Quantifying Air Pollution Removal By Green Roofs In Chicago

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Atmospheric Environment 42 (2008) 7266–7273Contents lists available at ScienceDirectAtmospheric Environmentjournal homepage: www.elsevier.com/locate/atmosenvQuantifying air pollution removal by green roofs in ChicagoJun Yang a, c, *, Qian Yu b, Peng Gong caDepartment of Landscape Architecture and Horticulture, Temple University, 580 Meetinghouse Road, Ambler, PA 19002, USADepartment of Geosciences, University of Massachusetts, 611 N Pleasant Street, Amherst, MA 01003, USAState Key Lab of Remote Sensing Science, Jointly Sponsored by Institute of Remote Sensing Applications, Chinese Academy of Science and Beijing NormalUniversity, Beijing 100101, Chinabca r t i c l e i n f oa b s t r a c tArticle history:Received 17 March 2008Received in revised form 30 June 2008Accepted 2 July 2008The level of air pollution removal by green roofs in Chicago was quantified using a drydeposition model. The result showed that a total of 1675 kg of air pollutants was removedby 19.8 ha of green roofs in one year with O3 accounting for 52% of the total, NO2 (27%),PM10 (14%), and SO2 (7%). The highest level of air pollution removal occurred in May andthe lowest in February. The annual removal per hectare of green roof was 85 kg ha 1 yr 1.The amount of pollutants removed would increase to 2046.89 metric tons if all rooftops inChicago were covered with intensive green roofs. Although costly, the installation of greenroofs could be justified in the long run if the environmental benefits were considered. Thegreen roof can be used to supplement the use of urban trees in air pollution control,especially in situations where land and public funds are not readily available.Ó 2008 Elsevier Ltd. All rights reserved.Keywords:Extensive green roofsIntensive green roofsDry depositionCost1. IntroductionCity air often contains high levels of pollutants that areharmful to human health (Mayer, 1999). The AmericanLung Association (ALA, 2007) reported that over 3700premature deaths annually in the United States could beattributed to a 10-ppb increase in O3 levels. Worldwide, theWorld Health Organization (WHO, 2002) estimated thatmore than 1 million premature deaths annually could beattributed to urban air pollution in developing countries.The United Nations Population Fund (UNFPA, 2007) predicted that the urban population worldwide wouldincrease from 3.3 billion in 2008 to 5 billion by 2030,meaning that there will be an increase in sensitive population groups such as children and the elderly. Therefore,cities with serious air pollution problems need to come upwith ways to control the problem and reduce the damages.* Corresponding author. Department of Landscape Architecture andHorticulture, Temple University, 580 Meetinghouse Road, Ambler, PA19002, USA. Tel.: þ1 267 468 8186; fax: þ1 267 468 8188.E-mail addresses: [email protected] (J. Yang), [email protected](Q. Yu), [email protected] (P. Gong).1352-2310/ – see front matter Ó 2008 Elsevier Ltd. All rights ional air pollution management programs focuson controlling the source of air pollutants (Schnelle andBrown, 2002). This strategy effectively reduces the emission of new air pollutants but does not address thepollutants already in the air. Innovative approaches can beadopted to remove existing air pollutants thereby reducingair pollution concentrations to an acceptable level. One wayto reach that goal is the use of urban vegetation which canreduce air pollutants through a dry deposition process andmicroclimate effects. The high surface area and roughnessprovided by the branches, twigs, and foliage make vegetation an effective sink for air pollutants (Beckett et al.,1998; Hill, 1971). Vegetation also has an indirect effect onpollution reduction by modifying microclimates. Plantslower the indoor air temperature through shading, thusreducing the use of electricity for air conditioning (Heisler,1986). The final result is that the emission of pollutantsfrom power plants decreases due to reduced energy use.Vegetation also lowers the ambient air temperature bychanging the albedos of urban surfaces and evapotranspiration cooling. The lowered ambient temperature thenslows down photochemical reactions and leads to lesssecondary air pollutants, such as ozone (Akbari, 2002;

J. Yang et al. / Atmospheric Environment 42 (2008) 7266–7273Rosenfeld et al., 1998). Studies show that trees couldcontribute significantly to air pollution reduction in cities(Nowak, 1994; Nowak et al., 2006; Rosenfeld et al., 1998;Scott et al., 1998). Nowak et al. (2006) estimated that urbantrees remove a total of 711 000 metric tons annually in theU.S. These findings led to the inclusion of tree planting asa state implementation strategy for improving air qualityby the United States Environmental Protection Agency(EPA) in 2004 (US EPA, 2004).While it is desirable to use trees for controlling airpollution, it is not always easy to plant trees in a denselypopulated city. For example, the percentage of imperviousarea in New York City is 64%; it can reach as high as 94% indistricts like Mid-Manhattan west (Rosenzweig et al., 2006).The green roof can be a solution to this dilemma since itmakes use of rooftops, usually 