${?}_{{?}_{{?}_{?}}{?}_{{?}_{?}}{?}_{{?}_{?}}is\text{t}\text{h}\text{e}\text{l}\text{a}\text{r}\text{g}\text{e}\text{s}\text{t}\text{c}\text{a}\text{u}\text{s}\text{e}of\text{u}\text{p}\text{p}\text{e}\text{r}\text{o}\text{z}\text{o}\text{n}\text{e}\text{d}\text{e}\text{p}\text{l}\text{e}\text{t}\text{i}\text{o}\text{n}\text{.}}$

Carbon dioxide

Nitrous oxide

Sulfur dioxide

Acid rain