Photochemical and classical smog formation: Smog episodes are driven by precursors such as sulphur dioxide, nitrogen oxides, hydrocarbons, sunlight, and stagnant air. Which gas listed below does not directly contribute to smog formation mechanisms?

Introduction / Context:
Smog is a mixture of pollutants and secondary products that impair visibility and harm health. Two main types are recognized: classical (sulphurous) smog and photochemical (oxidant) smog. While many gases coexist in urban air, only some are reactive precursors that drive smog chemistry. This question asks which listed gas is not a direct participant in smog formation.

Given Data / Assumptions:

• Classical smog involves SO2 and particulates under humid conditions.
• Photochemical smog involves NOx and volatile organic compounds (VOCs) under sunlight.
• CO2 is abundant but relatively unreactive on urban smog timescales.

Concept / Approach:
Sulphur dioxide oxidizes to sulphates and contributes to haze and acidity; nitrogen oxides are central to ozone formation via photolysis and radical chemistry. Carbon dioxide is a stable greenhouse gas that does not partake in the rapid chain reactions forming ozone, peroxyacyl nitrates, or secondary aerosols during smog events. Therefore, CO2 is not a smog precursor even though it is co-emitted from combustion sources.

Step-by-Step Solution:
List known smog precursors: SO2, NOx, VOCs, ammonia, and particulates.Assess each option's role: SO2 and NOx participate directly; CO2 does not.Select CO2 as the gas not forming smog.

Verification / Alternative check:
Urban photochemical modeling frameworks (e.g., ozone isopleths) exclude CO2 from precursor control strategies, confirming its non-participation in smog chemistry.

Why Other Options Are Wrong:
SO2: Oxidizes to sulphate aerosol; key in classical smog and acid rain chemistry.NOx: Essential for ozone production and oxidant cycles.Any of these: Incorrect because not all listed gases drive smog; CO2 is the exception.

Common Pitfalls:

• Equating greenhouse gases with air quality drivers.
• Assuming any combustion product contributes to smog chemistry.

Final Answer:
CO2