Carbon savings resulting from the cooling effect of green areas:A ca study in Beijing
Wenqi Lin a ,*,Tinghai Wu b ,Chengguo Zhang b ,Ting Yu a
a Key Laboratory of the Ministry of Education for Urban-rural Ecological Planning and Green Architecture,School of Architecture,Tsinghua University,Beijing 100084,China b
Institute of Architectural and Urban Studies,Tsinghua University,Beijing 100084,China
a r t i c l e i n f o
Article history:
Received 14August 2010Received in revid form 2February 2011
樱桃用英语怎么说Accepted 24February 2011Keywords:
Carbon savings Cooling effect Green areas Beijing
a b s t r a c t
Green areas cool the climate of a city,reduce the energy consumption caud by the urban heat island (UHI)effect,and bring along carbon savings.However,the calculation of carbon savings due to the cooling effect of green areas is still not well understood.We have ud a Landsat Enhanced Thematic Mapper Plus (ETM þ)image of Beijing,to identify the cooled areas,compute the possible energy ud to maintain the temperature differences between cooled areas and their surrounding heated areas,and calculate the carbon savings owing to the avoidance of energy u.Results show that a total amount of 14315.37tons carbon savings was achieved in the study area and the amount was related to the biomass,the size and the shape of green areas.The results demonstrate the importance of carbon savings resulting from green areas ’cooling effect.
Ó2011Elvier Ltd.All rights rerved.
1.Introduction
Increa in atmospheric temperature is thought by many to be resulted from incread atmospheric carbon dioxide (CO 2),which is largely attributable to fossil fuel combustion (about 75percent)(McPherson et al.,1994).About two-thirds of the world ’s total electric power is generated from fossil fuels (Dreslhaus and Thomas,2001),of which about one-sixth is ud to air-condi-tion buildings in t
he United States.Of this one-sixth electricity,about half is ud in cities classi fied as ‘‘heat islands ’’(Akbari,2002).For example,Akbari et al.(1992)reported that 1 C increa in temperature would increa peak electricity demand by 2e 4%when temperature exceeded 15e 20 C.The same phenomenon occurred in China.According to the calculation of Beijing Electric Power Corporation,Beijing ’s summer air-condi-tioning peak demand accounted for 40percent of the total peak electricity load (Beijing Municipal Commission of Development and Reform,2006).Air-conditioning demand is higher in urban areas than in the surrounding suburbs becau temperature in urban areas is usually much higher than that of suburbs.There-fore,the cooling effect of urban green areas will decrea the emission of atmospheric CO 2.
In reducing atmospheric CO 2,urban green areas offer double bene fits.First,trees in green areas directly quester and store atmospheric carbon.Second,green areas cool local urban areas by trees ’transpiration and shade,reducing air-conditioning energy u and avoiding carbon emission,that is,carbon savings.Ronfeld et al.(1995)estimated that 27million tons of carbon emission reduction would be achieved if 20%of air-conditioning could be avoided in all US cities by cool surfaces and tree shade.US Environmental Protection Agency (EPA)(2008)recommended increasing the percentage of vegetated area as a common mitiga-tion strategy of UHI.Rowntree et al.(1982)estimat
ed that about 25e 50%of the incread temperature attributed to urban heat island (UHI)could be mitigated via vegetation.Nowak (1993)estimated that carbon emissions avoided annually due to energy conrvation from existing trees throughout the city of Chicago and the Chicago area of Cook and Dupage counties was approxi-mately 12,600tons.However,few studies have attempted to calculate the carbon savings resulting from the cooling effect of green areas on the park scale as a whole,which covers green areas and their surroundings.Only a few experimental investigations were performed on the carbon emission avoided of one or veral trees (Akbari,2002;Jo and McPherson,2001),and the carbon savings that resulted from green areas ’cooling effect still remains unclear.
This paper provides an alternative holistic method,which is different from the method of measuring single trees and then by accumulation calculating the carbon savings resulted from urban
*Corresponding author.Tel.:þ8613911587258;fax:þ861062771154.E-mail address:linwq@mail.tsinghua.edu (W.
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0269-7491/$e e front matter Ó2011Elvier Ltd.All rights rerved.doi:10.vpol.2011.02.035
Environmental Pollution 159(2011)2148e 2154put down
green areas’cooling effect.Though urban green areas offer the double benefits of direct carbon storage and indirect carbon savings,the latter is the main concern of this paper.
