Influence of vertical gardens on the air quality of a city historic center

Quito is an Andean city with 2.7 million inhabitants that regularly exceeds the WHO air quality guidelines for O3, SO2, PM2.5, and PM10. Within the historic center in an area of 920.000 m2, only 4% is green space. However, 14.000 m2 of vertical walls exist that could potentially host vertical gardens. The present study evaluates the ability of four vertical gardens to improve air quality and quantifies the area of viable spaces to host vertical gardens in the Historic Center. The air quality was monitored with continuous measuring systems near each vertical garden and in areas outside the area of influence. The capacity for retention of gaseous emissions from an internal combustion engine in an active garden was also evaluated. The results were a mixture of advantages and uncovering possible myths: A) the presence of vertical gardens causes a significant decrease in O3 (up to 99%), NO2 (up to 80%), SO2 (up to 83%), PM2.5 (up to 79%) and PM10 (up to 85%); b) however, a poor choice of plant species in vertical gardens may increase the formation of O3; and c) in the case of exposing an active vertical garden to emissions injected directly into the garden by a combustion engine, the particle size distribution influences its removal, being more efficient with a size greater than 4 um but not effective for smaller diameters.