Animated Solution for Chemistry - Environmental Chemistry: The primary pollutant that leads to photochemical smog is

Imagine you are standing at a busy traffic intersection on a warm, sunny day. The air feels heavy, and there is a yellowish-brown haze hanging over the city. This irritating haze is known as photochemical smog. But where does it come from? To understand this, we have to look at the very source of the problem: the burning of fossil fuels in automobile engines.

When petrol and diesel burn inside an engine, the environment is incredibly hot. Normally, the nitrogen (${\text{N}}_2$) and oxygen (${\text{O}}_2$) present in the air do not react with each other because nitrogen has a very strong triple bond. However, the intense heat inside the engine forces them to combine.

This reaction produces nitric oxide ($\text{NO}$), which is expelled from the exhaust pipe along with unburnt hydrocarbons.

These gases—nitric oxide and unburnt hydrocarbons—are emitted directly from the source into the atmosphere. Because they are the direct products of combustion, they are classified as primary pollutants. They act as the raw materials or the 'seeds' for the smog that will eventually form.

Once released into the atmosphere, nitric oxide does not remain stable for long. It quickly reacts with the abundant atmospheric oxygen to form nitrogen dioxide (${\text{NO}}_2$), which is a reddish-brown gas. This gas is responsible for the characteristic brown color of photochemical smog.

Sunlight and Secondary Pollutants

Here is where the chemistry gets really interesting. The formation of photochemical smog requires a trigger, and that trigger is sunlight. The ultraviolet (UV) radiation from the sun possesses enough energy to break the bonds in the nitrogen dioxide molecule.

This photolysis reaction produces nitric oxide and a highly reactive atom of nascent oxygen ($[\text{O}]$). This nascent oxygen is incredibly unstable and hungry for a reaction. It immediately combines with molecular oxygen (${\text{O}}_2$) in the air to form ozone (${\text{O}}_3$).

Ozone, along with other complex molecules like Peroxyacetyl Nitrate (PAN) formed by the reaction of ozone with unburnt hydrocarbons, are known as secondary pollutants. They are called 'secondary' because they are not emitted directly but are formed through chemical reactions in the atmosphere.

The accumulation of these toxic secondary pollutants—ozone, PAN, and nitrogen dioxide—creates the dangerous photochemical smog that irritates our eyes and damages our lungs.

So, if we trace the entire chain reaction back to its roots, the primary pollutant that initiates this cascade is the nitrogen oxides (${\text{NO}}_x$) released from vehicle exhausts. Therefore, the correct answer to our question is nitrogen oxides.