Q.High-sulfur coal and diesel release sulfur dioxide ($\text{SO}_2$) on combustion. Write the sequence of reactions by which atmospheric $\text{SO}_2$ is converted into sulfuric acid, and name the environmental problem this ultimately causes.

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.