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Dept for Environment Food and Rural Affairs

Answer

Sarah Teather: To ask the Secretary of State for Environment, Food and Rural Affairs what assessment she has made of the link between trees in urban areas and air quality. [214937] Alun Michael: Trees provide benefits to urban air pollution, but can also have adverse effects on air quality. Adverse effects of trees on air quality Trees can have adverse effects on air quality. They can release biogenic emissions known as volatile organic compounds (VOCs), which in combination with oxides of nitrogen (NOx), can contribute to the formation of other pollutants, especially ozone and particles. Ozone and particles are known to damage human health when in the lower atmosphere. In addition, trees release pollen, which can cause allergic reactions in sensitive people, and can be associated with hay fever and asthma. Beneficial effects of trees on air quality Trees deliver air quality benefits in two ways. Firstly, the cooling effect of their shade can directly reduce evaporative emissions from vehicles and other fuel storage, and by cooling homes and offices, power generation emissions are also lessened. Cooling also reduces the speed at which atmospheric chemical reactions occur. Secondly, trees can remove various pollutants from the atmosphere via deposition to their leaves and needles. Trees can remove pollutants typically associated with urban air pollution such as ozone, nitrogen dioxide, sulphur dioxide, and ammonia reducing atmospheric concentrations and improving air quality. The pollutants generally collect in the leaves or bark which are then shed as part of the tree's life cycle. Trees also reduce concentrations of harmful particulate matter when leaf surfaces collect airborne particles, that are subsequently washed to the ground when it rains. The trees most suited to removal of particles are those with large leaf areas, or sticky or hairy leaves. Studies indicate that the hairy leaves of lime trees can collect four times as many PM10 particles than plane trees. In addition, trees can have a positive benefit in terms of global warming by acting as a sink for carbon dioxide, which is the most common greenhouse gas. Trees remove carbon dioxide from the air and release oxygen during photosynthesis. The carbon is stored in the tree and the oxygen is released back into the atmosphere. A recent report entitled "Trees and Sustainable Urban Air Quality", produced under the Natural Environment Research Council's Urban Regeneration and the Environment (URGENT) programme by the University of Lancaster and the Centre for Ecology and Hydrology Edinburgh, investigated the role of trees in reducing urban air pollution. The report concluded that trees that emit the least reactive VOCs and have large leaf surface areas have the best effect on air quality. Scots pine, common alder, larch, Norway maple, field maple, ash and silver birch were found to remove the most pollutants without contributing to the formation of new pollutants, whereas oaks, poplars and willows were found to have detrimental effects on air quality downwind. In general, the report concludes that the effects on air quality of large scale planting of most tree species in cities would be positive and for example, by doubling the number of trees in the West Midlands, the concentration of small particles could be reduced by 25 per cent., avoiding 140 deaths that are currently caused by airborne particles each year in the West Midlands.

Secondary information

Type

Written question

Reference

431 c187-8W; 214937

Session

2004-05

Subjects

Air pollution Environment Pollution control Urban areas Trees West Midlands

Contains statistics

Yes

Link

View this Written question on www.publications.parliament.uk