Becau the cooling extent of a green area goes beyond its boundary,and extends into its surrounding areas(Jauregui,1991; Oke,1982;Upmanis et al.,1998),calculation of the carbon savings caud by green areas’cooling effect should not only take into account the green areas themlves,but also their extended cooled areas.By using ETMþ,wefirst identified the cooled areas resulted from urban green areas’cooling effect bad on the land surface temperature(LST).The cooled areas consist of green areas and their extended cooled areas.Then we calculated the energy that would be ud to maintain the temperature differences between the cooled areas and their surrounding urban heated areas.Finally,we calculated the avoided carbon emissions by accumulating the energy conrvation of unit green areas to get the total carbon savings.
Furthermore,green areas’cooling extent is influenced by both the characteristics of the green area itlf,such as its quality,size and shape,and tho of the surrounding area,such as building density and height,and street direction(Lee et al.,2009;Upmanis et al.,1998).It is esntial to identify the relation of carbon savings with the factors.However,until now few investigations have been made to reveal the relations.
To investigate what factors affected the magnitude of carbon savings,we systematically analyzed the relationship between carbon savings and the characteristics of green areas,such as biomass,size and shape.The factors affect the green area’s cooling effect and will also alter the amount of carbon savings.The results showed that there were significant correlations between carbon savings and the above-mentioned factors,which demon-strated:(1)more biomass would achieve better cooling effect and more carbon savings;(2)more small green areas would be better than fewer large green areas under the same sum of size;and(3) green areas of complicated shapes would perform better than simple-shaped ones of the same size.
This paper is organized into the following three ctions.In Section1,the study area and the methods are introduced,by which step-by-step calculation is outlined.The amount of carbon savings of the study area and its relation with the green areas’characteristics are demonstrated in Section2,and the
results of carbon savings are analyzed and discusd in Section3.
2.Study area and methods
2.1.Study area
Beijing(39 540N,116 230E)is a metropolis located at the North China Plain.A branch range of the Tai-hang Mountains runs to the west of the city and the Yan-shan Mountains to the north.In2009, Beijing’s residential population reached17.55million(Beijing Municipal Bureau of Statistics,2010),with the population density being1069per km2.Our study area covers2268.89km2,who boundary roughly coincides with the city’s6th Ring Road(Fig.1). Urban built-up areas,green belts,agricultural land,and water bodies are included in the study.According to official statistics, during the past ten years,the monthly mean temperature of the study area has been21.3 C in September,while the amounts of evaporation and precipitation have been194.9mm and316.5mm respectively.
2.2.Methods
2.2.1.Data preparation
First,land surface temperature was retrieved with mono-window algorithm(as shown in the following
equation)(Qin et al., 2001),and relevant parameters calibrated.
T s¼½að1ÀCÀDÞþðbð1ÀCÀDÞþCþDÞT6ÀD T a =C(1) C¼s3(2) D¼ð1ÀsÞ½1þsð1À3Þ (3) where T a is the average atmospheric effect temperature,s is atmospheric transmission ratio,and e is land surface emissivity.We had T a¼286.418K,s¼0.806903836,and the land surface emis-sivity values of different underlying surface categories were
as Fig.1.Location of Study Area.
W.Lin et al./Environmental Pollution159(2011)2148e21542149
follows:water bodyÀ0.995,surface of buildingÀ0.971,and vegetationÀ0.986(Qin et al.,2001).
Then,NDVI(Normalized Difference Vegetation)and MNDWI (Modified Normalized Difference Water Index)were calculated respectively.The calculation methods were(Han-qiu,2005; Nemani and Running,1989):
NDVI¼ðr NIRÀr RedÞ=ðr NIRþr RedÞ(4) where r NIR is the reflection value of wave band TM4,and wave band TM30s is denoted by r Red;and
MNDVI¼ðr GreenÀr MIRÞ=ðr Greenþr MIRÞ(5) where r Green is the reflected value of Green-light wave band,and r MIR is that of mid-infrared wave band.We classified land cover bad on the values of NDVI and MNDWI(Pixels with NDVI!0.157 were treated as vegetation,while who NDVI<0.157were divided into water bodies with MNDWI!0,and buildings with MNDWI<0).
girlicious2.2.2.Determine green area’s cooling extent
Green area’s cooling extent consists of two types of“sub-region-s”d one is green area(GA)itlf,and the other is extended cooled area(ECA)around GA.As GAs would form the lower-temperature cores on the land surface,depending on different natures of the underlying surfaces,ECAs were in fact the collecting areas around GAs,or if described metaphorically such structure is analogous to the rervoir and its surrounding catch basin.GAs were demarcated on satellite images of the study area according to“the Prent Green Area Map of the Central City”in the Master Plan for Beijing.In the meantime,ECAs were delineated by determining the catch basins of GAs on the land surface temperature map of the study area.
