Q.What is smog? How is classical smog different from photochemical smogs?

Smog is a type of air pollution named from the combination of the words 'smoke' and 'fog'. There are two distinct kinds:

Classical smog (reducing smog):

• Occurs in cool, humid climates.
• It is a mixture of smoke, fog and sulphur dioxide (SO2), formed largely from the burning of coal.
• Chemically it is a reducing mixture, since SO2 is a reducing agent.
• Common in cities with high coal use, especially in winter (e.g. historically London).

Photochemical smog (oxidising smog):

• Occurs in warm, dry and sunny climates.
• It is produced by the photochemical reaction of sunlight with unburnt hydrocarbons and nitrogen oxides (NO, NO2) released mainly by automobile exhaust.
• The reaction sequence produces ozone (O3), nitric oxide, acrolein, formaldehyde and peroxyacetyl nitrate (PAN).
• Chemically it is an oxidising mixture, since these products (especially O3 and PAN) are strong oxidising agents.
• Common in cities with heavy vehicular traffic and abundant sunlight (e.g. historically Los Angeles).

Key differences: …

Define smog as a smoke-fog mixture, then contrast the two types on four points: climate, composition, chemical nature (reducing vs oxid …

• Mixing up which type is reducing and which is oxidising (classical = reducing due to SO2; photochemical = oxidising due to O3/PAN). …