Q.Write down the reactions involved during the formation of photochemical smog.

Photochemical smog is formed through a sequence of photochemical reactions between nitrogen oxides, hydrocarbons and sunlight in the troposphere.

Step 1 — Photolysis of NO2:

Sunlight splits nitrogen dioxide (from vehicle exhaust) into nitric oxide and a free oxygen atom.

Step 2 — Formation of ozone:

The reactive oxygen atom combines with molecular oxygen to form ozone.

Step 3 — Ozone reacts back with NO:

Normally this step would consume the ozone as fast as it forms, keeping O3 levels low. However, in the presence of unburnt hydrocarbons from vehicle exhaust, NO is instead consumed by reacting with hydrocarbon free radicals, so it is not available to remove O3 — this allows ozone to build up to high (harmful) concentrations.

Step 4 — Reaction of hydrocarbons and free radicals:

Unburnt hydrocarbons react with oxygen atoms, ozone and other radicals to give a range of secondary pollutants: …

Write the sequence of reactions in order: NO2 photolysis, ozone formation, the O3–NO equilibrium reaction, and the hydrocarbon oxidati …

• Skipping the role of unburnt hydrocarbons, which is why O3/PAN accumulate instead of the O3+NO reaction simply cancelling ozone out. …