Urban Tree Planting as an Aid to Air Pollution Abatement: Cost-Effective Analysis of the West Midlands Case

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The objectives of this project are to calculate the benefits of large-scale urban tree planting programmes as a means of reducing human exposure to air pollutants. This will involve:

  • Mapping existing vegetation cover in the conurbation.
  • Discussing with planning authorities and other local government departments possible scenarios for large-scale tree planting on both public and private land in the conurbation.
  • Estimating the removal rates of air pollutants by trees under present tree cover conditions and under various future planting programme scenarios. The pollutants to be considered include SO2, NO, NO2, PM10, O3 and volatile organic compounds (VOCs) including benzene.
  • Calculating the emission rates of VOCs from trees under present and future scenarios.
  • Calculating the incremental amount of ozone and aerosol likely to be formed as a result of these enhanced VOC emission rates arising from large-scale tree planting.


West Midlands


The West Midlands will be used as a case study because of the data and knowledge base that will arise from the URGENT PUMA project which will provide an essential input to this project. There will also be interaction with one other URGENT project, Thermal climatology of the West Midlands. The models derived will, however, be applicable elsewhere.

The atmospheric transport and chemistry model that will be used will extend previous urban plume models, particularly with respect to those parts of the chemistry, emission and deposition schemes that are most sensitive to changes in tree population. A simple Lagrangian treatment of transport in the planetary boundary layer will be used. This will be coupled to a complex chemistry scheme which is compatible with state-of-the-art explicit structure-reactivity relationships and also compatible with global modelling studies.

Start date and duration

January 1999 Three years, plus 6 months extension

Lead Organisations

Lancaster University

CEH Edinburgh


  • Quantitative information on the present and possible future tree cover in the West Midlands conurbation, together with estimates of the financial costs of a large-scale tree planting programme.
  • Estimates of the possible reductions in air pollutant concentrations, and hence, reductions in human exposure to air pollutants achievable by large-scale tree planting.
  • Estimates of the detrimental effects of tree planting to VOC emissions and the resultant likely increases in downwind ozone and aerosol particle concentrations under a number of possible future emission control and planting scenarios.
  • Surveying, measurement and modelling methodologies that can be applied to other conurbations and to other similar problems of environmental cost-benefit analysis.
  • Major publication in the peer-reviewed literature.

Progress to date

  • An urban classification has been derived for the 900 km2 of West Midland conurbation using principal component analysis and cluster analysis based on 27 attributes describing the land cover of each km2. Thus each km2 has been allocated one of 8 urban classes (villages and farms, open water and farms, woodlands, very light suburban, light suburban, dense suburban urban, dense urban).
  • Urban morphology types have been mapped onto urban land class using the definitions in the "Trees in Towns" report (HMSO, 1994). From this mapping, a first estimate has been made of tree cover in the West Midlands area.
  • First estimates of biogenic VOC and anthropogenic PM10 emission potentials from the West Midlands area have been made at 1 km2 resolution. These estimates were made using the urban classification tree cover estimate, data from existing biogenic VOC emissions and biomass databases (Stewart et al., 1995), and anthropogenic pollutant concentrations and emissions inventories for the West Midlands (LRC, 1996; Veal et al., 1997). A first estimate has also been made of PM10 deposition and removal by trees in the West Midlands area.
  • A tree survey methodology has been devised and surveying of trees in the West Midland area is underway. Twelve one-hectare plots are being surveyed in km2 samples for each of the 8 urban classes. Each tree within the survey plots are recorded with details of height, age, condition, aspect, crown spread etc. More than 5,000 trees in 79 ha have been surveyed to date. This will provide current tree data for more accurate estimates of biogenic VOC emissions, and pollution deposition.
  • Meetings have been held with each of the 7 West Midland Metropolitan Borough Councils to discuss their existing tree data and tree planting policies and to encourage councillors to consider their requirements from our project. A systematic mining of each councilís existing tree data is currently underway. This will supplement and complement the field survey data.
  • Workshops are planned for councils and other end-users in early 2001 to discuss future planting scenarios and to determine how we can accommodate end-user requirements.


Birmingham City Council

Other local authorities

Further Details

Further information is available from the following contacts:

Lead Researcher: Prof C N Hewitt

Tel: 01524 593931, Fax: 01524 593985, e-mail: n.hewitt@lancaster.ac.uk 

URGENT Programme Manager: Graham Leeks

Tel: 01491 692203, Fax: 01491 692313, e-mail: gjll@ceh.ac.uk