Impact of particulates, oxidants and other pollutants
Particulates, oxidants and other pollutants are a serious problem in the industrial, urban and peri-urban areas of developed countries and countries in transition. For example, Mootgavhar & Hutchinson (2000) identified a correlation between hospital admissions from chronic respiratory diseases and the index of respirable particulates in the air of Seattle, USA. Furthermore, a recent World Health Organisation report suggested that more people are killed prematurely by the effects of pollutants than through car accidents (WHO, 1999).
How urban and peri-urban trees and greenspace can improve air quality
As a result of their large leaf area and the turbulent air movements created by trees, woods hedges and shelterbelts take up more pollution from the atmosphere than shorter vegetation or other land uses (Fowler et al, 1989, Beckett et al, 2000b). The ability of trees to take up particles has been characterised through measurement of deposition velocities and capture efficiencies in wind tunnels and through micro-meteorological measurement in the field with good comparability of data from different approaches (Freer-Smith et al, 2004). There is now sufficient understanding to produce an integrated model of how urban and peri-urban trees and greenspace can improve air quality.
Emission characteristics, atmospheric chemistry and local factors (site and climate) affect the way in which urban trees and greenspace influence air quality. It is necessary to consider all major urban pollutants in an integrated study since their interactions are complex. For example, urban trees take up ozone, ammonia and particles but also release biogenic volatile organics (VOCs). VOCs can contribute to the downwind production of secondary organic aerosols and ozone. Recent modelling of pollutant indicates that some tree species will have net beneficial effects while others may not, particularly under some extreme climatic conditions.
Building a model
We have been able to use established stomatal conductance values (gs) for open pasture, Scots pine, spruce and oak to model the effects of pasture/park grass and tree planting scenarios on ozone, sulphur dioxide and nitrogen dioxide removal from urban air in a Community Forest (Broadmeadow & Freer-Smith, 1996). Data are now available on the concentrations of particulates in the UK urban environment (NETCEN) and also on deposition velocities (Beckett et al, 2000b; Freer-Smith et al, 2004) for a significant number of tree species. We also monitor gaseous and wet deposited pollutants at a number of woodland sites (Level 2 data). Thus a model which considers gaseous air pollution, particulates, aerosols and wet deposited pollutants could be developed for the first time.
Once intercepted by leaf surfaces it is important to consider the ultimate fate and ecological impact of particulates. Particulates can form a reservoir for chemicals moving into the plants, herbivores can ingest particulate matter on leaves, particles can be washed off on to soil and road surfaces or be redispersed to the atmosphere.
The design of urban and peri-urban tree planting
The design of urban and peri-urban tree planting to improve the appearance of towns, provide shaded areas for relaxation and to lessen the adverse noise, pollution and wind-buffering from vehicles (urban screening) has been widely considered in Europe and the US. Such issues are of increasing concern in both developed and developing countries.
Work on pollutant uptake indicates that current designs of the urban environment and of specific developments might be significantly improved in the extent to which they remove pollutants from urban air. Quite subtle changes such as proximity of planting to school playgrounds or the mix of deciduous and evergreen species might have rather significant impacts on health and other benefits. The end users are thus planners, architects and those responsible for maintaining urban green space (town/city authorities, etc). There are also potential benefits in trapping spray drift and other aerosols through windbreak incorporation in agriculture, roadside and industrial sites.
Long term sequestration
For pollutants such as sulphur dioxide, nitric acid, ammonia and nitrous oxides trees and other vegetation offer the possibility of long term sequestration. Anthropogenically derived particles include a wide range of elements (lead, cadmium, nickel and chromium) and many organic species (e.g. poly-nuclear aromatic hydrocarbons – PAH and dioxins which are potential carcinogens) so that field measurements and analysis is likely to be required if budgets and fates are to be properly considered. This is particularly the case in the urban environment for which limited data are available, contrasting with the position for conventional woodland, where deposition of these pollutants is recorded at over 300 sites across Europe.