2.2.
3.Calculate the possible energy ud(energy conrvation)to maintain the temperature differences
The temperature of a GA and its ECA is lower than their surrounding urban heated areas becau of tree shade and evapo-transpiration in the GA.To maintain the temperature differences, how much energy may possibly be ud?
上海夜校To calculate roughly the possible energy ud to maintain the temperature differences,an ideal cylindrical model is established as follows(Fig.2):imagine GA is a round-shaped zone,and its ECA for
ms an outer ring-shaped area surrounding GA.The latter’s land surface temperature is lower than built-up areas.Starting from the centre of GA,land surface temperature increas roughly in a linear fashion outwards.Temperature differences start to disappear when reaching the outer edge of the ECA.If viewed from a vertical angle, when reaching the upper limit of the influence sphere of GA,the temperature difference will disappear accordingly.
To calculate the amount of energy needed to sustain the temperature difference of such a cylindrical area,integral method was ud as shown in the following equation:
Q¼Q GAþQ ECA¼C p r$
Z H
Z r1
2p r V T d r d h
Z H
Z r2
r1
2p r V T d r d h
!
(6)
where C p stands for specific heat under constant air pressure with a value of1004.68J kgÀ1TÀ1,air density r¼1.2923kg mÀ3,vertical influence sphere of lower-temperature zone H¼70m,temperature gradient V T¼D T=D r,r1the radius of the cylindrical bottom surface (size equal to GA),and r2the radius of the bottom surface of the hollow cylindrical area(size equal to ECA).
2.2.4.Carbon savings and influencing factors
Urban GAs play the same role as the functioning of air-condi-tioner.Hence,the energy consumed by air-conditioners can be reduced or avoided by GA’s trees shade and evapotranspiration.If the same effect is to be realized by employing electric measures,the amount of CO2to be generated when producing the needed energy for air-conditioning will be just equivalent to what is to be saved by the GAs.Becau vegetation can achieve the same cooling effect through its bio-chemical process,the
zero-consumption of elec-tric energy will make CO2emissions from power generation avoidable.
The amount of atmospheric CO2emission of per unit energy (power)generation varies in different countries.In2008,China’s CO2emission rate of coal-fired power was863g/kW h.According
to
Fig.2.The Cylindrical Model for Calculating GA’s Heat Absorption.
W.Lin et al./Environmental Pollution159(2011)2148e2154
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the Comparative Carbon Dioxide Emissions from Power Generation (World Nuclear Association),CO2emission rate of coal-fired power in the UK was891g/kW h in the year of2006.The value of863 g/kW h was ud in our study to estimate carbon savings owing to the cooling effect of green areas on air-conditioning energy u by the assumption that the thermal power was generated by coal. Carbon savings was calculated by the avoided energy multiplying China’s CO2emission rate of coal-fired power.
Through statistical analysis,attempts were made to identify the main factors that might influence the carbon saving effect of GAs. The factors included:size,shape and biomass.Among them,the biomass factor was reprented by the NDVI index.The shape index (SI)of GA was calculated by the ratio of its perimeter to area size.
By respectively analyzing the above characteristics of GAs and ECAs,we discovered not only the main characteristics that had affected the cooling effect of GAs,but also how and to what extent they had done so.
3.Results
3.1.Total amount of carbon savings
Total amount of carbon savings is the sum of carbon savings of each GA.Firstly we substituted the bottom surface radius and temperature gradient of the GA cylinder and tho of ECA into the above Equation(6),and obtained the amount of energy ud to maintain the temperature differences by each GA.Secondly,we added up the amount of energy of each GA in the study area,and obtained the total amount as5.97Â1010kJ,which was equal to 16.59MKWh.
If using electric equipment to replace GAs,the projected amount of CO2emission to achieve the above temperature differences would be14,315.37tons.Thus it could be en that,in order to sustain a lower-temperature zone within the study area,when comparing the functioning of GAs’cooling effect with artificial refrigeration,the former would not only save 16.59MKWh of electric power,but also reduce14,315.37tons of CO2emission.
