Q.What is peroxyacetyl nitrate (PAN)? Name the two classes of primary pollutant that combine, in the presence of sunlight, to eventually produce it, and state one harmful effect it has on living things.

Smog comes in two chemically distinct forms. Classical (London-type) smog forms in cold, humid,

still winter weather over coal-burning industrial cities: smoke, fog and sulfur dioxide combine, the

fog's moisture converting $\text{SO}_2$ into a fine sulfuric-acid mist. It is chemically

reducing, because $\text{SO}_2$ itself is readily oxidised rather than being an oxidant.

Photochemical smog, by contrast, forms over sunny, traffic-heavy cities and needs no fog at all:

its ingredients are $\text{NO}_x$ and unburnt hydrocarbons from vehicle exhaust, activated by strong

sunlight. Sunlight splits $\text{NO}_2$ into $\text{NO}$ and a free oxygen atom

($\text{NO}_2 \xrightarrow{h\nu} \text{NO} + \text{O}$), which combines with $\text{O}_2$ to give

ozone; hydrocarbon radicals separately react with oxygen and $\text{NO}_2$ to give peroxyacetyl

nitrate (PAN). Because $\text{NO}_2$ is consumed in one step and regenerated in the next

($2\text{NO} + \text{O}_2 \rightarrow 2\text{NO}_2$), it behaves catalytically, sustaining ozone

production through the day. Photochemical smog is chemically oxidising -- the opposite character

to classical smog -- and irritates the eyes and lungs while damaging plant tissue and reducing crop

yield.