Tropospheric Ozone Formation Mechanism

Tropospheric ozone (O₃) is a secondary air pollutant formed through complex photochemical reactions, not directly emitted into the atmosphere. It is a major component of smog.

Source of Oxygen Atoms

The formation of ozone requires individual oxygen atoms (O). In the troposphere, these oxygen atoms are primarily generated from the breakdown of nitrogen dioxide (NO₂) by sunlight (photolysis).

The key reactions are:

1. Photolysis of Nitrogen Dioxide:
   $NO_2$ + UV light $\rightarrow$ $NO$ + $O$
2. Ozone Formation:
   $O$ + $O_2$ $\rightarrow$ $O_3$

Therefore, nitrogen dioxide (NO₂) acts as the precursor molecule that provides the necessary oxygen atoms (O) for tropospheric ozone formation.

Analysis of Options

• $O_2$: While ozone is composed of three oxygen atoms and involves molecular oxygen ($O_2$), $O_2$ itself is not the direct source of the *individual oxygen atoms* needed for ozone formation in the troposphere via the dominant photochemical pathways.
• $H_2O$: Water vapor plays a role in atmospheric chemistry but does not directly provide the oxygen atoms for ozone formation in this context.
• $NO_2$: Nitrogen dioxide photolysis yields oxygen atoms (O), which then react with $O_2$ to form $O_3$. This is the primary source relevant to the question.
• $CO_2$: Carbon dioxide is not involved in generating oxygen atoms for ozone formation.

Based on the photochemical mechanism, $NO_2$ is the correct source of oxygen atoms required to form tropospheric ozone.