Which of the following gases is not a greenhouse gas?

Some reactions of $NO_2$ relevant to photochemical smog formation are:
$NO_{2(g)} \xrightarrow{hv} NO_{(g)} + X$
$X + O_2 \rightarrow Y$
$NO + Y \rightarrow NO_2 + A$
$O + O_2 + M \rightarrow B + M$
Identify $A, B, X$ and $Y$.

What are the reactions involved in the ozone layer depletion in the stratosphere?

Which of the following components is present in the troposphere?

The main cause of air pollution in large cities is ........

Which is the greatest source of air pollution these days?