Role of vegetation in deposition and dispersion of air pollution in urban parks

Urban parks have long been considered places of refuge from the bustle, noise and pollutants of the surrounding city. Parks were labelled the lungs of the city, a metaphor which gained support as studies revealed the deposition of pollutants to leaf surfaces. More recently air purification by urban vegetation came to be seen as an ecosystem service, that add to the other important services it provides. However, the significance of deposition to foliage in reducing air pollutant concentrations in cities and the service this provides is often questioned. Although pollutants deposit on vegetation, the change in turbulence and wind speed brought about by increased friction can often limit dispersion and means that pollutant concentrations can be higher than expected. This paper re-analyses pollutant transects away from roads and into parks to compare the relevance of deposition and dispersion to lowered concentrations in park interiors and shows that pollutant concentrations typically decline more rapidly along transects with vegetation than those without. This may support the notion that deposition to vegetation rapidly reduces pollutant concentrations, but simulations using ENVI-MET reveal that even dense foliage offers little improvement to air quality via removal of pollutants in small urban parks with length scale less than 100 m. New field observations show the dominance of aerodynamic factors in changing air quality in urban parks and the way vegetation affects pollutant concentrations through altering air flows. Deposition is generally not important for parks although it plays a role in the rapid removal of large particles with rapid settling velocities (> 10 cm s(-1)) at the park borders (< 5 m). Across cities as a whole rarely more than a percent or so of individual pollutants are removed by urban vegetation. Deposition on foliage is not an important contribution to improved air quality. Vegetation can alter the dispersion of pollutants from roads into parks and effectively increase pollutant concentrations. Nevertheless stands of trees or flower beds can affect the distribution of park users, so may change exposure to air pollutants by altering the spatial distribution of occupants.