Green City

Filtering for fresh air

Health

Climate

Improve regional air quality with large scale green spaces

The filtering effect of large-scale green contributes to improving air quality at the regional level. Forests are especially suitable for reducing background concentrations before the pollution even reaches the city. By providing as much leaf volume as possible in an area as big as possible, the general air quality can improve. Particulate matter is only captured at the edges and tops of a forest, but because they usually cover a large surface area, they are very effective.

Plant big trees with more leaf area

Bigger trees and more leaf area are more effective at cleaning the air. An average tree in the city (with a trunk diameter of +/- 30 cm) is able to capture about 100g particulate matter (PM10) while a mature tree captures as much as 1.4kg in the Netherlands. 100g PM10 equals the particulate matter emission from a private car that travels 1,500 km. 1.4kg PM10 equals 20,000 km. The porosity of the canopy should be above 50% so the leaves inside the canopy can also help. Combine trees with broad canopies and undergrowth of herbaceous plants and shrubs to have effective leaves at all levels. Green roofs and green walls also contribute to air filtering.

Some trees are better at cleaning the air

Trees and vegetation in urban landscapes provide noticeable benefits on a local scale. Some gaseous air pollution such as nitrous oxide (NOX), sulfur dioxide (SO2) and ozone (O3) are absorbed by the stomata of leaves. Deciduous trees with a large leaf volume are most effective.

In practice, vegetation has limited benefit for reducing citywide concentrations of nitrous dioxide (NO2), despite scientific studies reporting the ability of plants to capture NO2. However, in the still, polluted air created by street canyons, increased planting at street level can reduce NO2 levels by as much as 40% and particulate matter by as much as 60%.

Organic compounds such as polychlorinated biphenyl (PCBs), dioxins and furans are taken up by the cuticles of leaves (even at night). Ozone (O3) concentrations are reduced in the presence of trees as a result of direct ozone absorption and increased humidity and lower temperatures that reduce ozone formation.

Plant a variety of trees and plants

Include a mix of evergreen and deciduous tree species in the design with different characteristics so that many benefits are achieved. Include green at different heights / levels for ground to canopy benefits.  Particulate matter (PM10) is captured on the surface of leaves, called impaction, and travels to the ground through wind, rainwater or fallen leaves. At ground level, particulates are either washed away with runoff or fixed in the soil by organic decomposition. Conifers are most effective because of the large surface area of needles and the fact that the trees keep their needles all year round.

Avoid using any tree species that are sensitive to air pollution (NOX) and limit the use of trees which emit biogenic volatile organic compounds (BVOC).

Green roofs and green walls also filter the air

Green on roof gardens and green walls contributes to removing pollutants from the air. Cool air flowing down off green roofs and walls encourages air movement at street level and contributes to dispersal of pollutants.

Green roofs, with careful plant selection, can remove 70-85kg ha -1 of aerial pollutants per annum. Citywide planning offers the capacity for green roofs to represent as much as 32% of the horizontal surface area of the urban landscape.  Plant capacity varies due to differences in micro-and macro-morphology: the grasses Agrostis stolonifera and Festuca rubra are more effective than the broadleaves Plantago lanceolata and Sedum album at capturing PM10. In general, intensive green roofs have a higher impact than extensive green roofs.

Green walls planted with vines have a very dense leaf area per square metre and thus are good in removing PM10. A wall with Parthenocissus tricuspidata can catch 4g of PM10 per m2 of wall and Hedera helix can catch 6g.


Use trees and plants to reduce background concentrations of air pollution. All plants contribute to the improvement of air quality. Some species are more effective than others.

 

 

 

 

Air pollution types and leaf effectiveness

SO2, NOx, O3PM10VOC
Methodabsorptionimpactionadsorption
Best tree typebroad leaved evergreen treesconifer trees (evergreen)conifer trees
Leaf characteristicsflat, wide, glossy leavescone-shaped needlesneedles with a fatty top layer (cuticle)
Other good tree typesdeciduous treesdeciduous trees
Leaf characteristicsflat, wide, glossy leavescourse, hairy, sticky leaves

