Secondary Pollutants and Chemical Reactions in the Atmosphere

Not every harmful substance in polluted air was ever directly emitted. A secondary pollutant is

one that is manufactured, so to speak, by the atmosphere itself: a primary pollutant undergoes a

chemical transformation -- oxidation, hydrolysis, or a reaction triggered by sunlight -- after it

has already been released, and the product of that reaction is what actually causes the harm.

Three atmospheric reactions are especially important:

Oxidation of $\text{SO}_2$ to sulfuric acid. Sulfur dioxide released from burning coal is

gradually oxidised in air, catalysed by trace metals or by reaction with hydroxyl radicals, to

sulfur trioxide, which then dissolves readily in atmospheric water droplets:

The resulting sulfuric acid aerosol is a major component of acid rain and of the sulfate haze seen

over industrial regions.

Photolysis of $\text{NO}_2$ to ozone. Sunlight of the right wavelength splits nitrogen dioxide

into nitric oxide and a highly reactive free oxygen atom, which immediately combines with

molecular oxygen to give ozone:

Ground-level ozone produced this way is a powerful irritant and oxidant, unlike the protective

ozone found naturally much higher up in the stratosphere.

Radical reactions forming PAN. Free radicals generated from unburnt hydrocarbons react with

oxygen and with $\text{NO}_2$ in a multi-step radical chain to yield peroxyacetyl nitrate (PAN), an

eye- and lung-irritating secondary pollutant discussed further with photochemical smog. …