40–50% of the impermeablearea in a city (Dunnett and Kingsbury, 2004). Nevertheless,the limited number of studies on the air pollutant removalcapacity of green roofs does not provide enough information for people to judge their effectiveness in air pollutioncontrol. The methods and main findings of the few reportedstudies are summarized in the following section.Currie and Bass (2005) estimated that 109 ha of greenroofs in Toronto could remove a total of 7.87 metric tons ofair pollutants annually. They pointed out in their paper thatthe urban forest effects (UFORE) model they used wasdeveloped specifically for trees and shrubs. The majority ofplants used on green roofs are herbaceous plants whichwould have an impact on estimates when using this model.Deutsch et al. (2005) conducted a simulation of differentplanting scenarios of green roofs in Washington, DC, usingthe UFORE model. They showed that 58 metric tons of airpollutants could be removed if all the roofs in the city wereconverted to green roofs. Corrie et al. (2005) estimated theannual reduction of NO2 by green roofs in Chicago andDetroit. Their study showed by covering 20% of the roofsurface in Chicago the reduction of NO2 was between 806.48and 2769.89 metric tons depending on the type of plantsused. These estimates were reached by assuming the NO2uptake rates by green roof plants were constant. This couldbe problematic because NO2 uptake is influenced by manyfactors (e.g., meteorological conditions, concentration ofNO2, plant physiology). In one field study, Tan and Sia (2005)measured the concentrations of acidic gaseous pollutantsand particulate matters on a 4000 m2 roof in Singaporebefore and after the installation of a green roof. They foundthat the levels of particles and SO2 in air above the roof werereduced by 6% and 37%, respectively, after installation of thegreen roof. This field measurement proved that green roofscan reduce certain air pollutants but it is difficult toextrapolate their results to other places or to a larger scale.The measurement was site specific and the volume of airthat was influenced by the green roof was not given.The cases discussed above have shown the potentialbenefit of using green roofs in air pollution control.However, there are many aspects of this mitigationmeasure that remain unclear. More studies are needed tohelp cities decide whether the green roof can be an effective way to improve air quality. We believe the followingquestions need to be answered: How can we quantify thelevel of air pollutant removal after installing green roofs in7267one city? Is there a difference between different types ofgreen roofs in the level of air pollutant removal? How doesthe green roof compare to other mitigation measures suchas planting trees? In this paper, we will address thosequestions with a case study in Chicago, Illinois.2. Study site and methods2.1. Study siteThis study took place in Chicago, Illinois, which islocated along the southwest shore of Lake Michigan with a center coordinate of 41 530 N and 87 390 W. The total areaof the city is 588.3 km2. Chicago is the third most populouscity in the U.S with a population of 2.9 million in 2000.According to ALA (2007), over 2 million people in Chicagowere at heightened risk for health problems resulting fromacute exposure to O3 and particulate matters.Chicago is ranked number one in terms of total area ofinstalled green roofs among North American cities.According to Taylor (2007), green roofs were installed on300 buildings resulting in a total area of 27.87 ha by June2007. There are three types of green roofs in Chicago:extensive green roofs, intensive green roofs, and semiintensive green roofs. Extensive green roofs are plantedwith low height and slow growing plants. The depth of thegrowth media is less than 15 cm. Intensive green roofsconsist of large perennial herbaceous plants and, occasionally, shrubs and small trees. The depth of growth mediaon an intensive green roof usually varies between 20 cmand 1.2 m. The semi-intensive green roof is a mixture ofextensive and intensive green roof with 25% or less of thearea as extensive green roof.2.2. Survey of green roofs in ChicagoA request for information was submitted to Chicago’sDepartment of Environment for a list of green roofsresulting in a list of 170 green roofs. Two steps were takento verify the list. First, information including the address ofthe green roof, type of the green roof, size, and the date itwas completed was gathered from various sources. Wethen searched the address of each green roof through animage database hosted by Pictometry International Crop.Digital aerial photographs covering Chicago were taken byPictometry International Corp in July 2006. Because thephotographs have a ground resolution of 16 cm and weretaken from multiple angles, the location, size, type of thegreen roof, and the type of building could be clearly interpreted. For each green roof, the area of grass, trees, andother surfaces was measured and the percentage to thetotal area was calculated. Pictometry software allows usersto directly measure distances and areas on those georeferenced images. The error margin of the measurement wasestimated to be 1% or smaller (Federal EmergencyManagement Agency, 2005).2.3. Removal of air pollutants by green roofsIn this study, a big-leaf resistance model was used toquantify the dry deposition of air pollutants. The structure

