https://phys.org/news/2018-07-particulate-drought-vulnerability-trees.html

Tiny pores in the leaves, the stomata, are the gatekeepers that regulate how much water vapor is released to the atmosphere. But stomata also admit carbon dioxide, which is essential for photosynthetic production of energy storing sugars. This causes a dilemma for plants, between thirst and starvation: Closing the stomata saves water but restricts the uptake of carbon dioxide. Vice versa, open pores allow carbon dioxide to enter, but at the cost of increased water loss. Burkhardt: "Plants have adapted stomatal regulation to prevailing ambient conditions over evolutionary timescales, but particulate matter concentrations were generally much lower than they are today."

Trees transpire less in filtered air