The acceptable level of carbon monoxide gas $(CO)$ in the atmosphere in $ppm$ level is

What is smog? How is classical smog different from photochemical smog?

The smog is essentially caused by the presence of

Assertion : Photochemical smog is produced by nitrogen oxides.
Reason : Vehicular pollution is a major source of nitrogen oxides.

The part of the Earth where life exists,known as the biosphere,is composed of which components?

Identify the correct statements from the following:
$I$. Photochemical smog has high concentration of oxidising agents
$II$. $NO_2$ is present in classical smog
$III$. Higher concentration of $SO_2$ in air can cause stiffness of flower buds
$IV$. pH of rain water is approximately $7.5$