The tooltip says
Put together - this seems to be suggesting that 100% of pollution will be absorbed by trees, which I don't think is the intention. Surely this means 100% of the usual amount of absorpotion, rather than 100% absorption, which also invalidates the meaning of any higher value.
Suggestion for alternative description.
Modifies how much trees and tiles absorb pollution. 100% implies normal absorption. Reduce value for a more difficult challenge.