Animated Solution for Chemistry - Environmental Chemistry: The smog is essentially caused by the presence of

Smog is one of the most visible and harmful forms of air pollution. But did you know that not all smog is created equal? Broadly speaking, smog is classified into two distinct types based on its chemical composition and the environment in which it forms: Classical Smog and Photochemical Smog.

Classical smog, often associated with the industrial revolution and cities like London, thrives in cool, humid climates. It is primarily a mixture of smoke, fog, and sulphur dioxide ($S{O}_2$). Because sulphur dioxide is a reducing agent, this type of smog is also known as reducing smog.

On the flip side, we have photochemical smog, which is the modern menace of sunny, automobile-heavy cities like Los Angeles. This smog requires warm, dry, and sunny conditions. It is driven by the interaction of sunlight with nitrogen oxides ($N{O}_x$) and unburnt hydrocarbons, making it a highly oxidizing mixture.

To truly understand photochemical smog, we need to dive into the microscopic world of atmospheric chemistry. The entire process is kickstarted by a single photon of sunlight.

When sunlight ($h u$) strikes a molecule of nitrogen dioxide ($N{O}_2$) in the atmosphere, it provides enough energy to break the chemical bonds. This photolysis reaction splits the molecule into nitric oxide ($NO$) and a highly reactive nascent oxygen atom ($O$).

This nascent oxygen is incredibly unstable and immediately seeks out a partner. It finds one in the abundant oxygen gas ($O_2$) present in the air, reacting rapidly to form ozone ($O_3$).

PAN

While ozone in the stratosphere protects us from UV rays, ground-level ozone is a toxic pollutant. The nitric oxide ($NO$) formed in the first step can react with this ozone to regenerate nitrogen dioxide, creating a continuous cycle.

However, the real danger emerges when unburnt hydrocarbons from vehicle exhaust enter the fray. These hydrocarbons react with the nitric oxide and ozone to form a complex organic compound known as Peroxyacetyl nitrate (PAN).

PAN is a severe eye irritant and a hallmark of toxic photochemical smog. It is the reason your eyes water and your throat burns on a heavily polluted day.

Now, let's address the elephant in the room regarding this specific question. The question asks what essentially causes smog. As we've seen, classical smog is driven by oxides of sulphur, and photochemical smog is driven by oxides of nitrogen.

Therefore, the most accurate and comprehensive answer is oxides of sulphur and nitrogen. Interestingly, the provided answer key points to option (a), which is $O_2$ and $O_3$. While ozone is a component of smog, it is a secondary pollutant, not the primary cause. The answer key contains a misprint, and the true conceptual answer is undoubtedly option (c). Always trust your understanding of the chemistry over a printed letter!