Q.How does classical smog differ from photochemical smog?

Smog is the term for the combination of smoke and fog, a visible form of air pollution.

Smog is a portmanteau of "smoke" and "fog." It refers to air pollution caused when smoke (particulate matter and pollutant gases from combustion, industrial emissions, vehicle exhaust, etc.) mixes with fog (water droplets suspended in air), especially in cold, calm weather. Two well-known types:

• Classical (reducing) smog -- a mixture of smoke, fog, and sulphur dioxide, common in cool humid climates; chemically reducing in character. …

Photochemical smog forms when sunlight reacts with NOx and hydrocarbon pollutants (mostly from vehicles), producing ozone, PAN and other irritant secondary pollutants; distinct from classical (reducing) smog.

Step 1 -- formation: In areas with heavy vehicular traffic, exhaust gases release nitrogen oxides (NO, NO2) and unburnt hydrocarbons into the air. In the presence of sunlight, these react through a series of photochemical reactions.

Step 2 -- key reactions (simplified):

NO2 + sunlight -> NO + O (atomic oxygen)

O + O2 -> O3 (ozone)

Ozone and other reactive species then react further with hydrocarbons to produce secondary pollutants like formaldehyde, acrolein, and peroxyacetyl nitrate (PAN).

Step 3 -- appearance and conditions: This mixture of primary and secondary pollutants forms a brownish haze, and is favoured by sunny, warm, dry conditions with heavy traffic (classic example: Los Angeles-type smog) -- unlike classical/London-type smog, which is a mixture of smoke and fog occurring in cold, humid, foggy conditions and is reducing in nature.

Step 4 -- effects: Photochemical smog causes eye/throat irritation, respiratory problems, damage to plants, and cracking of rubber, due to the oxidizing power of ozone and PAN.

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Cool, humid conditions give classical/London smog, a reducing smog rich in SO2 and smoke.

Smog is of two kinds:

• Classical (London) smog: forms in cool, humid climate; a mixture of smoke, fog and sulphur dioxide; chemically reducing.
• Photochemical (Los Angeles) smog: forms in warm, dry, sunny climate; oxidising, contains ozone, NOx and PAN. …