New York City’s Trees Work Hard. Still, They Could Use a Little Help. | A chemical reaction involving emissions from cars and buildings can negate their environmental benefits.

Cleaning up a lot of local air pollution is a major co-benefit to getting off fossil fuels:

[oak trees] emit a chemical called isoprene. Alone, it is harmless. But when isoprene meets nitrogen oxides, a group of gasses emitted when fossil fuels are burned to power buildings and vehicles, the chemical reaction creates ground-level ozone, a harmful pollutant that can cause respiratory issues.