Many cities attempt to mitigate poor air quality by increasing tree canopy cover. Trees can indeed capture pollutants and reduce their dispersion, but they can also negatively impact urban air quality. For example, trees emit biogenic volatile organic compounds (BVOCs) that participate in both ozone (O<sub>3</sub>) and secondary organic aerosol (SOA) formation, yet these emissions have been little studied in urban contexts.</p> <p>We sampled BVOCs from the leaves of mature urban trees using lightweight enclosures and adsorbent tubes in two cities: Montreal, Canada and Helsinki, Finland. In both cities, we targeted five common broadleaved species, comparing their standardised BVOC emission potentials 1) between parks and streets and 2) to nonurban BVOC emission potential estimates from emission databases. Finally, we calculated the potential O<sub>3</sub> and SOA formation by urban trees at the leaf scale and upscaled to the neighbourhood.</p> <p>We found that the BVOC emission potentials differed slightly between park and street trees. Compared to park trees, street tree emissions were higher in Montreal (specifically isoprene and sesquiterpenoids) and lower in Helsinki (specifically green leaf volatiles). However, the measured BVOC emission potentials generally deviated little from the emission database estimates, supporting the use of database estimates for urban trees. In addition, we found that O<sub>3</sub> formation from urban tree BVOC emissions was dominated by isoprene, while SOA formation was also affected by lower monoterpenoid and sesquiterpenoid emissions. These findings highlight the importance of species selection and management strategies that protect trees from BVOC-inducing stresses.