Question

The major photochemical oxidant is:

• A. H\(_2\)O\(_2\)
• B. O\(_3\)
• C. NO
• D. Peroxy acetyl nitrate (PAN)

Hint:

Photochemical smog is often referred to as ozone smog due to the dominance of O\(_3\).

Answer 1

The Correct Option is B

Photochemical smog forms when sunlight drives reactions between nitrogen oxides and hydrocarbons in vehicle and industrial emissions. Nitric oxide is only a starting material, not an oxidant, since it is what gets converted during the reaction. Hydrogen peroxide and PAN do appear as by-products of this chemistry, but only in comparatively small concentrations. The dominant end-product of this entire reaction sequence, present in the largest amount and responsible for most of the oxidising damage associated with smog, is ground-level ozone.

Therefore, the correct answer is O\(_3\).

Answer 2

Air-quality monitoring standards are a useful angle here, since regulators track a specific gas as the indicator of photochemical smog rather than tracking all four of these substances equally.

1. H\(_2\)O\(_2\): This is not a pollutant that air-quality standards use to track smog severity, since it is only a minor transient species.
2. O\(_3\): Ground-level ozone is the pollutant that air-quality indices specifically monitor as the marker of photochemical smog, precisely because it builds up to the highest concentration among all the oxidising species formed in this reaction chain.
3. NO: Regulators track this as a precursor emission from combustion sources, not as an oxidant product, since it reacts further rather than persisting as an end product.
4. PAN: While recognised as a smog component, its concentration and regulatory emphasis are both far smaller than that of ozone.

Since regulatory and monitoring practice treats ozone as the representative and most abundant photochemical oxidant, this confirms it as the major one among the options given.

Therefore, the correct answer is O\(_3\).