It's impossible with the current functionality.

Hi robs074,

since you make not the most simple kind of scripts, I was wondering.
Do you think, you could make a script that allows for trees to reduce pollution?
A long time ago, I played Factorio and found a script mod which did add this. I'm not sure if it is possible in W&R as well and you would need to make the tree tool cost money as a side effect. Still, I believe this would add some depth to the game and make the cities in our republics much more beautiful.
Afterall, the "green lungs" of cities is a newer concept but the practice of lining roads with trees is very old.

Thx for listening to me :)