Leaves Leave Air Cleaner


The UK, like many other European countries, has a problem with high levels of pollution – specifically nitrogen dioxide (NO₂) - in urban areas. Although EU air quality limit-values for NO₂ are already met across 99% of the UK land area and 97% of the population, there are pockets within 40 of the 43 zones that don’t yet meet the limits. The Government was intending apply to the EU for a time extension from 2010 to 2015 for meeting these limit values; however, in recent weeks Defra has abandoned this strategy, acknowledging that the problem is so serious it will take up to 2025 in some areas to meet these limits and leaving itself open to possible legal action and fines. This means that for many years to come, planning applications for developments in these areas will continue to require an Air Quality Assessment – and appropriate mitigation measures.

Trees can remove some pollutants by direct mechanical-filtration action and also through gas exchange in the leaf stomata. However, trees can also emit pollutants, most notably volatile organic compounds – which can lead to the formation of ozone. Improved understanding of how trees and vegetation can remove pollutants means they are increasingly accepted as mitigation measures in certain circumstances – the Major of London has made trees and green walls a central element of his London Air Quality Strategy.

Where pollution levels are marginally above acceptable levels, or where the air quality mitigation options are limited, tree and vegetation planting schemes can sometimes show that a development site could be made suitable in air quality terms for, say, residential use. RPS assisted developers Big Yellow and Greenacre Homes obtain planning consent for a development in London after our Air Quality Assessment demonstrated the beneficial effects of the planting scheme in reducing poor air quality. In this case the pollutant of concern was NO₂, but trees and vegetation can reduce airborne concentrations of a wide variety of other pollutants – including particulate matter (PM10). This is important, because whereas the health benefits of further reducing NO₂ below its Limit Value are uncertain, particulate matter has no lower effect threshold, so any reduction in PM10 – even if it is already below its Limit Value – will bring health benefits. This can be a powerful argument for a scheme.

Trees and vegetation can also lower urban temperatures – reducing evaporative pollutant emissions and inhibiting chemical reactions – as well as reducing building-energy usage by providing shade and blocking winds. Trees, vegetation and green spaces also provide a wide range of economic, social and psychological health benefits. However, trees can prevent air movement and, in some cases, emit more pollutants than they retain.

Expert advice can influence positively the planning decision by demonstrating there will be a net local improvement in air quality associated with the planting scheme. RPS air quality experts work closely with landscape and arboriculture specialists to provide the optimal solution, based on a thorough understanding of the mechanisms and efficiencies of tree and vegetation species.