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The ability of urban vegetation to improve air quality for the benefit of urban residents is often considered fact since plants can absorb and capture air pollutants. However, there is little empirical evidence that urban air quality at the local scale is improved by the presence of, e.g. trees, especially in northern climatic regions. We studied the impact of urban forest vegetation on the levels of five types of air pollutants (NO 2 , ground-level O 3 , anthropogenic and biogenic VOCs, and particulate matter) in near-road environments during summer (June) using passive samplers in Helsinki, Finland. Concentrations of gaseous pollutants did not differ significantly between tree-covered and adjacent open areas, while particle pollutant levels were significantly lower in tree-covered areas than in adjacent open, treeless areas. Vegetation-related variables (canopy closure, tree number and size, and ground vegetation) did not explain differences in air quality. Our results suggest that the role of urban, mostly deciduous, vegetation is negligible in improving local air quality, in terms of the anthropogenic pollutants measured here, in northern climates. However, air particulate pollution, which is likely to be dominated by large-sized particles in our study, can be reduced by urban vegetation. Highlights:  Gaseous pollutant concentrations did not differ between tree-covered and open areas  Particle pollutant levels were significantly lower in tree-covered areas  Vegetation-related variables did not explain the difference in particulate levels

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Trees and other vegetation absorb and capture air pollutants, leading to the common perception that they, and trees in particular, can improve air quality in cities and provide an important ecosystem service for urban inhabitants. Yet, there has been a lack of empirical evidence showing this at the local scale with different plant configurations and climatic regions. We studied the impact of urban park and forest vegetation on the levels of nitrogen dioxide (NO 2) and ground-level ozone (O 3) while controlling for temperature during early summer (May) using passive samplers in Baltimore, USA. Concentrations of O 3 were significantly lower in treecovered habitats than in adjacent open habitats, but concentrations of NO 2 did not differ significantly between tree-covered and open habitats. Higher temperatures resulted in higher pollutant concentrations and NO 2 and O 3 concentration were negatively correlated with each other. Our results suggest that the role of trees in reducing NO 2 concentrations in urban parks and forests in the Mid-Atlantic USA is minor, but that the presence of tree-cover can result in lower O 3 levels compared to similar open areas. Our results further suggest that actions aiming at local air pollution mitigation should consider local variability in vegetation, climate, microclimate, and traffic conditions.

While the potential of plants to uptake polycyclic aromatic hydrocarbons (PAHs) is widely acknowledged, empirical evidence of the effects of this process on local atmospheric PAH concentrations and human health is tenuous. We measured gaseous PAH concentrations using passive samplers in urban tree-covered areas and adjacent open, treeless areas in a near-road environment in Finland to gain information on the ability of urban vegetation to improve air quality. The ability of urban, mostly deciduous, vegetation to affect PAHs was season dependent: during summer, concentrations were significantly higher in tree-covered areas, while in the fall, concentrations in open areas exceeded those in tree-covered areas. During winter, concentrations in tree-covered areas were either lower or did not differ from those in open areas. Results of this study imply that the commonly believed notion that trees unequivocally improve air quality does not apply to PAHs studied here.