Improving air quality has been an imperative of environmental policies throughout the 20th century. Installation of filtering devices in power plants, the switch to less polluting fuels and other technical measures have achieved significant improvements in this respect. Yet air quality remains a major concern. While the concentration of air pollutants such as sulfur dioxide has been successfully reduced in urban areas, other - mainly car induced - pollutants such as nitrogen oxides, ozone and volatile organic compounds are of increasing concern. Climate change is likely to add further to these air quality problems because rising air temperatures and higher levels of radiation can lead, for instance, to higher concentrations of ozone in the air. Particular attention also needs to be paid to ultraviolet radiation, which can cause skin cancer, in the southern parts of Europe.
A particular challenge lies in the fact that the sources of air pollutants are often diffuse. In this context, the role of urban green space and in particular that of the urban forest in removing air pollutants in urban areas has become of interest. Green spaces and trees are widespread in urban areas and thus could provide an effective means to improve air conditions locally and provide shelter from ultraviolet radiation. A number of studies in the United States have shown the potential of the urban forest for improving air quality (e.g., McPherson 1994; Nowak et al. 2002).
In Europe, evidence is still limited but results of previous studies clearly show that trees have an important role to play in removing air pollutants in urban areas. A woodland in Nottingham was estimated to reduce concentrations of sulfur dioxide and nitrogen oxides in the air by 4-5% (Freer-Smith and Broadmeadow 1996). More important, however, is the function of trees to capture dust. Evergreen tree species, and in particular conifers, filter more dust than deciduous species, but conifers are also more sensitive to damage caused by air pollutants (Dafiler 1991; Beckett et al. 1998).
Single trees filter less dust than groups or rows of trees. Trees capture air pollutants most efficiently when they are planted close to the source of emissions. Woodland canopies are more effective than other vegetation types at trapping particle pollutants because of their greater surface roughness (Manning and Feder 1980). Studies in North American cities have estimated the overall removal of air pollutants by trees (McPherson 1994).
While there can be little doubt that the urban forest has a largely beneficial effect on air quality, the emission of volatile organic compounds (VOC; Beckett et al. 1998) as precursors of ozone has recently gained attention. This may be an issue in hot climates with intensive solar radiation such as are experienced in Mediterranean cities. Also, the production of pollen from tree species such as birch needs to be carefully considered when tree species are selected for urban plantings, because of its allergenic effect.
A recent large-scale study in the West Midland region of England estimated the overall removal rates of air pollutants by the urban forest (Stewart et al. 2001). The study was based on a sample of over 30 000 trees. An air quality model was developed to assess the potential of the urban forest to remove air pollutants such as ozone, nitrogen oxides and carbon monoxide and also to estimate potential emissions of volatile organic compounds. At the time of writing, results had yet to be published in their entirety. However, the data that are available suggest that planting additional trees on land theoretically available for this purpose could lead to a significant reduction in concentrations of airborne particles in the West Midlands once these trees have matured. Planting of all available land could achieve a reduction of up to 25% of small airborne particles.
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