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Abstract

As public health concerns have increased due to the rising number of studies linking adverse health effects with exposures to traffic-related air pollution near large roadways, interest in methods to mitigate these exposures have also increased. Several studies have investigated the use of roadside features in reducing near-road air pollution concentrations since this method is often one of the few short-term options available. Since roadside vegetation has other potential benefits, the impact of this feature has been of particular interest. The literature has been mixed on whether roadside vegetation reduces nearby pollutant concentrations or whether this feature has no effect or even potentially increases downwind air pollutant concentrations. However, these differences in study results highlight key characteristics of the vegetative barrier that can result in pollutant reductions or increase local pollutant levels. This paper describes the characteristics of roadside vegetation that previous research shows can result in improved local air quality, as well as identify characteristics that should be avoided in order to protect from unintended increases in nearby concentrations. These design conditions include height, thickness, coverage, porosity/density, and species characteristics that promote improved air quality. These design considerations can inform highway departments, urban and transportation planners, and developers in understanding how best to preserve existing roadside vegetation or plant vegetative barriers in order to reduce air pollution impacts near transportation facilities. These designs can also be used to mitigate impacts from other air pollution sources where emissions occur near ground-level.
Roadside vegetation design characteristics that can improve
local, near-road air quality
Richard Baldauf
U.S. Environmental Protection Agency, Research Triangle Park, NC, USA
article info
Article history:
Keywords:
Air pollution
Traffic
Vegetation
Particulate matter
Highway landscape design
abstract
As public health concerns have increased due to the rising number of studies linking
adverse health effects with exposures to traffic-related air pollution near large roadways,
interest in methods to mitigate these exposures have also increased. Several studies have
investigated the use of roadside features in reducing near-road air pollution concentrations
since this method is often one of the few short-term options available. Since roadside veg-
etation has other potential benefits, the impact of this feature has been of particular inter-
est. The literature has been mixed on whether roadside vegetation reduces nearby
pollutant concentrations or whether this feature has no effect or even potentially increases
downwind air pollutant concentrations. However, these differences in study results high-
light key characteristics of the vegetative barrier that can result in pollutant reductions or
increase local pollutant levels. This paper describes the characteristics of roadside vegeta-
tion that previous research shows can result in improved local air quality, as well as iden-
tify characteristics that should be avoided in order to protect from unintended increases in
nearby concentrations. These design conditions include height, thickness, coverage, poros-
ity/density, and species characteristics that promote improved air quality. These design
considerations can inform highway departments, urban and transportation planners, and
developers in understanding how best to preserve existing roadside vegetation or plant
vegetative barriers in order to reduce air pollution impacts near transportation facilities.
These designs can also be used to mitigate impacts from other air pollution sources where
emissions occur near ground-level.
Published by Elsevier Ltd.
1. Introduction
Numerous health studies have linked adverse health effects with spending significant amounts of time near high-traffic
roads with elevated air pollution levels of particulate matter, gaseous pollutants, and air toxics emitted by nearby motor
vehicle activity (HEI, 2010). The significant impact of traffic emissions on urban populations all over the world has motivated
research on methods to reduce exposure to these pollutants. While vehicle emission control techniques and programs
directly reduce pollutants emitted to the air from transportation sources, these programs often take a long time to fully
implement and may be offset by increases in vehicle activity. Thus, other mitigation options will also be needed to fully
and comprehensively reduce air pollution exposures for these urban populations.
Recent studies have investigated how roadside vegetation may provide an opportunity to reduce near-road pollutant
concentrations in urban areas. This roadside vegetation can include the preservation of existing trees and bushes, as well
http://dx.doi.org/10.1016/j.trd.2017.03.013
1361-9209/Published by Elsevier Ltd.
E-mail address: Baldauf.Richard@epa.gov
Transportation Research Part D 52 (2017) 354–361
Contents lists available at ScienceDirect
Transportation Research Part D
journal homepage: www.elsevier.com/locate/trd
as planting vegetation, which may be some of the few near-term mitigation strategies available for urban developers and
facilities already subject to high pollution levels near roads. These mitigation methods, if successful, can complement exist-
ing pollution control programs and regulations, as well as provide measures to reduce impacts from sources that are difficult
to control such as brake and tire wear and re-entrained road dust (EPA, 2016).
In general, vegetation and green infrastructure has been shown to have overall health benefits including increased phys-
ical activity, lower obesity, improved mental health, overall improved birth outcomes, lower adverse cardiovascular illness,
and decreased mortality (James et al., 2015, 2016). Dadvand et al. (2015) found an improvement in school children’s cogni-
tive development associated with an increase in surrounding greenness, particularly at schools. The authors partly attributed
this association to reductions in nearby air pollution.
In addition to air quality and general health benefits, roadside vegetation can improve aesthetics, increase property val-
ues, reduce heat, control surface water runoff, and reduce noise pollution (with dense, thick and tall stands). However, veg-
etation can also affect driver sight lines, protrude into clear zones along highway right-of-ways, contribute to debris on
roads, present fire hazards, and be pathways for pests and invasive species. Thus, all of the benefits and potential unintended
consequences of roadside vegetation need to be considered for any application.
This paper provides insight into roadside vegetation design characteristics that have been shown to most effectively
reduce near-road air pollutant levels downwind of major highways in order to implement this feature as an air pollution
mitigation strategy. The recommendations focus on general considerations applicable to multiple development types and
scenarios, and do not address site-specific siting or permitting requirements that might be required in certain locations such
as planting along a particular highway right-of-way or within a city park. This paper also focuses on how the vegetation char-
acteristics impact air quality, but does not review models and other methods used to quantify these impacts, which is still an
area of active research.
2. Vegetation effects on air quality
Trees and other vegetation have been shown to reduce regional air pollution levels through the interception of airborne
particles or through the uptake of gaseous air pollution through leaf surfaces (Janhall, 2015; Gallagher et al., 2015). Pollution
removal (O
3
,PM
2.5
,PM
10
,NO
2
,SO
2
, CO) by urban trees has been estimated across the continental United States (US) using
the US Forest Service’s i-Tree model, which suggested that nationwide vegetation can reduce air pollution levels by approx-
imately one percent (equating to over 10 million tons of air pollution removed) (Nowak et al., 2014).
Removal of gaseous pollutants by trees can be permanent, while trees typically serve as a temporary retention site for
particles. The removed particles can be re-suspended to the atmosphere during turbulent winds, washed off by precipitation,
or dropped to the ground with leaf and twig fall (Nowak et al., 2000). These removal mechanisms can impact local air, water
and soil pollution; thus, careful consideration of the land uses that surround roadside vegetation are needed when choosing
species.
At the local level, trees can also act as barriers between sources and populations, although vegetation is inherently more
complex to study than solid structures. For example, the effectiveness of vegetative barriers at reducing ultrafine particle
(UFP) concentrations has been shown to be variable (Janhall, 2015; Tong et al., 2015; Hagler et al., 2012; Pataki et al.,
2011). This variability is due to a number of confounding factors. The complex and porous structure of trees and bushes can
modify near-road concentrations via pollutant capture or through altering air flow, which can result in either lower dispersion
through the reduction of wind speed and boundary layer heights (Nowak et al., 2000; Wania et al., 2012; Vos et al., 2013), or in
enhanced dispersion due to increased air turbulence and mixing as the pollutant plume is lofted up and over the vegetation
(Bowker et al., 2007). Recirculation zones have also been observed immediately downwind of forested areas with a flow struc-
ture consistent with an intermittent recirculation pattern (Frank and Ruck, 2008). Janhall (2015) summarized that vegetation
like hedges, can filter out PM when located close to an emission source such as a road, while higher, canopy vegetation like
trees can reduce mixing and turbulence and result in increased concentration levels near the ground. Thus, vegetation type,
height, and thickness can all influence the extent of mixing and pollutant deposition experienced at the site. The built environ-
ment also matters greatly – air flow and impacts of trees are substantially different for a street canyon environment than an
open highway environment (Gromke et al., 2008, 2016; Buccolieria et al., 2009, 2011; Pugh et al., 2012; Li et al., 2016).
