Quantifying air pollution removal by green roofs in Chicago

Abstract

The level of air pollution removal by green roofs in Chicago was quantified using a dry deposition model. The result showed that a total of 1675 kg of air pollutants was removed by 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 and the 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 in Chicago were covered with intensive green roofs. Although costly, the installation of green roofs could be justified in the long run if the environmental benefits were considered. The green 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.

Full text

Quantifying air pollution removal by green roofs in Chicago
Jun Yang
a
,
c
,
*
, Qian Yu
b
, Peng Gong
c
a
Department of Landscape Architecture and Horticulture, Temple University, 580 Meetinghouse Road, Ambler, PA 19002, USA
b
Department of Geosciences, University of Massachusetts, 611 N Pleasant Street, Amherst, MA 01003, USA
c
State Key Lab of Remote Sensing Science, Jointly Sponsored by Institute of Remote Sensing Applications, Chinese Academy of Science and Beijing Normal
University, Beijing 100101, China
article info
Article history:
Received 17 March 2008
Received in revised form 30 June 2008
Accepted 2 July 2008
Keywords:
Extensive green roofs
Intensive green roofs
Dry deposition
Cost
abstract
The level of air pollution removal by green roofs in Chicago was quantified using a dry
deposition model. The result showed that a total of 1675 kg of air pollutants was removed
by 19.8 ha of green roofs in one year with O
3
accounting for 52% of the total, NO
2
(27%),
PM
10
(14%), and SO
2
(7%). The highest level of air pollution removal occurred in May and
the 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 in
Chicago were covered with intensive green roofs. Although costly, the installation of green
roofs could be justified in the long run if the environmental benefits were considered. The
green 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.
Ó2008 Elsevier Ltd. All rights reserved.
1. Introduction
City air often contains high levels of pollutants that are
harmful to human health (Mayer, 1999). The American
Lung Association (ALA, 2007) reported that over 3700
premature deaths annually in the United States could be
attributed to a 10-ppb increase in O
3
levels. Worldwide, the
World Health Organization (WHO, 2002) estimated that
more than 1 million premature deaths annually could be
attributed to urban air pollution in developing countries.
The United Nations Population Fund (UNFPA, 2007)pre-
dicted that the urban population worldwide would
increase from 3.3 billion in 2008 to 5 billion by 2030,
meaning that there will be an increase in sensitive pop-
ulation groups such as children and the elderly. Therefore,
cities with serious air pollution problems need to come up
with ways to control the problem and reduce the damages.
Conventional air pollution management programs focus
on controlling the source of air pollutants (Schnelle and
Brown, 2002). This strategy effectively reduces the emis-
sion of new air pollutants but does not address the
pollutants already in the air. Innovative approaches can be
adopted to remove existing air pollutants thereby reducing
air pollution concentrations to an acceptable level. One way
to reach that goal is the use of urban vegetation which can
reduce air pollutants through a dry deposition process and
microclimate effects. The high surface area and roughness
provided by the branches, twigs, and foliage make vege-
tation an effective sink for air pollutants (Beckett et al.,
1998; Hill, 1971). Vegetation also has an indirect effect on
pollution reduction by modifying microclimates. Plants
lower the indoor air temperature through shading, thus
reducing the use of electricity for air conditioning (Heisler,
1986). The final result is that the emission of pollutants
from power plants decreases due to reduced energy use.
Vegetation also lowers the ambient air temperature by
changing the albedos of urban surfaces and evapotranspi-
ration cooling. The lowered ambient temperature then
slows down photochemical reactions and leads to less
secondary air pollutants, such as ozone (Akbari, 2002;
*Corresponding author. Department of Landscape Architecture and
Horticulture, Temple University, 580 Meetinghouse Road, Ambler, PA
19002, USA. Tel.: þ1 267 468 8186; fax: þ1 267 468 8188.
E-mail addresses: juny@temple.edu (J. Yang), qyu@geo.umass.edu
(Q. Yu), gong@irsa.ac.cn (P. Gong).
