Step 1: Identifying what makes smog “photochemical”.
Photochemical smog forms only when chemical reactions take place in the presence of sunlight.
So, the responsible pollutants must be those that can absorb sunlight and start chain reactions in the atmosphere.
Step 2: Finding the primary trigger.
Among air pollutants, nitrogen dioxide \( \text{NO}_2 \) is highly sensitive to sunlight.
Under strong sunlight, it breaks down as:
\[ \text{NO}_2 \xrightarrow{\text{sunlight}} \text{NO} + \text{O} \]
This reaction acts as the starting point of photochemical smog formation.
Step 3: Formation of secondary pollutants.
The free oxygen atom produced reacts with molecular oxygen to form ozone:
\[ \text{O} + \text{O}_2 \rightarrow \text{O}_3 \]
Ozone then reacts further with hydrocarbons to form harmful compounds like PAN and aldehydes.
Step 4: Checking other gases.
Carbon monoxide does not undergo photochemical reactions.
Sulphur dioxide mainly causes industrial smog and acid rain.
Methane is too stable to initiate rapid photochemical reactions.
Only nitrogen oxides actively start the sunlight-driven process.
Final Conclusion:
The gas primarily responsible for the formation of photochemical smog is:
\[ \boxed{\text{Nitrogen oxides}} \]