Report No. 10/2005. Retrieved April 12, 2008 from, http://reports.eea.europa.eu/eeareport-2005-10/en/EEA-report-10-2005.pdf; 2005.

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