Best trees, shrubs and vines for filtering

Trees, shrubs and vines* that are effective in capturing PM10

Picea abies,  Hedera

Pinus mugo, Prunus padus

Pinus nigra,  Betula pubescens

Pinus sylvestris, Ilex x. meservae

Taxus sp., Corylus colurna

Metasequoia glyptostroboides,  Acer pseudoplatanus

Trees, shrubs and vines* that are effective in absorbing NOX

Robinia pseudoacacia, Prunus ‘Yoshino’

Sophora japonica, Zelkova serrata

Magnolia, Populus nigra

Salix babylonica ‘Tortuosa’

Trees, shrubs and vines* that are effective in absorbing O3

Chamaecyparis lawsoniana,  Betula pendula

Crataegus monogyna,  Acer campestre

Larix decidua,  Pinus nigra

Prunus laurocerasus,  Alnus glutinosa

Acer platanoides

Trees and shrubs that emit biogenic volatile organic compounds (limit widespread planting)

* 1 Cercidiphyllum japonicum

1 Fagus spp.

1 Ginkgo

1 Ilex spp.

*Koelreuteria paniculata

* 1 Liquidambar styraciflua

1 Magnolia spp.

* 1 Platanus

1 Populus spp.

* 1 Quercus (particularly Q. robur 2 and Q. rubra 2 )

*Robinia pseudoacacia

* 1 Salix (particularly S. alba 2 , S. caprea 2 , S, fragilis 2 )

1 Styphnolobium japonicum

1 Syringa spp.

*plants commonly used in The Netherlands

1 Tree species selection for Green Infrastructure: a guide for specifiers. Hirons and Sjoman, 2019. http://www.tdag.org.uk/uploads/4/2/8/0/4280686/tdag_treespeciesguidev1.3.pdf  Species known to be high BVOC-emitters produce more than 10 μg g-1 h-1 (microgram of BVOC per gram of dry weight per hour) and this information is based on Lancaster University’s BVOC dataset and Samson et al. (2017).

Samson, R., Ningal, T.F., Tiwary, A., Grote, R., Fares, S., Saaroni, H., Hiemstra, J.A., Zhiyanski, M., Vilhar, U., Cariñanos, P. and Järvi, L. (2017) Species-Specific Information for Enhancing Ecosystem Services. In: Pearlmutter, D., Calfapietra, C., Samson, R., O’Brien, L., Ostoic´, S.K., Sanesi, G. and del Amo, R.A. (eds.) The urban forest: cultivating green infrastructure for people and the environment (Vol. 7). Springer. Berlin, Germany.

 

2 An extensive list of tree species giving trait-specific information relevant to air pollution mitigation is given in a recent literature review of the capacity of vegetation to form a functional barrier to air pollution open road environments. This review also reports that Acer campestre, Acer platanoides, Alnus glutinosa, Betula pendula, Chamaecyparis lawsoniana, Crataegus monogyna, Larix decidua, Prunus laurocerasus and Pinus nigra were the most beneficial species, judged according to potential for pollutant deposition versus emission of BVOCs. Barwise, Y., Kumar, P. Designing vegetation barriers for urban air pollution abatement: a practical review for appropriate plant species selection. npj Clim Atmos Sci 3, 12 (2020). https://doi.org/10.1038/s41612-020-0115-3

Benefits of green infrastructure for air filtering

  • Fewer health complaints and premature deaths due to short-term exposure to air pollutants such as PM10 and ozone
  • Gaseous pollutants (O3, NOX, SO2 ) are absorbed out of the air
  • Particulate matter, ash, pollen and smoke are captured out of the air
  • Shade and water are provided which result in lower temperatures and less ozone (O3 ) forming
  • Photosynthesis results in more O2 in the air

 

In new and existing developments

In new development:

Place green strategically in new plans in order to maximize the filtering capacity of each tree and prevent conflicts between land uses. Provide enough room (both above and underground) to allow trees to grow to maturity and therefore maximise their filtering capacity.

In existing development:

When replacing or adding trees, add a variety of species which are especially good in filtering the air.

Consult best practice guidance for technical methods.

Trees in Hard Landscapes: A Guide for Delivery

Explores the practical challenges and solutions to integrating trees in 21st century streets, civic spaces and surface car parks, detailing process, design and technical options. Will be of particular interest to highway engineers, public realm professionals and tree specialists. Winner of the 2016 Landscape Institute Award for Policy and Research.

http://www.tdag.org.uk/trees-in-hard-landscapes.html