How can photochemical smog be controlled?

We need to identify how photochemical smog can be controlled.

Photochemical smog is formed when sunlight reacts with nitrogen oxides (NO$$_x$$) and volatile organic compounds (VOCs) in the atmosphere, producing ground-level ozone, PAN (peroxyacetyl nitrate), and other harmful secondary pollutants. The primary sources are automobile exhaust and industrial emissions.

Evaluate each option:

(1) Tall chimneys: These disperse pollutants over a wider area but do not reduce the actual emission of NO$$_x$$ or VOCs. Not effective for photochemical smog.

(2) Complete combustion of fuel: This helps reduce CO and unburnt hydrocarbons but does not significantly reduce NO$$_x$$ formation (which occurs at high temperatures regardless of combustion completeness).

(3) Catalytic converters in automobiles/industry: This is the most effective method. Catalytic converters contain platinum, palladium, and rhodium catalysts that simultaneously reduce NO$$_x$$ to N$$_2$$ and oxidize CO and hydrocarbons to CO$$_2$$ and H$$_2$$O. By removing both NO$$_x$$ and VOCs from exhaust, they directly prevent the precursors of photochemical smog from entering the atmosphere. ✓

(4) By using catalyst: Too vague and general to be the correct answer in this context.

The correct answer is Option (3): By using catalytic converters in the automobiles/industry.