Animated Solution for Chemistry - Environmental Chemistry: The correct set of species responsible for the photochemical smog is

Have you ever looked at the skyline of a bustling city on a warm, sunny day and noticed a yellowish-brown haze hanging in the air? That is not just dust; it is a complex chemical soup known as photochemical smog.

In this problem, we are asked to identify the primary culprits behind this environmental menace. Let's dive deep into the atmospheric chemistry and unravel the chain reaction that creates this smog.

Photochemical smog does not just appear out of nowhere. It requires specific conditions: a warm, dry, and sunny climate, and a steady supply of primary pollutants. The main sources of these pollutants are automobile exhausts and industrial emissions, which release nitrogen oxides (like $\text{NO}$ and ${\text{NO}}_2$) and unburnt hydrocarbons into the atmosphere.

These primary pollutants are relatively harmless on their own, but when they mix and are exposed to sunlight, the real trouble begins.

Photolysis of Nitrogen Dioxide

The entire chain reaction is kick-started by the sun. When ultraviolet (UV) radiation from sunlight hits a molecule of nitrogen dioxide (${\text{NO}}_2$), it provides enough energy to break the chemical bond.

This process, called photolysis, splits ${\text{NO}}_2$ into nitric oxide ($\text{NO}$) and a highly reactive nascent oxygen atom ($\text{O}$):

This nascent oxygen is the spark that ignites the smog formation.

Nascent oxygen is incredibly unstable. It desperately wants to bond with something. In the lower atmosphere (troposphere), it quickly collides and combines with the abundant diatomic oxygen gas (${\text{O}}_2$) to form ozone (${\text{O}}_3$):

While ozone in the stratosphere forms a protective shield against UV rays, down here in the troposphere, it is a toxic secondary pollutant that causes respiratory issues and damages plants.

Under normal, unpolluted conditions, this ozone would simply react back with the nitric oxide ($\text{NO}$) to regenerate ${\text{NO}}_2$ and ${\text{O}}_2$, keeping the ozone levels balanced.

However, the presence of unburnt hydrocarbons disrupts this natural cycle. These hydrocarbons react with the ozone and nitric oxide to form a variety of toxic, irritating chemicals, such as formaldehyde (${\text{CH}}_2=\text{O}$), acrolein, and peroxyacetyl nitrate (PAN).

Because the hydrocarbons consume the $\text{NO}$, the ozone is not broken down and begins to accumulate to dangerous levels. The mixture of ozone, nitric oxide, nitrogen dioxide, and these oxidized hydrocarbons forms the brown, irritating haze we call photochemical smog.

Looking at the entire mechanism, the essential species that drive the formation of photochemical smog are nitric oxide ($\text{NO}$), nitrogen dioxide (${\text{NO}}_2$), ozone (${\text{O}}_3$), and hydrocarbons.

Therefore, the correct set of species is given by option (c). Understanding this chemistry is the first step towards developing technologies, like catalytic converters, to mitigate this pollution and breathe cleaner air.