Contents lists available at ScienceDirect
Atmospheric Environment
journal homepage: www.elsevier.com/locate/atmosenv
1352-2310/$ – see front matter Ó2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.atmosenv.2008.07.003
Atmospheric Environment 42 (2008) 7266–7273

Rosenfeld et al., 1998). Studies show that trees could
contribute 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 urban
trees remove a total of 711000 metric tons annually in the
U.S. These findings led to the inclusion of tree planting as
a state implementation strategy for improving air quality
by the United States Environmental Protection Agency
(EPA) in 2004 (US EPA, 2004).
While it is desirable to use trees for controlling air
pollution, it is not always easy to plant trees in a densely
populated city. For example, the percentage of impervious
area in New York City is 64%; it can reach as high as 94% in
districts like Mid-Manhattan west (Rosenzweig et al., 2006 ).
The green roof can be a solution to this dilemma since it
makes use of rooftops, usually 40–50% of the impermeable
area in a city (Dunnett and Kingsbury, 2004). Nevertheless,
the limited number of studies on the air pollutant removal
capacity of green roofs does not provide enough informa-
tion for people to judge their effectiveness in air pollution
control. The methods and main findings of the few reported
studies are summarized in the following section.
Currie and Bass (2005) estimated that 109 ha of green
roofs in Toronto could remove a total of 7.87 metric tons of
air pollutants annually. They pointed out in their paper that
the urban forest effects (UFORE) model they used was
developed specifically for trees and shrubs. The majority of
plants used on green roofs are herbaceous plants which
would have an impact on estimates when using this model.
Deutsch et al. (2005) conducted a simulation of different
planting scenarios of green roofs in Washington, DC, using
the UFORE model. They showed that 58 metric tons of air
pollutants could be removed if all the roofs in the city were
converted to green roofs. Corrie et al. (2005) estimated the
annual reduction of NO
2
by green roofs in Chicago and
Detroit. Their study showed by covering 20% of the roof
surface in Chicago the reduction of NO
2
was between 806.48
and 2769.89 metric tons depending on the type of plants
used. These estimates were reached by assuming the NO
2
uptake rates by green roof plants were constant. This could
be problematic because NO
2
uptake is influenced by many
factors (e.g., meteorological conditions, concentration of
NO
2
, plant physiology). In one field study, Tan and Sia (2005)
measured the concentrations of acidic gaseous pollutants
and particulate matters on a 4000 m
2
roof in Singapore
before and after the installation of a green roof. They found
that the levels of particles and SO
2
in air above the roof were
reduced by 6% and 37%, respectively, after installation of the
green roof. This field measurement proved that green roofs
can reduce certain air pollutants but it is difficult to
extrapolate their results to other places or to a larger scale.
The measurement was site specific and the volume of air
that was influenced by the green roof was not given.
The cases discussed above have shown the potential
benefit of using green roofs in air pollution control.
However, there are many aspects of this mitigation
measure that remain unclear. More studies are needed to
help cities decide whether the green roof can be an effec-
tive way to improve air quality. We believe the following
questions need to be answered: How can we quantify the
level of air pollutant removal after installing green roofs in
one city? Is there a difference between different types of
green roofs in the level of air pollutant removal? How does
the green roof compare to other mitigation measures such
as planting trees? In this paper, we will address those
questions with a case study in Chicago, Illinois.
2. Study site and methods
2.1. Study site
This study took place in Chicago, Illinois, which is
located along the southwest shore of Lake Michigan with
a center coordinate of 41

53
0
N and 87

39
0
W. The total area
of the city is 588.3 km
2
. Chicago is the third most populous
city in the U.S with a population of 2.9 million in 2000.
According to ALA (2007), over 2 million people in Chicago
were at heightened risk for health problems resulting from
acute exposure to O
3
and particulate matters.
Chicago is ranked number one in terms of total area of
installed green roofs among North American cities.
According to Taylor (2007), green roofs were installed on
300 buildings resulting in a total area of 27.87 ha by June
2007. There are three types of green roofs in Chicago:
extensive green roofs, intensive green roofs, and semi-
intensive green roofs. Extensive green roofs are planted
with low height and slow growing plants. The depth of the
growth media is less than 15 cm. Intensive green roofs
consist of large perennial herbaceous plants and, occa-
sionally, shrubs and small trees. The depth of growth media
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 of Environment for a list of green roofs
resulting in a list of 170 green roofs. Two steps were taken
to verify the list. First, information including the address of
the green roof, type of the green roof, size, and the date it
was completed was gathered from various sources. We
then searched the address of each green roof through an
image database hosted by Pictometry International Crop.