时刻表英文3.2.Relationship between carbon savings and NDVI(biomass)
NDVI is an indicator of a GA’s biomass.Wefirst ud all the GA samples to obtain the relationship between carbon savings and NDVI,and found that they emed to be unrelated.After further analysis,we realized that the relation might be concealed by location and factors of size and shape,which should have been excluded in this process.Hence,we lected tho GAs at almost the same location and with almost the same size and shape index, performed the regression analysis again,and found that there really existed a relationship between carbon savings and NDVI.Fig.3 shows that carbon savings of GAs increas with the increasing NDVI by Equation(7).However,becau it was difficult tofind more samples at almost the same location and with almost the same size and shape,more general equation could not be concluded from our current investigation.
Y¼15:14XÀ2:225
R2¼0:545
(7)
3.3.Relationship between carbon savings and size
3.3.1.Relationship between carbon savings and size of GAe hentai galleries
Fig.4shows that both carbon saving amounts of GA and ECA change with respect to their size statistically.Fig.4a demonstrates the relationship between amount of carbon savings and size of GAs by Equation(8).This result shows that carbon saving amount of GA is directly and significantly related to GA size.The slope of the trend line in Fig.4a is steeper than that of Fig.4b,which implies that
an Fig.3.Relationship between GA’s NDVI and its Carbon saving
Amount.
Fig.4.a Relationship between GA’s size and its carbon saving amount.b Relationship between ECA’s size and its carbon saving amount.
W.Lin et al./Environmental Pollution159(2011)2148e21542151
increa in the size of GA itlf would have a bigger effect for carbon savings than the same size-increa in its ECA.
Y ¼75:80X À1:679
R 2
¼0:9922020年7月六级真题及答案
(8)Y ¼34:99X À1:93 R 2
¼0:954
(9)
3.3.2.Relationship between carbon savings of ECA and size of GA
Fig.4b illustrates similar relationship between amount of ECA carbon saving and GA size by Equation (9),showing that GA size has direct and signi ficant in fluence on amount of ECA carbon saving.Fig.5shows that per unit-area carbon saving amount of ECA decreas with the increa in the size of as far as the carbon savings of ECA is concerned,when the total GA sizes are equal,the effect of more small-size GAs will be superior to that of fewer large-size ones.
The scatter graph in Fig.5also depicts that when the size of a GA was smaller than 0.1km 2,per unit ECA carbon savings will decline signi ficantly with the increa of GA ’s size;and while the GA ’s size is larger than 0.10km 2,changes in the per unit ECA ’s carbon saving amount will not be that evident.
Y ¼À14:1ln ðX Þþ26:39 R 2¼0:425
(10)
3.4.Relationship between carbon savings and shape
3.4.1.Relationship between carbon savings and shape of GA
To eliminate the in fluence of GA size on the relationship between carbon savings and GA shape,we ud the factor of per unit-area carbon savings amount of GA and ECA instead of total carbon savings amount of GA and ECA to delineate the relation.Fig.6shows that GA ’s per unit-area carbon savings decreas slowly with increasing of GA shape index by Equation (11),illus-trating that shape index does have an effect on per unit-area carbon savings amount of GA,but its impact strength is relatively weak if compared with GA size.
Y ¼À0:01ln ðX Þþ0:006
R 2¼0:167
(11)
3.4.2.Relationship between carbon savings of ECA and shape of GA
ECA ’s carbon savings of per unit GA increas together with the increa of GAs ’shape indices d although such a positive
correlation was not marked.The scatter graph in Fig.7depicts the relationship between GA ’s shape and ECA ’s carbon savings of per unit GA.Results show that when a GA ’s shape index is smaller tha
n 0.055,the distribution of dots reprenting ECAs ’carbon saving amount is comparatively converging d as a result of shape changes;and while SI is larger than 0.055,the dots will show a dispersing trend,which is to say,when the shape of GA tends to be quite irregular,factors in fluencing its ECA ’s carbon savings will be plural,and the shape index of GA alone d as a single factor d can no longer fully express its ECA ’s carbon-saving effect.
Y ¼À0:625X 0:730dairy
R 2¼0:133
(12)
4.Discussion
4.1.Total amount of energy conrvation
To maintain the temperature difference between the GAs ’cooled areas and their surrounding heated areas,the possible energy conrvation by GAs is estimated to be 5.97Â1010kJ.
This
Fig.5.Relationship between GA ’s size and per unit-area carbon saving amount of
ECA.
Fig.6.Relationship between GA ’s shape index and its per unit-area carbon saving
amount.
欺侮Fig.7.Relationship between GA ’s shape index and ECA ’s carbon saving amount of per unit GA.
W.Lin et al./Environmental Pollution 159(2011)2148e 2154
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