Q.Distinguish between a primary air pollutant and a secondary air pollutant. Give one example pair (a primary pollutant and the secondary pollutant that forms from it in the atmosphere) and briefly state the chemistry that links them.

Air pollution is driven by two classes of pollutant. Primary pollutants -- carbon monoxide

($\text{CO}$), sulfur dioxide ($\text{SO}_2$), oxides of nitrogen ($\text{NO}_x$), unburnt

hydrocarbons and particulate matter -- are emitted directly from combustion sources such as vehicle

engines, power plants and industrial stacks, and are still in their original chemical form when

they cause harm. $\text{CO}$ forms by incomplete combustion ($2\text{C} + \text{O}_2 \rightarrow 2\text{CO}$) and is toxic because it binds haemoglobin roughly 200 times more strongly than oxygen,

blocking the body's oxygen supply. $\text{SO}_2$ comes from burning sulfur-containing coal and

diesel; $\text{NO}_x$ forms when engine heat forces atmospheric $\text{N}_2$ and $\text{O}_2$ to

combine. Secondary pollutants are not emitted at all -- they form when a primary pollutant

reacts further in the atmosphere. $\text{SO}_2$ oxidises and hydrates to sulfuric acid

($2\text{SO}_2 + \text{O}_2 \rightarrow 2\text{SO}_3$; $\text{SO}_3 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_4$), the acid responsible for acid rain, while $\text{NO}_2$ and hydrocarbons

react under sunlight to give ozone and PAN, the defining pollutants of photochemical smog. The

primary/secondary distinction matters practically: cutting a primary pollutant at its source

automatically prevents every secondary pollutant that would otherwise have formed from it downstream.