Digital aerial photographs covering Chicago were taken by
Pictometry International Corp in July 2006. Because the
photographs have a ground resolution of 16 cm and were
taken from multiple angles, the location, size, type of the
green roof, and the type of building could be clearly inter-
preted. For each green roof, the area of grass, trees, and
other surfaces was measured and the percentage to the
total area was calculated. Pictometry software allows users
to directly measure distances and areas on those geore-
ferenced images. The error margin of the measurement was
estimated to be 1% or smaller (Federal Emergency
Management Agency, 2005).
2.3. Removal of air pollutants by green roofs
In this study, a big-leaf resistance model was used to
quantify the dry deposition of air pollutants. The structure
J. Yang et al. / Atmospheric Environment 42 (2008) 7266–7273 7267

of the model and how the input parameters were fitted are
explained below.
The removal of a particular air pollutant at a given place
over a certain time period was calculated as (Nowak, 1994):
Q¼FLT(1)
where Qis the amount of a particular air pollutant removed
by certain area of green roofs in a certain time period (g), F
is the pollutant flux (g m
2
s
1
), Lis the total area of green
roof (m
2
), and Tis the time period (s). The pollutant flux Fis
calculated as in Eq. (2):
F¼VdC108(2)
where V
d
¼dry deposition velocity of a particular air
pollutant (cm s
1
), and C¼concentration of that pollutant
in the air (
m
gm
3
). The dry deposition process can be
described as the inverse of total resistance (Baldocchi et al.,
1987):
Vd¼1
RaþRbþRc
(3)
where R
a
¼aerodynamic resistance, R
b
¼quasi-laminar
boundary layer, and R
c
¼canopy resistance. The algorithms
for calculating R
a
and R
b
were reported in Yang et al. (2005).
In this study, the roughness length z
0
and displacement
length dfor short grasses were used to represent extensive
green roofs. The intensive green roofs were treated as
mixtures of short grass, tall herbaceous plants, and small
deciduous tree. The z
0
and dvalues used in the model are
listed in Table 1.
The hourly canopy resistances R
c
for O
3
,SO
2
, and NO
2
are calculated as (Walmsley and Wesely, 1996).
Rc¼hðRsx þRmxÞ1þR1
lux
þðRdc þRclxÞ1þRac þRgsx 1i1(4)
In Eq. (4),R
sx
is leaf stomata resistance, R
mx
is leaf meso-
phyll resistance, R
lux
is leaf cuticles resistance, R
dc
is the
resistance for gas-phase transfer by buoyant convection in
canopies, R
clx
is resistance by leaves, twigs, bark or other
exposed surfaces in the lower canopy, R
ac
is transfer resis-
tance which depends only on canopy height and density,
and R
gsx
is ground surface resistance. Resistance compo-
nents can vary with solar intensity, seasons, and vegetation
types. Algorithms are available for calculating resistance
components for grass and deciduous trees. The tall herba-
ceous plants were modeled as crops in this study. Details of
the algorithms were described in Wesely (1989);Walmsley
and Wesely (1996);Zhang et al. (2002).
The deposition velocity of PM over green roofs was
calculated as (Zhang et al., 2001).
Vd¼Vgþ1
ðRaþRsÞ(5)
Where V
g
is the gravitational settling velocity, R
a
is the
aerodynamic resistance above the canopy, R
s
is the surface
resistance.
The gravitational settling is calculated as.
Vg¼
r
d2
pgC
18
h
(6)
Where
r
is the density of the particle, in this study, a value
of 1800 kg m
3
was used as suggested by Lim et al. (2006),
d
p
is the particle diameter, gis the acceleration of gravity, C
is the correction factor for small particles and is calculated
as (Zhang et al., 2001),
h
is the viscosity coefficient of air.
The aerodynamic resistance R
a
is calculated as before.
The surface resistance R
s
is based on the size of deposition
particles, atmospheric conditions, and surface properties. It
was calculated as (Zhang et al., 2001).
