This is a good question and deserves a thorough explanation.
THRIFTEE has seen his planet be negatively affected from both smoggy air and the acidity in rain (Acid Rain) that affects his trees and nuts. He has spoken to several of his science friends and they feel so strongly that they publish reports that explain that in a drop of rain, Nitric Acid makes up 73-80% of the acid component. In fact, the American chestnut tree had become extinct due to a progressive fungus from acid rain killing most of them 25 years ago. THRIFTEE has more friends now with asthma than he knew of 50 years ago. And some of his friends in Asia are being seriously affected by very poor air quality.