Great question, Dexter! Yes, the air can become a bit more polluted without leaves on trees! Trees remove gaseous air pollution directly through their leaves, which improves local air quality. When leaves of deciduous trees, like maples and oaks, fall off in autumn and winter, those trees can no longer remove pollution and other airborne particulates until they regrow their leaves in the spring. Evergreen trees, like pine and spruce trees, keep their leaves (aka, needles) all year and continue removing pollution throughout the fall and winter. So places with more evergreen trees have better pollution removal during the fall and winter!

Best,