Green filter areas are vertical green infrastructure interventions, constructed to provide avisual barrier and/or and pollution filter between roads or industrial operations and publicspace or walkways. Green filter areas may take the form of street trees, green walls (screens),shrubs or hedges. Trees, shrubs or climbers may be planted directly into the ground or intocontainers. Green filter planting may be combined with solid barrier construction to reducenoise impact.

Choice of plant species and urban layout is important in designing vertical vegetationinterventions for air pollution mitigation. Higher rates of particulate capture are associatedwith the complex shoot structure and finer leaves of conifer species. Whilst trees and shrubs can remove gaseous pollutants from theatmosphere and provide large surface areas for deposition of particulate pollutants, carefuldesign of the vertical green filter area, accounting for local conditions, is required to avoidexacerbation of local pollutant concentrations through reducing airflow and slowing dispersal.Increased height (over 5m) and width (10m) will in general increase the effectiveness of greenfilter areas in reducing concentrations of airborne pollutants. Both density (toallow maximum deposition) and porosity (to allow penetration rather than deflection ofairflow) of low vegetation barriers close to pollution source are important for effectivecapture of particulate pollutants. Gaps in green vegetation barriers can resultin increased downwind ambient pollution concentrations.

Design and maintenance considerations include: avoiding obstruction of driver andpedestrian lines of site and access; irrigation during drought; pruning annual growth; andremoval and replacement of dead vegetation; Choice of woody vegetation should take into consideration: stress tolerance; selection of non-deciduous species to provide a year-round barrier; and avoidance of poisonous, invasive or allergenic species, or species associated with pollutant emissions (including VOCs).