Trees and parks as “the lungs of cities”

The metaphor “lungs of cities”, initially a slogan for the preservation of urban parks, has been retained almost as common sense to the present. It implies that parks provide urban dwellers spaces for breathing in polluted cities. Observations on air pollutants in urban parks detect imprints left by emissions from local vehicles and industries, although they also reveal cleaner park interiors. There has been divergence about the way enhanced air quality in urban parks has been interpreted, some seeing this as the result of pollutant dispersion, while others believe it arises through pollutant uptake by vegetation. A bibliometric analysis suggests that studies considering only deposition found pollutant reduction, while those which account dispersion are less consistent, but street trees often fail to improve air quality. The balance between pollutant dispersion and deposition processes varies with spatial scale and is an important determinant of the roles played by vegetation in improving air quality. In small parks, common in dense cities, pollutant removal by vegetation is unlikely to make the major contribution to improved air quality in their interiors. Moreover, dense tree canopies supress dispersion so can increase localised pollutant concentrations. The contemporary understanding of air-vegetation interactions has yet to be widely adopted in park design and urban planning. Although the metaphor “lungs of cities” may inspire enthusiasm for urban parks, it should not frustrate the use of emerging research in design.