Many plants, particularly conifers, emit abundant terpenes, especially in hot weather. When combined with pollution, terpenes radically increase ozone to levels considered dangerous. This is probably why Ronald Reagan told the tree huggers that trees pollute.
I was a greenie, 1976-84, during which time I personally planted 1,000 trees. Of course, this area at the time was exurbia, not urban, and now I guess you'd call it somewhere between suburbia and exurbia, altho this particular valley is still mostly agri. I wonder how much my efforts have contributed and will contribute to pollution. Maybe should've left the area remain as meadow.