Rs¼1
3
0
m
ðEBþEIM þEINÞR1
(7)
Where
3
0
is an empirical constant and taken as 3 here,
m
*
is
the friction velocity. E
B
,E
IM
, and E
IN
are collection efficiency
from Brownian diffusion, impaction and interception,
respectively. The re-suspension of particles after hitting
a surface was modeled by modifying the total collection
efficiency by the factor of R
1
, which represents the fraction
of particles sticking to the surfaces. The extensive green
roofs and intensive green roofs were modeled in the same
manner as in calculating R
c
. Details on how those param-
eters were fitted can be found in Zhang et al. (2001).
The final deposition velocity for PM
10
was the weight-
averaged V
d
for all particles with a size less than 10
m
m.
Information on size classes and mass concentration of
particles in Chicago were obtained from Offenberg and
Baker (2000).
Hourly air pollution data including NO
2
,SO
2
,O
3
, and
PM
10
concentration from an air pollution monitoring
station in central Chicago between 8/1/2006 and 7/31/2007
were obtained from the U.S. EPA. Hourly surface meteo-
rology data including sky condition, air temperature, rela-
tive humidity, atmospheric pressure, wind speed,
precipitation, and snow cover measured by a station located
at O’Hara International Airport for the same time period
was obtained from the National Climatic Data Center. The
hourly solar radiation intensity was simulated by using the
meteorological/statistical solar radiation model (METSTAT,
Maxwell et al., 1995). During precipitation and when the
ground was covered by snow, the value of V
d
was set as zero
because the dry deposition process could not occur. Hourly
fluxes of NO
2
,SO
2
,O
3
, and PM
10
to green roofs in Chicago
were calculated by using weather data, concentration of
pollutants, and the modeled deposition velocities.
2.4. Additional removal with different planting
scenarios and costs
Three future planting scenarios were assumed and the
amount of air pollutionremoval for each scenario calculated.
Table 1
Value of roughness lengths and displacement heights used in the model
Vegetation
type
Average
height h
0
(m)
Z
0
¼0.1h
0
(m) d¼0.7h
0
(m)
Short grass 0.15 0.015 0.105
Tall herbaceous plants 1.0 0.1 0.7
Deciduous trees 5.0 0.5 3.5
J. Yang et al. / Atmospheric Environment 42 (2008) 7266–72737268

The first scenario assumed planting all roofs in Chicago with
the same ratio of extensive vs. intensive green roofs used
currently. The second scenario assumed the remaining roofs
would only be planted with extensive roofs. The third
scenario assumed only intensive roofs would be used in
future projects. In all these scenarios, the intensive roof was
treated as a mixture of tall herbaceous plants and small
deciduous trees and shrubs at a ratio of 50:50. The total area
of roofs in Chicago was obtained from Gray and Finster
(2000) study, which showed that Chicago’s roof surface was
27.86% of the urban area. According to information gathered
from the green roof companies and the literature, the
average installation cost for green roofs are as follows:
extensive green roofs between $107.64 and $161.46 per m
2
($10–$15 per ft
2
); intensive green roofs between $161.46
and $269.1 per m
2
($10––$25 per ft
2
). The medians of those
ranges were used in the calculation. The maintenancecost of
green roofs was not included in this calculation.
3. Results
Among the 170 green roofs included in the list, detailed
information for 71 green roofs was obtained and verified
through aerial photographs. The total area of those 71
green roofs is 19.8 ha, 71% of the total area of green roofs in
Chicago reported by Taylor (2007).
The information about those green roofs is shown in
Table 2.
The green roofs surveyed were located mainly on
commercial building and the size of each individual roof
was relatively large. Among the 71 green roofs, half had an
area larger than 500 m
2
and 23 green roofs were larger than
1000 m
2
. The green roof in the Soldier Field was 22 445 m
2
while the one in Millennium Park was 99 983 m
2
.
Based on the analysis of aerial photographs, the 19.8 ha
of green roof consisted of 63% short grass and other low
growing plants, 14% large herbaceous plants, 11% trees and
shrubs, and about 12% various structures and hard surfaces.