Several studies have shown significant reductions in air pollution concentrations behind roadside vegetation barriers
(Tong et al., 2016; Al-Dabbous and Kumar, 2014; Brantley et al., 2014; Steffens et al., 2012). Each of these studies compared
air quality concentrations near a large roadway with and without the presence of roadside vegetation. For all of these studies,
the roadside vegetation was dense and a mixture of trees and bushes, with full coverage from the ground to the top of the
canopy, although the heights, thickness and species varied. Two recent modeling studies demonstrated that hedgerows can
improve air quality in street canyons if the bushes provide full coverage with no openings or gaps (Gromke et al., 2016; Li
et al., 2016).
2.1. Roadside vegetation barrier physical design characteristics
The seemingly contradictory results of previous field studies investigating how roadside vegetation affects near-road air
quality actually provide useful insights on the characteristics needed for such a barrier to improve local air quality. The fol-
R. Baldauf / Transportation Research Part D 52 (2017) 354–361 355
lowing sections review how height, thickness and the porosity of the vegetation interact to either provide an effective barrier
against air pollution impacts or potentially have no or even a negative effect. These recommendations and characteristics
focus on the presence of vegetation along highways and motorways that carry large volumes of traffic but are not located
in urban street canyons. These characteristics will also be pertinent for other similar air pollution source types where emis-
sions occur near ground-level. Although there are similarities in results from street canyon studies to this type of application,
other recent publications provide more detailed information on designs for street canyons (Gallagher et al., 2015; Gromke
et al., 2016; Li et al., 2016).
Generally, a vegetation barrier along a high-volume highway should be tall, thick, and dense to achieve greater reductions
in downwind pollutant concentrations. Each of these factors is discussed below with an emphasis on roadside vegetation
characteristics that promote improved near-road air quality.
2.1.1. Vegetation barrier height
Near-road vegetation barrier studies in non-street canyon settings that measured air pollution reductions behind the veg-
etation typically had heights ranging from 4 to 5 m or higher (Al-Dabbous and Kumar, 2014; Brantley et al., 2014; Steffens
et al., 2012). At these heights, the barrier will be above the exhaust release height of typical motor vehicles operating on the
adjacent roads, forcing the pollutant plume to loft above and over or pass through the vegetation. Heights lower than
approximately 4 m may allow for the pollutant emissions to proceed downwind of the low barrier unimpeded, although
studies evaluating varying heights of vegetation barriers have been minimal.
Vegetation barriers will only be effective at reducing air pollution in urban areas with limited space for planting if there’s
full coverage from the ground to top of the canopy. Ornamental trees with large openings under the canopy can result in
higher downwind concentrations by allowing the plume to pass through while also reducing wind speeds as in the example
of Tong et al. (2015). Thus, the vegetation barrier should be similar to a solid noise barrier by impeding the entire plume air
flow from the highway. Solid noise barrier studies also suggest walls at least 4 m or taller provide a sufficient height for air
quality improvement near roads (Baldauf et al., 2008, 2016; Heist et al., 2009; Finn et al., 2010). As discussed in Gallagher
et al. (2015) and Li et al. (2016), vegetation heights for barriers in street canyons may be effective at lower heights due to the
differences in air flow patterns.
2.1.2. Vegetation barrier thickness
The thickness of the barrier will provide the residence time to allow for particulate removal by impaction or diffusion, as
well as reduce turbulence and wind speed, increasing the amount of air flow blocking. The vegetation thickness also forces
air flow over the barrier for a longer distance, as well as provides increased distance from the air pollution source to the
downwind receptors. The thickness of the barrier needed for effective air pollution mitigation will vary depending on the
porosity/density of the vegetation. In general, studies reporting decreased near-road pollutant concentrations with vegeta-
tion were a minimum of approximately 5 m thick, with most approaching 10 m or more (Neft et al., 2016).
2.1.3. Vegetation porosity/density
The porosity or density of the vegetation comprising the barrier will determine air movement through the interior of the
barrier. Generally, the lower the porosity (or higher the density) and thicker the barrier, the more air flow forced over the
structure. At extremely low porosities, the vegetation will affect pollutant transport and dispersion in a similar manner as
a solid noise barrier. At higher porosity, the vegetation can reduce wind speeds, allowing pollutants to stagnate within or
behind the vegetation, potentially leading to higher pollutant concentrations near ground-level. Thus, the vegetation poros-
ity should be high enough that the combination of particle loss within the vegetation and the particle removal mechanisms
dominate the lowering wind speed and stagnation effect, leading to reduced concentrations behind the barrier. Since the
measurement of porosity and/or density along the horizontal air flow from a highway is very difficult, a quantitative tech-
nique in the field has not been implemented. Modeling and wind tunnel analyses have used Leaf Area Index (LAI) and Leaf
Area Density (LAD) to estimate the porosity/density of vegetation (Tong et al., 2016; Steffens et al., 2012; Neft et al., 2016; Lin
and Khlystov, 2012). These studies suggest thicker and denser vegetation promotes increased pollution reductions, although
none provide a quantitative relationship that has been shown effective in the field. Fig. 1 shows examples of effective and
not-effective vegetation barriers that provides a qualitative understanding of the porosity and density needed for pollution
reductions.
2.1.4. Vegetation barrier coverage
As described, gaps in vegetation barriers, whether from high porosity, missing or dead trees, or space under ornamental
trees, can lead to increased pollutant concentrations downwind, sometimes higher than concentrations are if no barrier were
present. These increases can occur because pollutant emissions from the road funnel through the gaps or cause winds to
stagnate. Thus, the vegetation should provide full coverage from the ground to the top of the canopy as shown in Fig. 1. This
characteristic is important in planning the barrier design as well as maintaining existing or planted roadside vegetation. In
order to achieve sufficient coverage, multiple rows and types of vegetation may be most feasible. For example, a barrier could
consist of a row of bushy plants and hedges followed by a row of trees to enable a barrier with full coverage from the ground
to top of canopy at the initial planting, yet achieve higher canopy heights than feasible by bushy plants alone. In addition,
rows of multiple vegetation types may allow for sufficient downwind pollutant removal while the vegetation grows over
356 R. Baldauf / Transportation Research Part D 52 (2017) 354–361
time after first planting. This approach will ensure sufficient density for pollutant removal at the initial planting, while
allowing for increased pollutant removal as the vegetation matures. This process will also limit concerns of promoting plant
monocultures.
