Animated Solution for Chemistry - Environmental Chemistry: The type of pollution that gets increased during the day time and in the presence of ${\text{O}}_3$ is

Daytime and Ozone

When tackling environmental chemistry questions, the specific conditions provided are your biggest hints. In this problem, we are asked to identify a type of pollution that specifically increases during the day time and thrives in the presence of ozone ($O_3$).

Let's break down what "day time" implies chemically. The presence of the sun means there is an abundance of ultraviolet (UV) radiation. This sunlight acts as the energetic trigger required to drive photochemical reactions in the atmosphere. Therefore, any smog that peaks during the day is almost certainly photochemical in nature.

To understand how this smog forms, we have to trace it back to its source. When fossil fuels are burnt in automobile engines and industrial plants, they release primary pollutants into the air. The main culprits are unburnt hydrocarbons and nitric oxide ($NO$).

Once in the atmosphere, the nitric oxide encounters ozone ($O_3$). A rapid chemical reaction takes place:

Notice how the ozone is consumed to generate nitrogen dioxide ($N{O}_2$). Now, the atmosphere is loaded with $N{O}_2$, remaining $O_3$, and unburnt hydrocarbons.

Photochemical Smog

In the presence of sunlight, these components undergo a complex series of reactions to form a toxic soup of secondary pollutants. This mixture includes dangerous chemicals such as formaldehyde, acrolein, and Peroxyacetyl nitrate (PAN). This entire concoction is what we call photochemical smog.

But why is it called oxidising smog? Let's look at the chemical nature of its primary ingredients. Both ozone ($O_3$) and nitrogen dioxide ($N{O}_2$) are exceptionally strong oxidising agents. Because photochemical smog contains such a high concentration of these oxidising species, it is universally referred to as oxidising smog.

Photochemical Smog

To solidify this concept, it is crucial to contrast it with the other major type of smog: Classical Smog (often called London smog).

Classical smog occurs in cool, humid climates and is primarily a mixture of smoke, fog, and sulphur dioxide ($S{O}_2$). Because sulphur dioxide is chemically a reducing agent, classical smog is known as reducing smog.

In stark contrast, our daytime, ozone-rich photochemical smog occurs in warm, dry, and sunny climates and is fundamentally oxidising. Therefore, the correct answer to our question is oxidising smog.