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 in
city planning. Urban trees help to improve air quality by facilitating widespread deposition of various gases
and particles through the provision of large surface areas as well as through their influence on microclimate
and air turbulence. However, many of these trees produce wind-dispersed
pollen (a known allergen) and
emit a range of gaseous substances that take part in photochemical reactions - all of which can negatively
affect air quality. The degree to which these air-quality
impacts are manifested depends on species-specific
tree properties: that is, their "traits". We summarize and discuss the current knowledge on how such traits
affect urban air pollution. We also present aggregated traits of some of the most common tree species in
Europe, which can be used as a decision-support
tool for city planning and for improving urban air-quality
models.