Green Walls as Mitigation of Urban Air Pollution: A Review of Their Effectiveness

Mitigation of urban air pollution has been constrained by the availability of urban spaces for greening. Green walls offer the prospect of greening spaces and surfaces without requiring large areas. Green walls can largely be divided into green facades where the aboveground parts of plants rooted in soil and pots grow directly on, and living walls holding bags, planter tiles, trays and vessels containing substrates in which plants are grown. Green facades and living walls can be continuous or modular with repeating units that can be assembled for extension. This review aims to present the effectiveness of green walls in removing different types of air pollutants in indoor and outdoor environments. It examined more than 45 peer-reviewed recently published scholarly articles to achieve the aim. It highlights that most of the studies on green walls focus on particulate matter removal and green walls could effectively remove particulate matter though the effectiveness varies with plant types, air humidity, rainfall and its intensity, leaf area index and contact angle, green wall surface coverage ratio, as well as the height of green walls. Increasing the height of green walls and optimizing their distance from roadsides could promote the deposition of particulate matter. Washing off could regenerate plant surfaces for capturing pollutants. Green walls are also effective in removing NO2, O3, SO2 and CO. Indoor active living walls, when properly designed, could have air purifying performance comparable to a HVAC system. The performance of green walls could be optimized through polycultures, selection of plants, surface coverage and height, and air inflow.