The monthly air quality between August 2006 and July
2007 in Chicago is shown below (Fig. 1).
It can be seen from Fig. 1 that O
3
was the main air
pollutant in Chicago. PM
10
ranked second while the SO
2
pollution was low. PM
10
and O
3
pollution peaked in
summer while SO
2
and NO
2
peaked in winter.
The monthly mean deposition velocities for air pollut-
ants calculated for different vegetation types showed
a seasonal trend (Table 3). The deposition velocities for all
air pollutants were highest in May and lowest in February.
The modeled monthly uptake of air pollutants by green
roofs is shown in Fig. 2.
The total air pollution removal by 19.8 ha of green roofs
was 1675 kg between August 2006 and July 2007. If the
reported 27.87 ha of green roofs were all completed and
had 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 O
3
was the
largest, 52% of the total uptake followed by NO
2
(27%), PM
10
(14%), and SO
2
(7%). Seasonally, the highest uptake
occurred in May and the lowest in February. The annual
removal rate among different vegetation types is compared
in Table 4.
If all remaining roofs in Chicago were planted with
intensive green roofs, the direct removal of air pollutants
could reach as high as 2046.89 metric tons, assuming the
same level of air pollution as 2006–2007. However, the
installation cost would be $35.2 billion.
4. Discussion
4.1. Evaluation of results
The results showed that air pollutant removal by green
roofs in Chicago was affected by air pollutant concentra-
tions, weather conditions, and the growth of plants. The
highest air pollutant removal occurred in May when leaves
of plants were fully expanded and the concentration of
pollutants was high. The lowest removal was in February
when the vegetation was covered in snow. The reliability of
the estimate was evaluated by comparing it to values
reported in other studies.
The dry deposition velocities of air pollutants influence
the magnitude of air pollutant removal most. We found
that the modeled deposition velocities were within
a reasonable range compared to the measured values
reported in the literature (Tables 4 and 6). It should be
noted that the size of PM has a strong influence on the
deposition velocity. In Chicago, Offenberg and Baker (2000)
found that the bulk mass of PM was at particles with a d
p
less than 2
m
m. The modeled V
d
values for PM
10
in this
Table 2
Percentages of different type of green roofs in Chicago
Type of green roof On residential
buildings (%)
On commercial
buildings (%)
On office
buildings (%)
Total (%)
Extensive 4.05 20.55 7.98 32.58
Intensive/semi-
intensive
0.10 61.42 5.90 67.42
Sub total (%) 4.15 81.97 13.88 100.00
Aug/2006
Sept
Oct
Nov
Dec
Jan/2007
Feb
Mar
April
May
June
July
Month
31
42
6
10
30
60
20
30
Concentration (ug/m3)
NO2 by Month
SO2
O3
PM10
Fig. 1. Concentrations of criteria air pollutants in Chicago between August
2006 and July 2007. The monthly mean values were shown in the figure.
J. Yang et al. / Atmospheric Environment 42 (2008) 7266–7273 7269

study were comparable to the values for fine particles
reported in the literature.
The removal rate was compared to the removal rate of
air pollutants, including SO
2
,NO
2
,PM
10
, and O
3
, extracted
from similar studies. The results showed that the annual
removal per hectare of green roof was 85 kg ha
1
yr
1
and
the annual removal per hectare of canopy cover was
97 kg ha
1
yr
1
. The annual removal per hectare of canopy
cover reported in this study was higher than the removal
rate of 69 kg ha
1
yr
1
estimated for green roofs in Toronto
by Currie and Bass (2005).Deutsch et al. (2005) reported
a removal rate of 77 kg ha
1
yr
1
for Washington, DC. As
suggested by Nowak et al. (2006), the different pollution
removal rates among cities can be caused by factors such as
the amount of vegetation cover, pollution concentration,
length of growing season, and meteorological conditions.
Furthermore, the different methods used in modeling the
air pollution removal by grass and large herbaceous plants
in those studies also contributed to difference in results.
