Quantifying the effect of urban tree planting on concentrations and depositions of PM10 in two UK conurbations
Introduction
Atmospheric particles, especially those with an aerodynamic diameter of <10 μm (PM10), pose a long-term threat to human health, in particular to human respiratory functions. Elevated PM10 concentrations have also been linked to an increase in mortality rates (Powe and Willis, 2004); although it is increasingly thought that human health issues are more closely linked to even smaller particles (e.g. PM2.5, i.e. particles with an aerodynamic diameter of <2.5 μm), current European air quality guidelines are, for historical reasons, targeted at PM10 concentrations.
Particles in the atmosphere are classified as either primary or secondary: primary particles are those emitted directly as particles, including smoke or exhaust fumes from anthropogenic sources and particles generated by frictional processes, while secondary particles are formed by gaseous interactions in the atmosphere (Matsumoto and Tanaka, 1996; Owen et al., 2003).
Current PM Air Quality Standards for the protection of human health are exceeded in many urban areas across the globe. Abatement measures to reduce anthropogenic sources of primary and secondary aerosol are generally very costly, while natural sources (e.g. wind blown dust, sea salt and secondary particle formation from biogenic organic precursor gases) are difficult or impossible to control. It therefore becomes vital to explore all alternatives to lower PM concentrations in urban areas. Several studies have investigated the effects of trees on pollutant concentrations. For example, it has been found that trees can reduce concentrations of an ammonia plume, by deposition to plant cuticles and stomatal uptake, by around 3–13% (Sutton et al., 2004b; Theobald et al., 2004; Tiwary et al., 2006), and that forests are efficient sinks for most atmospheric pollutants (Fowler et al., 1989; Nowak et al., 2000; Nowak and Crane, 2002). It is important to note that different tree species have different properties, such as leaf size and stomata, which will affect the capture efficiency (Freer-Smith et al., 2004; Hewitt, 2003). In addition, trees also have the potential to affect air quality through the emission of volatile organic compounds (VOCs), which may contribute to ozone formation (Owen et al., 2003). More specific research on the effects of urban trees on pollutant concentrations has been reported in the UK and the USA. Urban trees can reduce concentrations of SO2 and O3 by 20% (Beckett et al., 1998), and it has been estimated that existing urban forests in Chicago have removed 212 ton of PM10 each year, which is equivalent to a 0.4% hourly average improvement in air quality (McPherson et al., 1994).
Particles are removed from the atmosphere by dry, wet or occult deposition (Fowler et al., 2001). Wet deposition is the removal of pollutants by precipitation and is independent of land cover. Dry deposition can occur by gravitational settling, impaction, interception or diffusion, depending on particle size. As trees have a larger collecting surface area than other land cover types and also promote vertical transport by enhancing turbulence, there is a greater opportunity for particles to be collected on the trees surface. Trees are therefore more efficient at capturing particles from the atmosphere by dry deposition relative to short vegetation (Gallagher et al., 1997). For example, it has been shown using 210Pb as a marker of aerosol deposition that the deposition velocity (Vd) in a woodland is three times that of adjacent grassland areas (Fowler et al., 2004).
This paper describes a numerical experiment to quantify the potential of urban tree planting schemes to reduce PM10 concentrations in urban areas. This work was conducted within the context of three UK projects: Sources and Sinks of Urban Aerosols (SASUA), URGENT Trees and Environmental Information System for Planners (EISP), all within the UK effort on urban regeneration and the environment (URGENT). The SASUA project focussed on aerosol production, transport and deposition in and around Edinburgh, including measurement and modelling of the fate of aerosols as they leave the city. A transport and deposition model was adapted to simulate the fate of aerosols as they are advected out of the city. By planting grassland with trees downwind of the urban areas, the model predicted a decrease in air concentration of between 5% and 50% (Fowler et al., 2002). Within the EISP project, the main aim was to develop methods to help planners assess the environmental impact of planning applications. To meet air quality targets, local councils need to develop strategies to reduce, among other pollutants, PM10 concentrations. For every planning application, environmental impacts need to be considered. As well as reducing and controlling emissions, planners are interested in more efficient ways of removing particles from the urban atmosphere. Since planting trees in urban areas is potentially a simple way of assisting a reduction in PM10 concentrations, the transport and deposition model used for SASUA was adapted to run over two other urban UK domains (Glasgow City and West Midlands County), and the land cover was manipulated to simulate the effects of tree planting on PM10 concentrations. Outputs from the model provided inputs to the EISP which provided a tool-box of decision aids for use by local authorities and planners (Culshaw et al., 2006), using a series of flow diagrams (Bealey et al., 2006). This enables planners to assess the potential impacts of new developments and possible ways to counter any negative effects, for example, by including tree planting as a condition of planning consent.
