Urban trees improve air quality

Planting trees in urban areas could cut particulate pollution in cities by as much as a quarter, according to a new study.

Small air particles, especially those less than 10?m in diameter (PM10) pose a long-term threat to human health. These can originate from human activities, including exhaust fumes or smoke, or from natural causes, such as dust. The study shows that trees are particularly good at capturing PM10 on their leaf surfaces. Scientists investigated the potential of trees to remove PM10 in Glasgow in Scotland, and also in the West Midlands area in England, which includes the cities of Birmingham, Wolverhampton and Coventry.

The researchers used an established statistical model to calculate how further tree planting in these urban areas would affect PM10 levels. The model generated a number of scenarios which showed that PM10 from human activity could be reduced by between seven and 26 per cent. The greatest reductions could be achieved by increasing tree cover in the West Midlands.

If tree cover was increased from 3.7 per cent to 16.5 per cent, PM10 levels could be cut by 19 per cent. Increasing tree cover to the theoretical maximum level of 54 per cent (achievable only by planting trees on available green space) would result in a 26 per cent drop in PM10 levels and result in the removal of 200 ton (203.21 metric tonnes) of PM10 per year.

In Glasgow, a six per cent reduction in PM10 could be achieved by increasing tree cover from 3.6 per cent to eight per cent.

In the West Midlands, trees currently remove seven per cent of the particulate pollution arising from human activity, or 39 ton (39.63 metric tonnes) of PM10 removed from the atmosphere. This is equivalent to a four per cent reduction in PM10 levels.

The study estimates that trees currently remove 4.9 ton (4.99 metric tonnes) of PM10 from the air in Glasgow. Tree planting appears to be an effective strategy to remove PM10 from urban air. If a quarter of the urban land available was planted with trees, then average PM10 could be cut by between two and ten per cent.

Tree species with the greatest overall leaf surface area, such as larch, pine and ash, are the best PM10 scavengers, and planting them individually rather than in large groups offers the biggest improvements in air quality. The downside is that some tree species emit volatile organic compounds (VOCs) which contribute to ozone formation. In addition, the PM10 that trees collect is not removed completely but can accumulate in the soil, which over the long term may lead to contaminated soils.

Source: A.G. McDonald et al (2007). Quantifying the effect of urban tree planting on concentrations and depositions of PM10 in two UK conurbations. Atmospheric Environment. 41(38): 8455–8467. Contact: [email protected]