Plants are involved in the uptake, transport and assimilation (or, in some cases, decomposition) of many gaseous or particulate pollutants. Therefore trees and vegetation can play an important role in influencing urban air quality, and in mediating some of the negative effects of pollutants. Vegetation also removes from the atmosphere carbon dioxide (CO₂) – the main greenhouse gas associated with climate change.

Background

A wide range of chemical pollutants and particulates (dust) can cause serious problems in industrial, urban and peri-urban environments. As well as affecting human health, many common pollutants may also have a detrimental effect on our urban ecosystems. There has been increasing recognition over the past 20 years that urban greenspace can act as an important sink (or reservoir) for many of these pollutants.

There are a number of common air pollutants in the atmosphere of urban environments, all of which may be influenced by the presence of trees and other greenspace.

• Chemical pollutants include ozone (O₃), oxides of nitrogen (NO_x), ammonia (NH₃), sulphur dioxide (SO₂).
• Particulate matter consists of tiny particles of solid or liquid suspended in a gas. Some particulates occur naturally; others are caused by human activities such as the burning of fossil fuels in vehicles, power plants and various industrial processes.

In terms of direct impacts on human health, the most important pollutants are particulate matter, sulphur dioxide and ozone.

Some tree species can also have a negative effect and can form pollutants in the atmosphere, for example willow and poplar. Trees may emit gases known as volatile organic compounds (VOCs). When these combine with the man-made oxides of nitrogen (NOx), they can contribute to the production of other pollutants, especially ozone.

Legislation

In UK, the Environment Act 1995, the Pollution Prevention and Control Act 1999 and the European emission standards control pollution emissions from transport and industry.

In July 2007, the UK Government and the devolved administrations published the latest Air Quality Strategy for England, Scotland, Wales and Northern Ireland, which defines air quality standards and objectives that need to be achieved.

Practical considerations

• Any planting strategy that maximises leaf area per unit ground area will increase the potential for uptake of pollutants by trees and other vegetation. The layered canopy structure of trees has evolved to maximise uptake of CO₂ through the process of photosynthesis, and provides a surface area 2–12 times greater than the area of land they cover.
• It is also important to consider site-specific factors such as soil quality and compaction, which may have an adverse effect on plant growth and so reduce the uptake of pollutants.
• Pollutant uptake is greatest at the canopy edge, as a result of reduced canopy resistance. The movement of water vapour from within any individual leaf to the atmosphere is governed by stomatal resistance (the inverse of which is stomatal conductance), at the whole tree or stand level this is collectively referred to as the canopy resistance.
• Deposition of pollutants is closely related to stomatal conductance (the speed at which water vapour can evaporate from pores called stomata in a plant’s leaves). Species with a high stomatal conductance are likely to be more efficient at pollutant uptake. So these species will also receive the largest pollutant dose, and therefore may be more susceptible to pollution damage.
• Pollutant uptake is maximised where plants are exposed to high concentrations of the gases or particulates in question. So vegetation corridors next to roads may be the most efficient pollutant scavengers.

Research case study

The use of vegetation to mitigate particulate pollution has been recognised for a number of years. Forest Research used an integrated modelling approach that utilised air dispersion (ADMS-Urban) and particulate interception (UFORE) to predict the PM₁₀ concentrations both before and after greenspace establishment, using a 10 x 10 km area of East London Green Grid (ELGG).

The corresponding health benefits, in terms of premature mortality and respiratory hospital admissions, as a result of the reduced exposure of the local population were then modelled.

It was found that PM10 capture from the scenario comprising 75% grassland, 20% sycamore maple (Acer pseudoplatanus L.) and 5% Douglas fir (Pseudotsuga menziesii (Mirb.) Franco) was estimated to be 90.41 t yr-1, equating to 0.009 t ha-1 yr-1 over the whole study area. The human health modelling estimated that 2 deaths and 2 hospital admissions would be averted per year.

Services

Forest Research provides research services to the Forestry Commission and external clients.

Site investigation and monitoring

Forest Research has extensive experience of conducting site-specific air pollution monitoring and surveys. Scientists provide advice and recommendations on species choice and planting regimes to help improve air quality in urban and industrial areas.

Research and modelling

Forest Research has a long track record in conducting research on the impacts of vegetation and trees on urban air quality, and has extensive modelling capabilities.

Further information

An extensive library of information on the different types of air pollution common in UK, as well as up-to-date information on air quality and forecasts, can be found at the UK Air Quality Archive. This website gives access to both national and regional air quality data.

Specific advice on the influence of tree species and different planting regimes on urban air quality

Department of the Environment (1996). Urban Woodland and the Benefits for Local Air Quality. Research for Amenity Trees No. 5. London: Department for Communities and Local Government.

Additional information

Tiwary, A., Sinnett, D., Peachey, C., Chalabi, Z., Vardoulakis, S., Fletcher, T., Leonardi, G., Grundy, C., Azapagic, A., Hutchings, T.R. (2009). An integrated tool to assess the role of new planting in PM10 capture and the human health benefits: A case study in London. Environmental Pollution 157, 2645-2653.

Peachey, C.J., Sinnett, D., Wilkinson, M., Morgan, G.W., Freer-Smith, P.H. and Hutchings, T.R. (2009). Deposition and solubility of airborne metals to four plant species grown at varying distances from two heavily trafficked roads in London. Environmental Pollution 1–9.