Functional traits of urban trees in relation to their air pollution mitigation potential: A holistic discussion

In an increasingly urbanized world, air pollution mitigation is considered one of most important issues incity planning. Urban trees help to improve air quality by facilitating widespread deposition of various gasesand particles through the provision of large surface areas as well as through their influence on microclimateand air turbulence. However, many of these trees produce wind-dispersedpollen (a known allergen) andemit a range of gaseous substances that take part in photochemical reactions - all of which can negativelyaffect air quality. The degree to which these air-qualityimpacts are manifested depends on species-specifictree properties: that is, their "traits". We summarize and discuss the current knowledge on how such traitsaffect urban air pollution. We also present aggregated traits of some of the most common tree species inEurope, which can be used as a decision-supporttool for city planning and for improving urban air-qualitymodels.