2.1.5. Vegetation barrier Length
In addition to passing through gaps, pollutants can also meander around the edges of a roadside vegetative barrier. Thus,
if a vegetative barrier will be constructed for a specific neighborhood or facility (e.g. school, daycare, nursing home), it should
extend sufficiently beyond the area of concern. Research on solid noise barriers suggests that the barrier should extend at
least 50 m laterally beyond the area of concern in order to maximize reductions in downwind concentrations (Baldauf
et al., 2008, 2016). If extending the barrier laterally is not feasible, extending it perpendicularly from the road, wrapping
around the area of interest, has been shown to be effective as well (Li et al., 2016; Brantley et al., 2014); however, if air pol-
lution sources are present upwind of the road, this design may trap pollutants emitted from this source when winds emanate
from this source toward the road.
2.2. Roadside vegetation species considerations
Certain types and species of vegetation will provide more air quality benefits compared to other types of vegetation.
When considering the design and construction of a vegetation barrier, optimal species’ physical characteristics should be
favored to the extent feasible. However, given the vast number of vegetation species, and the regional differences in the fea-
sibility and effectiveness of specific species for a roadside barrier, specific recommendations cannot be made in this docu-
ment. The U.S. Forest Service’s i-Tree model (see Nowak et al. (2014)) can provide a list of potential species that best
meet the factors listed below for the United States and select other parts of the world. In addition, users need to identify
whether particular vegetation types can survive and prosper in a particular area of interest. Key factors to consider include.
2.2.1. Seasonal effects
The vegetation chosen for a barrier should not be subject to significant changes in characteristics and integrity during
changing seasons. Deciduous trees that lose leaves during the cold season should not be considered for a barrier to mitigate
air quality impacts year-round. Instead, trees that are not subject to significant seasonal changes, such as evergreen and
other similar coniferous plants, should be considered. Other shrubs and bushes that are not subject to seasonal changes
can also be considered as part of a roadside barrier.
2.2.2. Leaf surface characteristics
Leaf surfaces can also enhance particulate removal through diffusion and interception. Trees and bushes with waxy and/
or hairy surfaces have been shown to preferentially remove and retain particulates compared to smooth leaf surfaces. In
addition, vegetation with smaller leaves as well as leaf and branch structures that provide increased surface area for particle
diffusion are preferred (Tong et al., 2016; Petroff et al., 2009).
2.2.3. Vegetation air emissions
When selecting vegetation for a roadside barrier, especially at locations where sensitive populations may be spending sig-
nificant amounts of time, care must be taken to choose species that do not emit compounds which increase allergic
responses or air pollution. Compounds that can be emitted by vegetation include high-allergy pollens as well as volatile
organic compounds (VOCs) which can contribute to the formation of ozone. Both can exacerbate respiratory effects and
should be avoided for roadside barriers, especially in areas where sensitive populations may be present, such as children
and the elderly. The i-Tree model provides vegetation air emission data for many species found throughout the world.
2.2.4. Resistance to air pollution and other environmental stressors
Vegetation implemented in a roadside barrier must also be resistant to air pollution and other traffic stressors since con-
centration levels will be high. If the vegetation is not resistant and cannot maintain its integrity, gaps will form in the barrier,
potentially leading to increased pollutant concentrations downwind as discussed previously. Air pollutants emitted by traffic
can include the typical tailpipe emissions like CO, NO
x
, and particulates; materials from brake and tire wear; re-suspended
road dust; and salt and sand used for road surface treatment during winter weather conditions.
2.3. Other vegetation barrier considerations
In addition to air quality considerations, other potentially beneficial and adverse aspects of vegetation need to be consid-
ered in the development and use of a roadside barrier. These considerations include general physical and species-specific
factors. While location-specific factors will need to be addressed on an individual basis, some general considerations include.
2.3.1. Vegetation maintenance
The roadside vegetation will need to be maintained in order to provide a protective barrier from air pollution exposures
yet not lead to safety concerns from reduced visibility or falling debris. Maintenance requirements will depend on vegetation
R. Baldauf / Transportation Research Part D 52 (2017) 354–361 357
type and species, so a plan should be in place when selecting and constructing the barrier for optimal long-term perfor-
mance. These requirements include watering and fertilization needs, trimming and other pruning requirements, and overall
plant care. Maintenance should also include vegetation replacement due to die-off, disease, or damage from accidents. Prop-
erly designed roadside vegetation may also minimize the need for extensive mowing and trimming, saving money and
reducing air pollution emissions.
2.3.2. Water runoff control
An additional benefit of a roadside vegetation barrier can be the control and containment of surface water runoff from the
impervious road and supporting infrastructure. Roadside barriers constructed to provide water runoff control can prevent
localized flooding as well as improve water quality in the area. For certain regions of the country, drought resistant vegeta-
tion that can also resist high-water events may be most appropriate.
2.3.3. Native and non-invasive species
Whenever feasible, native species should be considered for implementing the roadside barrier. Native species will usually
be more robust and resistant to local climatic conditions. The vegetation barriers should also not be constructed from inva-
sive species that may not be contained within the project area of interest, and may create problems at other locations or at
the roadside. Non-poisonous species should also be used, especially if present near children or in locations that have the
potential to cause harm in other ways.
2.3.4. Roadway safety
Planting on or near a highway right-of-way (ROW) requires consideration of potential safety issues. In most cases, the
applicable highway department will require approvals for planting near roads due to these issues. Concerns may include cre-
ating undesirable wildlife habitat near roadways (e.g. deer and other animals that can exacerbate auto accidents), preserving
safe lines-of-sight and viewshed standards for drivers on the road, maintaining clear zones and horizontal clearance for dri-
ver safety, ensuring compatibility of the chosen vegetation species with existing species, and not obstructing outdoor
advertising.
Fig. 1. Examples of (a) effective vegetation barriers that are dense with full coverage from the ground to the top of canopy and (b) ineffective roadside
barriers due to gaps and high porosity.
Fig. 2. Examples of effective combinations of vegetation with solid noise barriers. Panel (a) shows vegetation behind the barrier (as studied in Baldauf et al.
(2008) while panel and (b) shows bushy vegetation in front of the barrier (no empirical evidence available).
358 R. Baldauf / Transportation Research Part D 52 (2017) 354–361
3. Vegetation with noise barriers
Noise barriers combined with mature vegetation have also been found to result in lower ultrafine particle concentrations
along and away from a highway compared to an open field or a solid noise barrier alone (Bowker et al., 2007; Baldauf et al.,
2008). For vegetation planted with a solid noise barrier, the overall considerations should be the same as for vegetation
alone. However, for the vegetation to have an additive effect for pollutant reductions, the vegetation should extend beyond
the top of the solid barrier by a sufficient height in order to allow air flow through and over the plants to enhance pollutant
Table 1
Factors affecting the effectiveness of roadside barriers in mitigating near-road air pollution impacts.
Barrier
characteristic
Recommendation Description
Physical characteristics
Height 5 m or higher (or extend 1+ meter above an
existing solid barrier)
The higher the vegetative barrier, the greater the pollutant reductions. A
minimum of 5 m will provide enough height to be above typical emission
elevations for vehicles on the road (4 m if little to no trucks use the road).