Currie and Bass (2005) did not model grass and large
herbaceous plants separately. Instead, they adjusted the
estimated V
d
value of air pollutants from trees to grasses by
using the ratio of leaf area index (LAI) of grasses to trees
(3:6). The ratio of 1:2 was supported by Shreffler (1978)
study on modeled deposition velocity for SO
2
over grass-
lands vs. forests. However, based on the V
d
values modeled
in this study, and also from the observed values reported in
the literature (Table 6), we found that the V
d
values of air
pollutants for trees may not always be two times those of
grass and large herbaceous plants. Finally, the UFORE
model tends to give conservative estimation of PM
10
removal because it assumes a fixed deposition velocity of
0.064 m s
1
and a 50% re-suspension rate for PM
10
(Nowak
et al., 2006). As pointed out by Ould-Dada and Baghini
(2001), the 50% re-suspension was much larger than the re-
suspension rate they measured for fine particles. All those
differences can lead to the relatively high removal rates
reported in this study.
4.2. Uncertainties of the approach
The estimated air pollutant removal for green roofs in
Chicago should be treated as an approximation rather than
an accurate estimation of actual air pollution removal.
Several uncertainties should be noted. The green roofs in
Chicago were generalized as continuous surfaces of short
grass, tall herbaceous plants, and deciduous trees with
uniform heights. This generalization was necessary for
running a big-leaf model at a city scale. Nevertheless,
small-scale effects such as the differing heights of green
roofs, arrangement of vegetation, and relation to the
geometry of street canyons could influence turbulence and
Aug/2006
Oct
Dec
Feb
Apr
June
Aug/2006
Oct
Dec
Feb
Apr
June
Month
0
60
120
0
60
120
Uptake by green roofs (kg)
NO2 by Month SO2
O3 PM10
Fig. 2. Monthly uptake of air pollutants by green roofs in Chicago between August 2006 and July 20 07.
Table 3
Annual removal rate of air pollutants per canopy cover by different vegetation types in Chicago between August 2006 and July 2007
Type of vegetation SO
2
(g m
2
yr
1
)NO
2
(g m
2
yr
1
)PM
10
(g m
2
yr
1
)O
3
(g m
2
yr
1
) Total (g m
2
yr
1
)
Short grass 0.65 2.33 1.12 4.49 8.59
Tall herbaceous plants 0.83 2.94 1.52 5.81 11.10
Deciduous trees 1.01 3.57 2.16 7.17 13.91
The non-vegetated surfaces were excluded from the calculation.
J. Yang et al. / Atmospheric Environment 42 (2008) 7266–72737270

transport in wind canopies (McDonald et al., 2007). The
concentrations of air pollutants were considered uniform
for the entire study area. This assumption works for situa-
tions where a well-mixed boundary layer exists in daytime
under unstable conditions (Colbeck and Harrison, 1985).
Nevertheless, the influence of buildings and the distances
to sources of emission could cause the concentrations of air
pollutants to vary spatially. Green roofs close to highly
polluted streets could have higher uptake of air pollutants
than those located in relatively clean areas.
Another source of uncertainty is the way the V
d
was
modeled. The resistance components were modeled by
simplifying all plants into three prototypes: grass, crops,
and deciduous trees. Values adopted from existing litera-
tures were used to represent the vegetation characteristics.
However, the differences among plant species (e.g.,
photosynthetic pathways, stomatal densities, LAI, growth
speed) can introduce uncertainties into the estimate of V
d
.
In the future, more field measurements on the dry depo-
sition velocities of pollutants on urban grass should be
conducted to calibrate the dry deposition model and verify
the modeling results.
Finally, green roofs can also become a source of pollut-
ants. Pollens produced by plants and erosion of growth
media under a strong wind can increase particle pollution
(Tan and Sia, 2005). Plants can also emit volatile organic
compounds (VOC) that can result in O
3
production
(Benjamin and Winer, 1998). Those factors were not
considered in this study but they can potentially lower the
estimate of air pollutant removal by green roofs.
4.3. Practical considerations
It can be seen from Table 5 that a large amount of air
pollutants can be removed if all roofs in Chicago were
converted to green roofs. However, it was also obvious that
the cost of constructing the specified area of green roofs
would be prohibitively high. Compared to the cost of
traditional air pollution controls, between $935 per metric
ton for CO and $4482 per metric ton for NO
2
(McPherson,
1994), the green roof is not an economically viable measure
in air pollution control.
