Tree selection in urban greening shapes air quality for global cities.

Mitigating ozone pollution remains a pressing challenge for megacities worldwide. Biogenic volatile organic compounds (VOCs) are proposed as pivotal precursors to urban ozone formation. However, direct observational evidence is scarce and the extent of this effect across cities is unclear. Here, by combining an unprecedented eddy covariance flux dataset from Beijing with modeling analysis, we find that urban vegetation contributes over half of organic reactivity essential for ozone production, predominantly driving ozone exceedances during hot periods. The unexpectedly high biogenic influences are attributed to the abundance of urban trees with strong emission potentials of VOCs within the city. Model extrapolations reveal that similar or even greater challenges exist for other cities across Asia and Oceania, where urban greening strategies often favor highly emitting trees. Our findings underscore the critical need for selective urban planning in greening programs to counteract a warming climate.