However, heights of 10 m or more would provide additional pollutant
reductions
Thickness 10 m or more The thicker the vegetative barrier, the greater the pollutant reductions. A
minimum thickness of 10 m should provide enough of a barrier to remove
particulate and enhance dispersion. However, gaps in the barrier should be
avoided. Multiple rows of different types of vegetation (e.g. bushes, shrubs,
trees) should be considered for maximum coverage and pollutant removal
during all stages of the barrier. A thickness of as little as 5 m may be sufficient
with low porosity (high density) vegetation
Porosity 0.5–0.9 Porosity should not be too high to allow pollutants to easily pass through the
barrier or cause wind stagnation. As the porosity gets lower, the vegetation
barrier will perform similarly to a solid barrier, which may limit the amount of
particulate removal since air is forced up and around the plants
Length 50 m or more beyond area of concern Extending the barrier beyond the area of concern protects against pollutant
meandering around edges. May also consider constructing the barrier
perpendicular from the road depending on land availability
Vegetation characteristics
Seasonal effects Vegetation not subject to change by season Vegetative barrier characteristics must be consistent throughout all seasons
and climatic conditions in order to ensure effective pollutant reductions
Leaf surface Complex waxy and/or hairy surfaces with high
surface area
Leaf surfaces with complex and large surface areas will capture and contain
more particulate pollutants as air passes through the structure
Air emissions Vegetation with low or no air emissions Vegetation used for roadside barriers should not be sources of air pollution,
either at the local or regional scale
Pollution and
stress
resistant
Resistant to effects of air pollution and other
stressors
Vegetation must be able to survive and maintain its integrity under the high
pollution levels and stress that can occur near roads in order to provide
effective pollution reductions from traffic emissions. In addition to air
pollution, other stressors can include salt and sand for winter road
conditioning and noise impacts
Other considerations
Maintenance Plan must be in place to properly maintain
vegetative barrier
Proper vegetation maintenance must be provided in order for the barrier to
survive and maintain its integrity to provide effective pollution reductions
from traffic emissions
Water runoff Contain surface water runoff and improve water
quality
Roadside vegetative barriers constructed appropriately can provide an added
benefit of controlling and containing surface water runoff from the road, which
can also improve local water quality
Drought
resistant
Choose species resistant to drought and flooding Many regions face climatic conditions of extended drought followed by
localized flooding. Vegetative barrier must maintain its integrity under these
conditions in order to provide effective pollution reductions
Native species Choose native species Native species will be more robust and resistant to climatic conditions in the
area of interest; thus, maintaining its integrity under these conditions in order
to provide effective pollution reductions
Non-invasive Choose non-invasive species The use of non-invasive species will ensure effective pollutant reductions
without potential unintended consequences from invasive species adversely
effecting nearby land uses
Non-poisonous Choose non-poisonous species if sensitive
populations will be nearby
Non-poisonous species are strongly encouraged and should be used if the
barrier will be at a location with sensitive populations, such as elementary
schools, parks, and recreation fields where small children may be active and in
close contact
Roadway safety Maintains safety for drivers on the road;
conforms to local safety and permit requirements
Prior to planting, ensure vegetation plan will meet all safety and other local
permit requirements (e.g. local highway department, city planning
department) to preserve sight-lines and vegetation compatibility while
avoiding potential wildlife/auto accidents and obstruction of outdoor
advertising
R. Baldauf / Transportation Research Part D 52 (2017) 354–361 359
removal and air mixing. Vegetation in combination with a solid barrier will also likely be effective at higher porosities than
for vegetation alone since the addition of the vegetation primarily enhances particulate removal through diffusion or impac-
tion as the solid barrier already enhances turbulence and mixing of the traffic plume.
Solid barriers can vary in height; research on air pollution reductions from these structures has been conducted for
heights between 4 and 6 m as previously discussed. A vegetation barrier should extend at least 1 m above the barrier,
although the higher and thicker the plants, the greater the downwind reduction as suggested by Hagler et al. (2011). For
shorter solid barriers, vegetation should extend above the barrier to a height of at least 6 m to maximize the potential for
downwind pollutant reductions. Fig. 2 provides examples of combinations of vegetation with solid noise barriers that could
lead to increased reductions in downwind air pollutant concentrations.
Previous research is based on vegetation planted behind the noise barrier (opposite side from the road), although bushes
or trees in front could provide an added reduction if sufficiently away from the solid barrier to allow air to flow through the
plants. Some modeling studies suggest that ‘‘green walls” such as ivy or other climbing vegetation that grows on the solid
barrier surface may also improve local air quality (Pugh et al., 2012); however, as noted above, air flow through the vege-
tation would be needed to enhance particulate diffusion and impaction.
No research has been done on whether gaps or spaces in vegetation along solid walls can lead to increased downwind
concentrations. Since solid noise barriers alone can reduce downwind pollutant concentrations, gaps in accompanying veg-
etation would likely not have the same detrimental effects as with vegetation alone, although the gaps would likely limit the
added benefit of particulate removal from the adjacent vegetation.
4. Summary
Research shows that roadside vegetation can affect nearby air quality in both a positive and negative way. If properly
designed, vegetation barriers can be used to improve near-road air quality, either alone or in combination with solid barriers.
Many factors must be considered in designing effective roadside vegetative barriers that are applicable for use by transporta-
tion departments, urban planners and local developers as summarized in Table 1. These factors can be evaluated along with
site-specific information to guide the planning and development of roadside vegetation barriers that improve local air qual-
ity while also providing other benefits associated with increased green infrastructure in urban areas.
Acknowledgements
Special thanks go to the many experts who provided advice and comments for the development of these recommenda-
tions as detailed in EPA (2016). These experts include David Nowak (U.S. Forest Service), Greg McPherson (U.S. Forest Ser-
vice), Kevin Jefferson (Urban Releaf), David Ralston (Bay Area Air Quality Management District), Tom Hanf (Michigan DOT),
Drew Buckner (Michigan DOT), Gorette Yung (Michigan DOT), Kevin Sayers (Michigan DEQ), Sheila Batka (U.S. EPA), Ken
Davidson (U.S. EPA), Bob Newport (U.S. EPA), Laura Jackson (U.S. EPA), Sue Kimbrough (U.S. EPA) and Vlad Isakov (U.S. EPA).
References
Al-Dabbous, A.N., Kumar, P., 2014. The influence of roadside vegetation barriers on airborne nanoparticles and pedestrians exposure under varying wind
conditions. Atmos. Environ. 90, 113–124.
Baldauf, R.W., Thoma, E., Khlystov, A., Isakov, V., Bowker, G., Long, T., 2008. Impacts of noise barriers on near-road air quality. Atmos. Environ. 42, 7502–
7507.
Baldauf, R.W., Isakov, V., Deshmukh, P.J., Venkatram, A., Yang, B., Zhang, K.M., 2016. Influence of solid noise barriers on near-road and on-road air quality.
Atmos. Environ. 129, 265–276.
Bowker, G.E., Baldauf, R.W., Isakov, V., Khlystov, A., Petersen, W., 2007. The effects of roadside structures on the transport and dispersion of ultrafine
particles from highways. Atmos. Environ. 41, 8128–8139.
Brantley, H.L., Hagler, G.S.W., Deshmukh, P.J., Baldauf, R.W., 2014. Field assessment of the effects of roadside vegetation on near-road black carbon and
particulate matter. Sci. Total Environ. 468–469, 120–129.
Buccolieria, R., Gromke, C., Di Sabatinoa, S., Ruck, B., 2009. Aerodynamic effects of trees on pollutant concentration in street canyons. Sci. Total Environ. 407,
5247–5256.
Buccolieria, R., Salimb, S.M., Leoa, L.S., Di Sabatinoa, S., Chanb, A., Ielpoc, P., Gennarod, G., Gromke, C., 2011. Analysis of local scale tree–atmosphere
interaction on pollutant concentration in idealized street canyons and application to a real urban junction. Atmos. Environ. 45, 1702–1713.