Although the removal rate of 97 kg ha
1
yr
1
is
comparable to the removal rates for urban forests reported
by Nowak et al. (2006) in 55 cities, which range between
59 kg ha
1
yr
1
and 168 kg ha
1
yr
1
, green roofs cost more
than planting trees. Based on the results of Nowak (1994),
a medium size tree can remove the same amount of air
pollutants as a 19 m
2
extensive green roof in one year but
the planting costs for them are around $400 and $3059,
respectively.
Even with their high cost, there are several reasons why
the green roof is a viable alternative to trees in air pollution
control. The high initial installation cost of a green roof can
be justified by its long-term benefits. Benefits contributed
by green roofs include reduction of storm water runoff,
saving energy, reducing urban heat islands, and extending
the life span of roofs (Carter and Keeler, 2007; Wong et al.,
2003). Acks (2005) did a cost-benefit analysis of several
planting scenarios of green roofs in New York City and
found the medium benefit/cost ratio was 1.02 over a period
of 55 years. The cost-benefit ratio of building green roofs
can be further improved by increasing the efficiency of air
pollutant removal and simultaneously lowering the
construction cost. Plant species used in green roofs can be
selected to increase the amount of air pollutants removed
and reduce the emission of VOC (Benjamin et al., 1996). The
construction and maintenance costs of a green roof can be
reduced if the industry is standardized and a complete
system for green roof production, delivery, and installation
is formed. Currently, as estimated by Philippi (2006), the
unit installation cost of the extensive green roof in the U.S.
was ten times that in Germany. Furthermore, unlike tree
planting programs where land has to be set aside for the
plantings, green roofs do not occupy land; they are built on
rooftops. This is an important factor for high-density urban
communities.
5. Conclusion
Air pollution in the urban environment is a major
threat to human health. As the global population is
becoming more concentrated in urbanized areas, new
ideas and approaches are needed to help maintain clean
air that is safe for everyone to breathe. This study eval-
uated one such innovative approach: using green roofs for
air pollution control. By using a big-leaf dry deposition
model, the air pollutants removed by green roofs in Chi-
cago were quantified. The result showed that the green
roofs in Chicago can remove a large amount of pollutants
from air. Currently, the green roof cannot be used as
a stand-alone measure in air pollution controls because of
its high cost. However, a comprehensive look at its envi-
ronmental benefits shows that it can be an effective
option to mitigate air pollution as well as other environ-
mental problems.
Table 5
Additional air pollution removal from planting more green roofs and the
estimated installation cost
Scenarios Total air
pollutants
removed
(metric tons)
Total installation
cost ($ million)
Cost of removal
($ million/
metric ton)
Current ratio 1835.23 3086.52 1.68
Extensive only 1405.50 2201.51 1.57
Intensive only 2046.89 3522.42 1.72
Table 4
Modeled deposition velocities of pollutants over different vegetation
types
Type of vegetation SO
2
(cm s
1
)
NO
2
(cm s
1
)
PM
10
(cm s
1
)
O
3
(cm s
1
)
Short grass 0.04 (0.005) 0.01 (0.001) 0.10 (0.005) 0.01 (0.001)
0.39 (0.006) 0.39 (0.006) 0.19 (0.003) 0.42 (0.007)
Tall herbaceous
plants
0.04 (0.006) 0.01 (0.001) 0.10 (0.006) 0.01 (0.001)
0.48 (0.007) 0.49 (0.007) 0.25 (0.004) 0.54 (0.008)
Deciduous trees 0.05 (0.006) 0.01 (0.001) 0.13 (0.008) 0.01 (0.001)
0.57 (0.007) 0.58 (0.008) 0.36 (0.006) 0.65 (0.008)
The minimum and maximum monthly average deposition velocities were
shown here. The numbers inside the parenthesis were standard errors.
J. Yang et al. / Atmospheric Environment 42 (2008) 7266–7273 7271

Acknowledgement
We thank two anonymous reviewers for their helpful
suggestions on the manuscript. Also, we express our
appreciation to Department of Environment, Chicago
City for directing us to information on green roofs in
Chicago. Finally, we thank Pictometry International Corp
for providing us the free trial of the image database.
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