7268J. Yang et al. / Atmospheric Environment 42 (2008) 7266–7273of the model and how the input parameters were fitted areexplained below.The removal of a particular air pollutant at a given placeover a certain time period was calculated as (Nowak, 1994):Q ¼ F L T(1)where Q is the amount of a particular air pollutant removedby certain area of green roofs in a certain time period (g), Fis the pollutant flux (g m 2 s 1), L is the total area of greenroof (m2), and T is the time period (s). The pollutant flux F iscalculated as in Eq. (2):F ¼ Vd C 10 8(2)where Vd ¼ dry deposition velocity of a particular airpollutant (cm s 1), and C ¼ concentration of that pollutantin the air (mg m 3). The dry deposition process can bedescribed as the inverse of total resistance (Baldocchi et al.,1987):Vd ¼1Ra þ Rb þ RcVd ¼ Vg þ(5)Where Vg is the gravitational settling velocity, Ra is theaerodynamic resistance above the canopy, Rs is the surfaceresistance.The gravitational settling is calculated as.Vg ¼rd2p gC18h(6)Where r is the density of the particle, in this study, a valueof 1800 kg m 3 was used as suggested by Lim et al. (2006),dp is the particle diameter, g is the acceleration of gravity, Cis the correction factor for small particles and is calculatedas (Zhang et al., 2001), h is the viscosity coefficient of air.The aerodynamic resistance Ra is calculated as before.The surface resistance Rs is based on the size of depositionparticles, atmospheric conditions, and surface properties. Itwas calculated as (Zhang et al., 2001).(3)Rs ¼where Ra ¼ aerodynamic resistance, Rb ¼ quasi-laminarboundary layer, and Rc ¼ canopy resistance. The algorithmsfor calculating Ra and Rb were reported in Yang et al. (2005).In this study, the roughness length z0 and displacementlength d for short grasses were used to represent extensivegreen roofs. The intensive green roofs were treated asmixtures of short grass, tall herbaceous plants, and smalldeciduous tree. The z0 and d values used in the model arelisted in Table 1.The hourly canopy resistances Rc for O3, SO2, and NO2are calculated as (Walmsley and Wesely, 1996).1ðRa þ Rs Þ130 m ðEB þ EIM þ EIN ÞR1(7)Table 1Value of roughness lengths and displacement heights used in the modelWhere 30 is an empirical constant and taken as 3 here, m* isthe friction velocity. EB, EIM, and EIN are collection efficiencyfrom Brownian diffusion, impaction and interception,respectively. The re-suspension of particles after hittinga surface was modeled by modifying the total collectionefficiency by the factor of R1, which represents the fractionof particles sticking to the surfaces. The extensive greenroofs and intensive green roofs were modeled in the samemanner as in calculating Rc. Details on how those parameters were fitted can be found in Zhang et al. (2001).The final deposition velocity for PM10 was the weightaveraged Vd for all particles with a size less than 10 mm.Information on size classes and mass concentration ofparticles in Chicago were obtained from Offenberg andBaker (2000).Hourly air pollution data including NO2, SO2, O3, andPM10 concentration from an air pollution monitoringstation in central Chicago between 8/1/2006 and 7/31/2007were obtained from the U.S. EPA. Hourly surface meteorology data including sky condition, air temperature, relative humidity, atmospheric pressure, wind speed,precipitation, and snow cover measured by a station locatedat O’Hara International Airport for the same time periodwas obtained from the National Climatic Data Center. Thehourly solar radiation intensity was simulated by using themeteorological/statistical solar radiation model (METSTAT,Maxwell et al., 1995). During precipitation and when theground was covered by snow, the value of Vd was set as zerobecause the dry deposition process could not occur. Hourlyfluxes of NO2, SO2, O3, and PM10 to green roofs in Chicagowere calculated by using weather data, concentration ofpollutants, and the modeled deposition velocities.VegetationtypeAverageheight h0 (m)Z0 ¼ 0.1h0 (m)d ¼ 0.7h0 (m)2.4. Additional removal with different plantingscenarios and costsShort grassTall herbaceous plantsDeciduous trees0. ¼ ðRsx þ Rmx Þ 1 þ R 1lux 1 i 1 1 þ ðRdc þ Rclx Þ þ Rac þ Rgsx(4)In Eq. (4), Rsx is leaf stomata resistance, Rmx is leaf mesophyll resistance, Rlux is leaf cuticles resistance, Rdc is theresistance for gas-phase transfer by buoyant convection incanopies, Rclx is resistance by leaves, twigs, bark or otherexposed surfaces in the lower canopy, Rac is transfer resistance which depends only on canopy height and density,and Rgsx is ground surface resistance. Resistance components can vary with solar intensity, seasons, and vegetationtypes. Algorithms are available for calculating resistancecomponents for grass and deciduous trees. The tall herbaceous plants were modeled as crops in this study. Details ofthe algorithms were described in Wesely (1989); Walmsleyand Wesely (1996); Zhang et al. (2002).The deposition velocity of PM over green roofs wascalculated as (Zhang et al., 2001).Three future planting scenarios were assumed and theamount of air pollution removal for each scenario calculated.