Dadvand, P., Nieuwenhuijsen, M.J., Esnaola, M., Forns, J., Basagaña, X., Alvarez-Pedrerol, M., Rivas, I., López-Vicente, M., Pascual, M.D.C., Su, J., Jerrett, M.,
2015. Green spaces and cognitive development in primary schoolchildren. Proc. Natl. Acad. Sci. 112 (26), 7937–7942.
EPA, 2016. Recommendations for Constructing Roadside Vegetation Barriers to Improve Near-Road Air Quality. EPA 600/R-16/072, Office of Research and
Development, Research Triangle Park, North Carolina.
Finn, D., Clawson, K.L., Carter, R.G., Rich, J.D., Eckman, R.M., Perry, S.G., Isakov, V., Heist, D.K., 2010. Tracer studies to characterize the effects of roadside noise
barriers on near-road pollutant dispersion under varying atmospheric stability conditions. Atmos. Environ. 44 (2), 204–214.
Frank, C., Ruck, B., 2008. Numerical study of the airflow over forest clearings. Forestry 81, 259–277.
Gallagher, J., Baldauf, R., Fuller, C.H., Kumar, P., Gill, L.W., McNabola, A., 2015. Passive methods for improving air quality in the built environment: a review
of porous and solid barriers. Atmos. Environ. 120, 61–70.
Gromke, C., Buccolieria, R., Di Sabatinoa, S., Ruck, B., 2008. Dispersion study in a street canyon with tree planting by means of wind tunnel and numerical
investigations – evaluation of CFD data with experimental data. Atmos. Environ. 42, 8640–8650.
Gromke, C., Jamarkattel, N., Ruck, B., 2016. Influence of roadside hedgerows on air quality in urban street canyons. Atmos. Environ. 139, 75–86.
Hagler, G.S., Tang, W., Freeman, M.J., Heist, D.K., Perry, S.G., Vette, A.F., 2011. Model evaluation of roadside barrier impact on near-road air pollution. Atmos.
Environ. 45 (15), 2522–2530.
Hagler, G.S.W., Lin, M.-Y., Khlystov, A., Baldauf, R.W., Isakov, V., Faircloth, J., 2012. Field investigation of roadside vegetative and structural barrier impact on
near-road ultrafine particle concentrations under a variety of wind conditions. Sci. Total Environ. 419, 7–15.
360 R. Baldauf / Transportation Research Part D 52 (2017) 354–361
Health Effects Institute (HEI), 2010. Traffic-Related Air Pollution: A Critical Review of the Literature On Emissions, Exposure, and Health Effects. HEI Special
Report 17. Health Effects Institute, Boston, MA.
Heist, D.K., Perry, S.G., Brixey, L.A., 2009. A wind tunnel study of the effect of roadway configurations on the dispersion of traffic-related pollution. Atmos.
Environ. 43 (32), 5101–5111.
James, P., Banay, R.F., Hart, J.E., Laden, F., 2015. A review of the health benefits of greenness. Curr. Epidemiol. Rep. 2, 131–142.
James, P., Hart, J.E., Banay, R.F., Laden, F., 2016. Exposure to greenness and mortality in a Nationwide Prospective Cohort Study of Women. Environ. Health
Perspect. http://dx.doi.org/10.1289/ehp.1510363.
Janhall, S., 2015. Review on urban vegetation and particle air pollution–deposition and dispersion. Atmos. Environ. 105, 130–137 (201).
Li, X.B., Lu, Q.C., Lu, S.J., He, H.D., Peng, Z.R., Gao, Y., Wang, Z.Y., 2016. The impacts of roadside vegetation barriers on the dispersion of gaseous traffic
pollution in urban street canyons. Urban Forest. Urban Green. 17, 80–91.
Lin, M.Y., Khlystov, A., 2012. Investigation of ultrafine particle deposition to vegetation branches in a wind tunnel. Aerosol Sci. Technol. 46 (4), 465–472.
Neft, I., Scungio, M., Culver, N., Singh, S., 2016. Simulations of aerosol filtration by vegetation: validation of existing models with available lab data and
application to near-roadway scenario. Aerosol Sci. Technol. 50 (9), 937–946.
Nowak, D.J., Civerolo, K.L., Trivikrama Rao, S., Gopal, S., Luley, C.J., Crane, D.E., 2000. A modeling study of the impact of urban trees on ozone. Atmos. Environ.
34, 1601–1613.
Nowak, D.J., Hirabayashi, S., Bodine, A., Greenfield, E., 2014. Tree and forest effects on air quality and human health in the United States. Environ. Pollut. 193,
119–129.
Pataki, D.E., Carreiro, M.M., Cherrier, J., Grulke, N.E., Jennings, V., Pincetl, S., Pouyat, R.V., Whitlow, T.H., Zipperer, W.C., 2011. Coupling biogeochemical cycles
in urban environments: ecosystem services, green solutions, and misconceptions. Front. Ecol. Environ. 9 (1), 27–36.
Petroff, A., Zhang, L., Pryor, S.C., Belot, Y., 2009. An extended dry deposition model for aerosols onto broadleaf canopies. J. Aerosol Sci. 40, 218–240.
Pugh, T.A., MacKenzie, A.R., Whyatt, J.D., Hewitt, C.N., 2012. Effectiveness of green infrastructure for improvement of air quality in urban street canyons.
Environ. Sci. Technol. 46 (14), 7692–7699.
Steffens, J.T., Wang, Y.J., Zhang, K.M., 2012. Exploration of effects of a vegetation barrier on particle size distributions in a near-road environment. Atmos.
Environ. 50, 120–128.
Tong, Z., Whitlow, T.H., MacRae, P.F., Landers, A.J., Harada, Y., 2015. Quantifying the effect of vegetation on near-road air quality using brief campaigns.
Environ. Pollut. 201, 141–149.
Tong, Z., Baldauf, R.W., Isakov, V., Deshmukh, P., Zhang, K.M., 2016. Roadside vegetation barrier designs to mitigate near-road air pollution impacts. Sci. Total
Environ. 541, 920–927.
Vos, P.E., Maiheu, B., Vankerkom, J., Janssen, S., 2013. Improving local air quality in cities: to tree or not to tree? Environ. Pollut. 183, 113–122.
Wania, A., Brse, M., Blond, N., Weber, C., 2012. Analysing the influence of different street vegetation on traffic-induced particle dispersion using microscale
simulations. J. Environ. Manage. 94, 91–101.
R. Baldauf / Transportation Research Part D 52 (2017) 354–361 361
... Only a few parts of atmospheric PM deposited on the plants are permanently immobilized and absorbed, the others would be washed away by heavy rain or blown off by strong wind, and they then start a new deposition process (Freer--Smith et al., 2005). Although the urban forest has been considered to have positive effects on PM capture and removal, their interactions do not follow a widespread or universal pattern (Al-Dabbous and Kumar, 2014;Baldauf, 2017;Buccolieri et al., 2009;Deshmukh et al., 2018;Jeanjean et al., 2017;Ottosen and Kumar, 2020;Yli-Pelkonen et al., 2017). The PM removal capacities reported by the scattered studies vary greatly and even are contradictory depending on the study region, evaluation method, meteorological parameter, and vegetation characteristics (Brantley et al., 2014;Fantozzi et al., 2015;Grundström and Pleijel, 2014;Hagler et al., 2012;Morakinyo and Lam, 2016;Tong et al., 2016a;Viippola et al., 2018). ...
