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

Step 1: Understanding photochemical oxidants.
Photochemical oxidants are secondary air pollutants formed by reactions of nitrogen oxides and hydrocarbons in the presence of sunlight.
Step 2: Formation of ozone.
Ground-level ozone is produced when NO\(_x\) and volatile organic compounds react under sunlight. It is the most abundant and reactive photochemical oxidant in the atmosphere.
Step 3: Evaluating the options.
(A) H\(_2\)O\(_2\): Not the major photochemical oxidant in ambient air.
(B) O\(_3\): Correct, ozone is the principal photochemical oxidant.
(C) NO: A precursor pollutant, not an oxidant.
(D) PAN: A secondary pollutant, but not the major oxidant.
Step 4: Conclusion.
The major photochemical oxidant present in photochemical smog is ozone (O\(_3\)).

Answer 2

Photochemical oxidants form through a chain of sunlight-driven reactions involving nitrogen oxides and volatile organic compounds, so identifying the major one means tracking what that reaction chain actually produces in the largest quantity.

1. H\(_2\)O\(_2\): Hydrogen peroxide can form in trace amounts during atmospheric photochemistry, but it is not the dominant or most abundant product of the smog-forming reaction chain.
2. O\(_3\): When nitrogen dioxide absorbs sunlight, it splits to release an oxygen atom that combines with molecular oxygen to form ozone, and this reaction is continuously reinforced by volatile organic compounds reacting with nitric oxide; the result is that ozone accumulates as the dominant oxidant in photochemical smog.
3. NO: Nitric oxide is a starting reactant and gets converted into nitrogen dioxide during the cycle; it is a precursor pollutant, not an oxidant produced by the reaction.
4. PAN: Peroxy acetyl nitrate does form as a secondary product and is itself an eye irritant, but it forms in much smaller quantities than ozone and is not considered the principal oxidant.

Tracing the sunlight-driven NO\(_x\)-hydrocarbon reaction chain to its main product shows that ozone is generated in far greater quantity than the other candidates, making it the dominant photochemical oxidant.

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