Effectiveness of green infrastructure for improvement of air quality in urban street canyons

A version of the CiTTyCAT model (CiTTy-Street) was used to estimate the contribution of vegetation to removing air pollutants in urban street canyons (streets flanked by uninterrupted buildings on both sides, creating a canyon-like environment). Previous estimates of the effect of urban vegetation on air quality have shown only minor improvements (1-5%), but these studies do not account for the complicated geometry of urban street canyons, which can lead to greatly increased residence time of air pollutants. Vegetated “green” walls, vegetated “green” roofs, and street trees were considered for their effects on individual street canyons and across a wider area. Green walls showed the greatest impact, reducing NO₂ concentrations by as much as 40% and PM₁₀ by as much as 60%. The effects, even at the scale of a single canyon, were substantial, and thus plantings that target specific areas could prove to be a valuable strategy. Green roofs had a much smaller effect, and the impact of street trees was ambiguous as trees increased deposition but reduced air mixing. The authors argue that the planting of trees in urban street canyons should be considered on a case-by-case basis, taking into account pollutant loads, wind speed and direction, species, and canyon geometry.