... Most studies have reported the positive effects of vegetation barriers and urban parks, with the reductions in PM concentrations at various sizes ranging from 20% to 60% (Al-Dabbous and Kumar, 2014;Baldauf, 2017;Buccolieri et al., 2009;Deshmukh et al., 2018;Jeanjean et al., 2017;Ottosen and Kumar, 2020;Yli-Pelkonen et al., 2017). However, a few studies have also indicated that vegetation barriers or hedges have limited (Brantley et al., 2014;Fantozzi et al., 2015;Grundström and Pleijel, 2014;Hagler et al., 2012) or even negative (Morakinyo and Lam, 2016;Tong et al., 2016a;Viippola et al., 2018) effects on PM removal under certain conditions. ...
... A series of vegetation characteristics significantly affect atmospheric PM removal (Baldauf, 2017) by changing paticle aerodynamics (Viippola et al., 2018;Mori et al., 2018). These characteristics include the canopy density (Abhijith et al., 2017;Gromke and Blocken, 2015;Wania et al., 2012), community porosity (Deshmukh et al., 2018;Ozdemir, 2019), physical dimensions (Al-Dabbous and Kumar, 2014;Vos et al., 2013), leaf area density (Jin et al., 2014;Morakinyo and Lam, 2016), plant species (Yan et al., 2016;Zhang et al., 2015), and configurational designs (Abhijith and Kumar, 2019;Gómez et al., 2016;Tiwari et al., 2019). ...
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Increasing studies worldwide have examined the impacts of urban forests on mitigating atmospheric particulate matter (PM) over the past decades. These scattered studies revealed the aerodynamics of atmospheric PM deposited on urban forests, as well as the various factors influencing the PM capture and removal by urban forests. However, these evidences are varying and even contradictory, and the affecting factors do not follow a universal pattern. In addition, these studies generally have been conducted in a specific scale such as leaf, stand and city without considering the multi-scale associations and incorporations. This literature review tried to address the associations of urban forest and PM removal across single tree, stand and regional scales, and summarized the confounding factors for PM capture and removal within each scale. Particle size and local meteorology have significant impacts across scales. For an individual tree, PM capture and removal capacity are largely determined by the leaf morphology and epidermal structures, but at the stand scale, the biophysical characteristics and configurational designs of urban forests are the essential factors. At the city and regional scale, the determinants are the fraction of forest coverage, as well as background pollution levels. The literature collation emphasizes the necessity of concerning the appropriate factors responding to the specific scale when quantifying and evaluating PM capture and removal by urban forests, and warrants a multi-scale research paradigm and inclusive modeling evaluation incorporating the confounding factors from multiple scales for PM capture and removal by urban forests.
... Undesired disturbing sounds caused by traffic are mainly made of a combination of non-harmonic vibrations. Noise is a significant problem while improving the transportation network and thus increasing the traffic status of cities (Fujiwara et al. 1998;Government of Hong Kong 2003;Van Renterghem et al. 2015;Baldauf 2017). To fix the problem at this point, the technique of noise barrier installation is one of the efficient and successful methods. ...
... Consequently, there is a third solution. A noise barrier is ensured with a sound-absorbent element on the side facing the highway so that reflection is reduced or entirely eliminated (Watts et al. 1994;Baldauf 2017). For instance, Figure-9 depicts a noise-absorbing barrier made of steel frame and steel grid in front (close up in the left corner) in Denmark. ...
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We are honored to present our new scientific book entitled "Recent Researches in Engineering Studies/2021" to you. This technical book covers recent selected studies in 2021 related to engineering numerical simulations and experiments. We believe that this reports will help young academicians and lead to further studies. In this book, we aimed to bring respected researchers overall the world and to share their recent researchers with the literature. Also, it is aimed to provide a fundamental reference source for administrative and technical personnel working at the research centre, universities and different technical sector. This work includes different scientific researchers, design and practical applications, is also a base work published in english for researchers and academicians who study in engineering fields and want to specialize in this field. The resulting work is also part of a common library pool for the international digital library, universities library, companies and its business partners and other industry stakeholders where they can always refer to technical knowledge. This fundamental work on different engineering fields is the first part of our work for the first quarter of 2021 and will continue to develop with the support of you, as our readers, with new editions and chapters. In order for our work to become widespread, it will be highly honored for you to announce to your surroundings and to convey your opinions and thoughts to us. As editor and authors of this book, we would like to send our warmest greetings to all of you and looking forward to having your contributions to future publications, pandemic-free and peaceful word. With warm regards,
... Greenery may reduce pollution concentrations because the plants themselves are a substitute for the pollution source (King et al., 2014;Wu et al., 2015). From the three-dimensional (3D) perspective, plants can decrease the concentration of PM 2.5 (Zeng et al., 2022) and can also increase it by altering air flows (Baldauf, 2017;Janhäll, 2015;Jeanjean et al., 2017;Wania et al., 2012). Most studies at the street scale use CFD simulations (Jeanjean et al., 2017;Wania et al., 2012), which simplify the built environment and give approximate rather than realistic and accurate predictions. ...
... Considering deposition and dispersion as two key direct effects of urban trees on air quality (Eisenman et al., 2019), these results may be attributed to the following two factors: 1) only a small amount of PM 2.5 at the street level is absorbed through the leaf pores of trees, and most of it is deposited on the surface of tree leaves, where wind flow resuspends it within the street area (Nowak et al., 2014;Weinbruch et al., 2014); and 2) trees on both sides of the street form a barrier that reduces the upward flowing wind speed within the street canyon, resulting in pollutants that cannot be effectively diffused out from the top of the street and circulate within a certain area of the street (Baldauf, 2017;Janhäll, 2015;Jeanjean et al., 2017;Wania et al., 2012). The effect of trees in reducing air movement was also confirmed in a recent paper on microclimate research . ...
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Greenery may be effective in mitigating particulate matter (PM) pollution. However, most previous studies have explored the effects of greenery on air quality from a two-dimensional rather than a three-dimensional (3D) perspective. In this study, a geographically weighted regression (GWR) model was constructed to explore the effect of street greenery on street-level PM 2.5 concentrations using street view imagery from a 3D perspective. Mobile monitoring of PM 2.5 concentrations was conducted in Wuhan. The results demonstrated that GWR is suitable for exploring street-level air quality. From a 3D perspective, street greenery strongly affects the air quality of the surrounding area within 300 m. Moreover, considering spatial nonstationarity, the negative impact of street greenery on air quality depends on the pollution and ventilation conditions. High pollution and canyon effects amplify the negative impact of street greenery on pollution dispersion. Thus, the negative effect of street greenery on air quality is sensitive to the density of second-level roads and of commercial areas, the number of intersections and the distance to rivers. Dense planting trees in these areas must be reconsidered based on the possibility of ventilation loss. These results provide insights into street-level PM 2.5 pollution and can contribute to the evidence-based design of urban green infrastructure.
... Firstly, regarding GI width, research suggests that thicker green barriers are more effective at AQ provisioning [28,67,68]. Some studies suggest up to a minimum width of 10 m, although such wide thickness approach seems to be more suitable for protecting populations near long open roads, such as motorways [12]. ...