Section snippets
The fine resolution atmospheric multi-pollutant exchange (FRAME) atmospheric transport model
The atmospheric transport model used for this work is based on the FRAME model. The FRAME model has been used to estimate deposition of nitrogen (Fournier et al., 2004; Singles et al., 1998), heavy metal deposition (McDonald et al., 2002) and the surface concentrations of greenhouse gases (Sutton et al., 2004a). The model is a multi-layer trajectory model, and uses statistical meteorology to calculate wet and dry deposition. The model uses 72 wind directions, with the concentration and
Future planting potentials (FPPs)
The calculated maximum FPPs for the West Midlands County are shown in Fig. 2 and show the spatial distribution of the calculated maximum FPPs (Donovan, 2003). This shows that the area between Birmingham (to the NW) and Coventry (to the E) has the highest maximum FPP values, as this area is mainly rural, though the area to the north and north-west of Birmingham is also available for a large amount of planting.
The calculated maximum FPP values using the method explained above for Glasgow City
Generalisation of the results
The results of the model allow the reductions in PM10 concentrations through tree planting to be quantified. We have shown that by modelling tree planting in cities, there is a significant effect on PM10 concentrations due to higher deposition velocities to forested areas. Few studies have attempted to quantify this effect. Work in Chicago in 1991 has estimated that localised improvements in air quality in highly wooded areas can be around 5–10%, and may in some cases be higher (McPherson et
Conclusions
We have found that tree planting in urban areas is a worthwhile benefit to the areas included in this study, and is a policy which could benefit other towns and cities. As PM10 has a major impact on human health, reducing PM10 air concentration is extremely beneficial, and we have shown that planting urban trees can have a significant impact, as the higher deposition rates provided by mature trees can reduce primary PM10 concentrations by between 7% and 26%. Even planting a quarter of the
Acknowledgements
This work was funded by the UK Natural Environment Research Council within the SASUA (Grant no. GST/02/2244), URGENT Trees (Grant no. GST/02/2236) and EISP programs. We are grateful to Phil Hickey, Paul Mellon and Lorraine Vogwell from Glasgow City Council for providing GIS land use data within the City of Glasgow, and, Tim Duffy, the EISP project manager of the British Geological Survey, Edinburgh. We would also like to acknowledge Bartholomew's local authority boundary files for use in the
References (46)
- et al.
Urban woodlands: their role in reducing the effects of particulate pollution
Environmental Pollution
(1998) - et al.
The role of web-based environmental information in urban planning—the environmental information system for planners
Science of the Total Environment
(2006) - et al.
Modelling the deposition of atmospheric oxidised nitrogen and sulphur to the United Kingdom using a multi-layer long-range transport model
Atmospheric Environment
(2004) - et al.
Urban-scale variability of ambient particulate matter attributes
Atmospheric Environment
(2006) - et al.
Measurements of aerosol fluxes to speulder forest using a micrometeorological technique
Atmospheric Environment
(1997) - et al.
Formation and dissociation of atmospheric particulate nitrate and chloride: an approach based on phase equilibrium
Atmospheric Environment
(1996) - et al.
Carbon storage and sequestration by urban trees in the USA
Environmental Pollution
(2002) - et al.
A modeling study of the impact of urban trees on ozone
Atmospheric Environment
(2000) - et al.
Air pollution removal by urban trees and shrubs in the United States
Urban Forestry and Urban Greening
(2006) - et al.
Urban land classification and its uncertainties using principal component and cluster analyses: a case study for the UK West Midlands
Landscape and Urban Planning
(2006)