Potential and limitation of air pollution mitigation by vegetation and uncertainties of deposition-based evaluations

The potential to capture additional air pollutants by introducing more vegetation or changing existing short vegetation to woodland on first sight provides an attractive route for lowering urban pollution. Here, an atmospheric chemistry and transport model was run with a range of landcover scenarios to quantify pollutant removal by the existing total UK vegetation as well as the UK urban vegetation and to quantify the effect of large-scale urban tree planting on urban air pollution. UK vegetation as a whole reduces area (population)-weighted concentrations significantly, by 10% (9%) for PM_2.5, 30% (22%) for SO₂, 24% (19%) for NH₃ and 15% (13%) for O₃, compared with a desert scenario. By contrast, urban vegetation reduces average urban PM_2.5 by only approximately 1%. Even large-scale conversion of half of existing open urban greenspace to forest would lower urban PM_2.5 by only another 1%, suggesting that the effect on air quality needs to be considered in the context of the wider benefits of urban tree planting, e.g. on physical and mental health. The net benefits of UK vegetation for NO₂ are small, and urban tree planting is even forecast to increase urban NO₂ and NOx concentrations, due to the chemical interaction with changes in BVOC emissions and O₃, but the details depend on tree species selection. By extrapolation, green infrastructure projects focusing on non-greenspace (roadside trees, green walls, roof-top gardens) would have to be implemented at very large scales to match this effect. Downscaling of the results to micro-interventions solely aimed at pollutant removal suggests that their impact is too limited for their cost-benefit analysis to compare favourably with emission abatement measures. Urban vegetation planting is less effective for lowering pollution than measures to reduce emissions at source. The results highlight interactions that cannot be captured if benefits are quantified via deposition models using prescribed concentrations, and emission damage costs. This article is part of a discussion meeting issue 'Air quality, past present and future'.