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... In general, the higher the canopy density and hierarchical structure, the higher the carbon sequestration capacity of the plant community [46], but its landscape function needs to be considered in the design of the green space. There is evidence that dense plant communities have high concentrations of pollutant uptake capacity and high allergenicity [47,48]. Therefore, it is recommended to choose a suitable density for plant growth, which not only attracts residents and provides multifunctional space, but also promotes carbon sequestration in the long term. ...
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Urban green space is considered to reduce the concentration of forcing factors of climate change such as the carbon dioxide in the atmosphere. Promoting carbon sequestration efficiency within a limited urban green space has become a practical challenge that must be faced in urban sustainability. This study proposed three design models and a list of high carbon sequestration plants. Based on the research on the distribution and change in carbon sequestration in urban green spaces, combined with field surveys and remote sensing images, this study analyzed the main factors affecting carbon sequestration. The results showed that the carbon sequestration capacity in urban green space tends to decrease gradually along with the change in forest structure in a time series of the years 2000, 2007, 2014, and 2019, and this trend was mainly related to the characteristic factors of plant communities in urban green spaces: the carbon sequestration of plants was significantly positively correlated with DBH (diameter at breast height) and community density; positively correlated with hierarchical structure. In addition, we put forward a list of plants with high carbon sequestration, including Styphnolobium japonicum, Salix babylonica, Pittosporum tobira, Spiraea salicifolia, and Iris pseudacorus, proposed three planting design models for different green spaces and habitats to improve the efficiency of carbon sequestration in urban green spaces, and established the community structure models of high carbon-fixing plants which can be directly applied to practical projects. It also explored the sustainable design approach of ecological processes in low-carbon cities.
... To standardize the control of particulate matter (PM) pollution concentrations, the "National Air Pollution Prevention and Control Law" was revised in 2015 in China [3]. In addition to promoting and using renewable energy and reducing motor vehicle emissions, programs that can alleviate road traffic pollution are being increasingly studied [4]. ...
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Determining the relationships between the structure and species of plant communities and their impact on ambient particulate matter (PM) is an important topic in city road greenbelt planning and design. The correlation between the distribution of plant communities and ambient PM concentrations in a city road greenbelt has specific spatial patterns. In this study, we selected 14 plant-community-monitoring sites on seven roads in Nanjing as research targets and monitored these roads in January 2022 for various parameters such as PM with aerodynamic diameters ≤ 10 µm (PM10) and PM with aerodynamic diameters ≤ 2.5 µm (PM2.5). We used a spatial model to analyze the relationship between the concentrations of ambient PM10 and PM2.5 and the spatial heterogeneity of plant communities. The consequences revealed that the composition and species of plant communities directly affected the concentrations of ambient PM. However, upon comparing the PM concentration patterns in the green community on the urban road, we found that the ability of the plant community structures to reduce ambient PM is in the order: trees + shrubs + grasses > trees + shrubs > trees + grasses > pure trees. Regarding the reduction in ambient PM by tree species in the plant community (conifer trees > deciduous trees > evergreen broad-leaved trees) and the result of the mixed forest abatement rate, coniferous + broad-leaved trees in mixed forests have the best reduction ability. The rates of reduction in PM10 and PM2.5 were 14.29% and 22.39%, respectively. We also found that the environmental climate indices of the road community, temperature, and traffic flow were positively correlated with ambient PM, but relative humidity was negatively correlated with ambient PM. Among them, PM2.5 and PM10 were significantly related to temperature and humidity, and the more open the green space on the road, the higher the correlation degree. PM10 is also related to light and atmospheric radiation. These characteristics of plant communities and the meteorological factors on urban roads are the foundation of urban greenery ecological services, and our research showed that the adjustment of plant communities could improve greenbelt ecological services by reducing the concentration of ambient PM.
... Similarly, in open road conditions, trees and hedges can reduce the concentration of pollutants near highways by serving as barriers between pollution sources and human receptors ( Figure 5; Abhijith et al. 2017;Xing and Brimblecombe 2020). Unlike vegetation in urban canyons, these barriers provide maximum benefits when the trees or hedges are tall (>0.5 m), thick (>10 m), and moderately porous (Baldauf 2017 Strategic plantings of trees along roadsides could shield sensitive populations from exposure to PM and other pollutants, especially in areas with insufficient green space. Because deposition of pollutants on plant surfaces decreases Trees as pollutant traps: dense tree canopies can trap polluted air (black arrows) and allergens (green dots) at ground level and prevent dilution with clean air from the atmosphere (white arrows). ...
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Better Forests, Better Cities evaluates how forests both inside and outside city boundaries benefit cities and their residents, and what actions cities can take to conserve, restore and sustainably manage those forests. This report is the first of its kind comprehensive resource on the connection between cities and forests, synthesizing hundreds of research papers and reports to show how all forest types can deliver a diverse suite of benefits to cities.
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Heather Henry explains how nature can boost public health, and how to dodge the downsides
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Background: Green, natural environments may ameliorate adverse environmental exposures (e.g. air pollution, noise, and extreme heat), increase physical activity and social engagement, and lower stress. Objectives: We aimed to examine the prospective association between residential greenness and mortality. Methods: Using data from the US-based Nurses' Health Study prospective cohort, we defined cumulative average time-varying seasonal greenness surrounding each participant's address using satellite imagery (Normalized Difference Vegetation Index (NDVI)). We followed 108,630 women and observed 8,604 deaths between 2000-2008. Results: In models adjusted for mortality risk factors (age, race/ethnicity, smoking, and individual- and area-level socioeconomic status), women living in the highest quintile of cumulative average greenness (accounting for changes in residence during follow-up) in the 250m area around their home had a 12% lower rate of all-cause non-accidental mortality (95% CI 0.82, 0.94) compared to those in the lowest quintile. Results were consistent for the 1,250m area, although the relationship was slightly attenuated. These associations were strongest for respiratory and cancer mortality. Findings from a mediation analysis suggest that the association between greenness and mortality may be at least partly mediated by physical activity, particulate matter less than 2.5 micrometers, social engagement, and depression. Conclusions: Higher levels of green vegetation were associated with decreased mortality. Policies to increase vegetation may provide opportunities for physical activity, reduce harmful exposures, increase social engagement, and improve mental health. While planting vegetation may mitigate effects of climate change, evidence of an association between vegetation and lower mortality rates suggests it also might be used to improve health.
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Public health concerns regarding adverse health effects for populations spending significant amounts of time near high traffic roadways has increased substantially in recent years. Roadside features, including solid noise barriers, have been investigated as potential methods that can be implemented in a relatively short time period to reduce air pollution exposures from nearby traffic. A field study was conducted to determine the influence of noise barriers on both on-road and downwind pollutant concentrations near a large highway in Phoenix, Arizona, USA. Concentrations of nitrogen dioxide, carbon monoxide, ultrafine particles, and black carbon were measured using a mobile platform and fixed sites along two limited-access stretches of highway that contained a section of noise barrier and a section with no noise barrier at-grade with the surrounding terrain. Results of the study showed that pollutant concentrations behind the roadside barriers were significantly lower relative to those measured in the absence of barriers. The reductions ranged from 50% within 50 m from the barrier to about 30% as far as 300 m from the barrier. Reductions in pollutant concentrations generally began within the first 50 meters of the barrier edge; however, concentrations were highly variable due to vehicle activity behind the barrier and along nearby urban arterial roadways. The concentrations on the highway, upwind of the barrier, varied depending on wind direction. Overall, the on-road concentrations in front of the noise barrier were similar to those measured in the absence of the barrier, contradicting previous modeling results that suggested roadside barriers increase pollutant levels on the road. Thus, this study suggests that noise barriers do reduce potential pollutant exposures for populations downwind of the road, and do not likely increase exposures to traffic-related pollutants for vehicle passengers on the highway.