J. Yang et al. / Atmospheric Environment 42 (2008) 7266–727320O36030SO2106NO2 by Aug/200631MonthFig. 1. Concentrations of criteria air pollutants in Chicago between August2006 and July 2007. The monthly mean values were shown in the figure.3. ResultsAmong the 170 green roofs included in the list, detailedinformation for 71 green roofs was obtained and verifiedthrough aerial photographs. The total area of those 71green roofs is 19.8 ha, 71% of the total area of green roofs inChicago reported by Taylor (2007).The information about those green roofs is shown inTable 2.The green roofs surveyed were located mainly oncommercial building and the size of each individual roofwas relatively large. Among the 71 green roofs, half had anarea larger than 500 m2 and 23 green roofs were larger than1000 m2. The green roof in the Soldier Field was 22 445 m2while the one in Millennium Park was 99 983 m2.Based on the analysis of aerial photographs, the 19.8 haof green roof consisted of 63% short grass and other lowgrowing plants, 14% large herbaceous plants, 11% trees andshrubs, and about 12% various structures and hard surfaces.The monthly air quality between August 2006 and July2007 in Chicago is shown below (Fig. 1).It can be seen from Fig. 1 that O3 was the main airpollutant in Chicago. PM10 ranked second while the SO2pollution was low. PM10 and O3 pollution peaked insummer while SO2 and NO2 peaked in winter.The monthly mean deposition velocities for air pollutants calculated for different vegetation types showeda seasonal trend (Table 3). The deposition velocities for allair pollutants were highest in May and lowest in February.The modeled monthly uptake of air pollutants by greenroofs is shown in Fig. 2.The total air pollution removal by 19.8 ha of green roofswas 1675 kg between August 2006 and July 2007. If theTable 2Percentages of different type of green roofs in ChicagoType of green roof On residential On commercial On officeTotal (%)buildings (%) buildings (%)buildings (%)ExtensiveIntensive/semiintensiveSub total (%)PM1030Concentration (ug/m3)The first scenario assumed planting all roofs in Chicago withthe same ratio of extensive vs. intensive green roofs usedcurrently. The second scenario assumed the remaining roofswould only be planted with extensive roofs. The thirdscenario assumed only intensive roofs would be used infuture projects. In all these scenarios, the intensive roof wastreated as a mixture of tall herbaceous plants and smalldeciduous trees and shrubs at a ratio of 50:50. The total areaof roofs in Chicago was obtained from Gray and Finster(2000) study, which showed that Chicago’s roof surface was27.86% of the urban area. According to information gatheredfrom the green roof companies and the literature, theaverage installation cost for green roofs are as follows:extensive green roofs between 107.64 and 161.46 per m2( 10– 15 per ft2); intensive green roofs between 161.46and 269.1 per m2 ( 10–– 25 per ft2). The medians of thoseranges were used in the calculation. The maintenance cost ofgreen roofs was not included in this 424.1581.9713.88100.00reported 27.87 ha of green roofs were all completed andhad the same ratio of extensive vs. intensive green roofs,the air pollutants removed could reach 2388 kg.Among the four air pollutants, the uptake of O3 was thelargest, 52% of the total uptake followed by NO2 (27%), PM10(14%), and SO2 (7%). Seasonally, the highest uptakeoccurred in May and the lowest in February. The annualremoval rate among different vegetation types is comparedin Table 4.If all remaining roofs in Chicago were planted withintensive green roofs, the direct removal of air pollutantscould reach as high as 2046.89 metric tons, assuming thesame level of air pollution as 2006–2007. However, theinstallation cost would be 35.2 billion.4. Discussion4.1. Evaluation of resultsThe results showed that air pollutant removal by greenroofs in Chicago was affected by air pollutant concentrations, weather conditions, and the growth of plants. Thehighest air pollutant removal occurred in M

on an intensive green roof usually varies between 20 cm and 1.2 m. The semi-intensive green roof is a mixture of extensive and intensive green roof with 25% or less of the area as extensive green roof. 2.2. Survey of green roofs in Chicago A request for information was submitted to Chicago’s Department