E10 Smog

E.10.1 State the source of primary pollutants and the conditions necessary for the formation of photochemical smog.

Smog is a poisonous mixture of smoke, fog, air, and other chemicals, is formed in large cities and is favoured by lack of wind and bowl-shape cities (land that is sounded by higher ground in all directions). Smog is likely to occur when there is a thermal inversion, whereby a layer of still warm air to blanket a layer of cooler air, so that convection currents cannot form). As a result, trapped pollutants cannot rise and can rise to dangerous levels if the stillness persists.

‘Pea smog’ (classically English) occurred before the introduction of clean air controls in the mid 1950s, whereby the combustion of coal and oil produced sulphur dioxide mixed with soot, fly ash and partially oxidising organic material (reducing smog). Photochemical smog is the more common smog now (e.g. Los Angeles) that is converted into secondary pollutants in sunlight, consisting of oxides of nitrogen and VOCs (brown/yellow colour).

E.10.2 Outline the formation of secondary pollutants in photochemical smog.

NO2(g) --> NO(g) + O ·(g)

Nitrogen dioxide is broken down to give an oxygen free radical and nitrous oxide. The oxygen radicals can react with oxygen to form ozone and with water to form hydroxyl radicals. These photochemical oxides can react with a variety of molecules including nitrous oxide to form nitric acid and VOCs to form peroxides (ROOR), aldehydes (RCHO), and ketones (RCOR). Peroxides are extremely reactive and aldehydes and ketones reduce visibility by condensing to form aerosols.

O·(g) + H2O (l) --> 2OH·(g) [hydroxyl radical]

OH·(g) + NO2(g) --> HNO3(aq) [nitric acid]

OH·(g) + RH(g) --> R·(g) + H2O [radical]

R·(g) + O2 --> ROO· [peroxide radical]