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Protecting the health of growing urban populations from air pollution remains a challenge for planners and requires detailed understanding of air flow and pollutant transport in the built environment. In recent years, the work undertaken on passive methods of reducing air pollution has been examined to address the question: “how can the built environment work to alter natural dispersion patterns to improve air quality for nearby populations?” This review brings together a collective of methods that have demonstrated an ability to influence air flow patterns to reduce personal exposure in the built environment. A number of passive methods exists but, in the context of this paper, are split into two distinct categories: porous and/or solid barriers. These methods include trees and vegetation (porous) as well as noise barriers, low boundary walls and parked cars (solid);, all of which have gained different levels of research momentum over the past decade. Experimental and modelling studies have provided an understanding of the potential for these barriers to improve air quality under varying urban geometrical and meteorological conditions. However, differences in results between these studies and real-world measurements demonstrate the challenges and complexities of simulating pollutant transport in urban areas. These methods provide additional benefits to improving air quality through altering dispersion patterns; avenue trees and vegetation are aesthetically pleasing and reduce noise pollution. Additionally, real-world case studies are considered an important direction for further verification of these methods in the built environment. Developing design guidelines is an important next stage in promoting passive methods for reducing air pollution and ensuring their integration into future urban planning strategies. In addition, developing channels of communication with urban planners will enhance the development and uptake of design guidelines to improve air quality in the built environment.
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Significance Green spaces have a range of health benefits, but little is known in relation to cognitive development in children. This study, based on comprehensive characterization of outdoor surrounding greenness (at home, school, and during commuting) and repeated computerized cognitive tests in schoolchildren, found an improvement in cognitive development associated with surrounding greenness, particularly with greenness at schools. This association was partly mediated by reductions in air pollution. Our findings provide policymakers with evidence for feasible and achievable targeted interventions such as improving green spaces at schools to attain improvements in mental capital at population level.
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Researchers are increasingly exploring how neighborhood greenness, or vegetation, may affect health behaviors and outcomes. Greenness may influence health by promoting physical activity and social contact; decreasing stress; and mitigating air pollution, noise, and heat exposure. Greenness is generally measured using satellite-based vegetation indices or land-use databases linked to participants’ addresses. In this review, we found fairly strong evidence for a positive association between greenness and physical activity and a less consistent negative association between greenness and body weight. Research suggests greenness is protective against adverse mental health outcomes, cardiovascular disease, and mortality, though most studies were limited by cross-sectional or ecological design. There is consistent evidence that greenness exposure during pregnancy is positively associated with birth weight, though findings for other birth outcomes are less conclusive. Future research should follow subjects prospectively, differentiate between greenness quantity and quality, and identify mediators and effect modifiers of greenness-health associations.
Article
Concentrations of ultrafine particles (UFP) are generally elevated in the near-roadway environment due to traffic-related pollution. Exposure to UFP has been linked to adverse health effects for communities living near major roadways. Strategies to mitigate near-roadway air pollution include vehicle emission regulations, as well as installation of physical barriers such as walls, tree stands, and shrubs. Numerical simulation tools can be very useful to investigate the effectiveness of these barriers in mitigating air pollution. In the present work, a Reynolds-Averaged Navier–Stokes (RANS) based computational fluid dynamics (CFD) solver is used to predict filtration of UFP by vegetation. The RANS equations for turbulent flow are combined with a dry deposition velocity model and three different wake turbulence models. Reasonably good predictions of pressure drop across the vegetation and particle penetration efficiency are obtained when compared with available wind tunnel experiments for high leaf area density (LAD) in the range 69–263 . It is found that the model predictions are sensitive to the choice of wake turbulence model and certain model parameters. The model predictions also suggest that thin roadside vegetation with local LAD ≤ 5 is only partially effective in filtering UFP, especially when the vegetation thickness is less than 10 m along the direction of the wind. Copyright 2016 American Association for Aerosol Research
Article
Understanding pollutant dispersion in the urban environment is an important aspect of providing solutions to reduce personal exposure to vehicle emissions. To this end, the dispersion of gaseous traffic pollutants in urban street canyons with roadside hedges was investigated. The study was performed in an atmospheric boundary layer wind tunnel using a reduced-scale (M = 1:150) canyon model with a street-width-to-building-height ratio of W/H = 2 and a street-length-to-building-height ratio of L/H = 10. Various hedge configurations of differing height, permeability and longitudinal segmentation (continuous over street length L or discontinuous with clearings) were investigated. Two arrangements were examined: (i) two eccentric hedgerows sidewise of the main traffic lanes and (ii) one central hedgerow between the main traffic lanes. In addition, selected configurations of low boundary walls, i.e. solid barriers, were examined. For a perpendicular approach wind and in the presence of continuous hedgerows, improvements in air quality in the center area of the street canyon were found in comparison to the hedge-free reference scenario. The pollutant reductions were greater for the central hedge arrangements than for the sidewise arrangements. Area-averaged reductions between 46 and 61% were observed at pedestrian head height level on the leeward side in front of the building for the centrally arranged hedges and between 18 and 39% for the two hedges arranged sidewise. Corresponding area-averaged reductions ranging from 39 to 55% and from 1 to 20% were found at the bottom of the building facades on the leeward side. Improvements were also found in the areas at the lateral canyon ends next to the crossings for the central hedge arrangements. For the sidewise arrangements, increases in traffic pollutants were generally observed. However, since the concentrations in the end areas were considerably lower compared to those in the center area, an overall improvement remained for the street canyon. The configuration of a sidewise arranged discontinuous hedgerow resulted in general in area-averaged increases in concentrations in the range of 3–19%. For a parallel approach wind, reduced concentrations of up to 30% at the facades and up to 60% at pedestrian level were measured with a sidewise continuous hedgerow arrangement. It is concluded that continuous hedgerows can effectively be employed to control concentrations of traffic pollutants in urban street canyons. They can advantageously affect the air quality at street level and can be a significant remedy to the pedestrians' and residents’ exposure in the most polluted center area of a street canyon.
Article
With increasing evidence that exposures to air pollution near large roadways increases risks of a number of adverse human health effects, identifying methods to reduce these exposures has become a public health priority. Roadside vegetation barriers have shown the potential to reduce near-road air pollution concentrations; however, the characteristics of these barriers needed to ensure pollution reductions are not well understood. Designing vegetation barriers to mitigate near-road air pollution requires a mechanistic understanding of how barrier configurations affect the transport of traffic-related air pollutants. We first evaluated the performance of the Comprehensive Turbulent Aerosol Dynamics and Gas Chemistry (CTAG) model with Large Eddy Simulation (LES) to capture the effects of vegetation barriers on near-road air quality, compared against field data. Next, CTAG with LES was employed to explore the effects of six conceptual roadside vegetation/solid barrier configurations on near-road size-resolved particle concentrations, governed by dispersion and deposition. Two potentially viable design options are revealed: a) a wide vegetation barrier with high Leaf Area Density (LAD), and b) vegetation–solid barrier combinations, i.e., planting trees next to a solid barrier. Both designs reduce downwind particle concentrations significantly. The findings presented in the study will assist urban planning and forestry organizations with evaluating different green infrastructure design options