Daily question review · 15 September 2026
Photochemical smog: a clarified practice question
Clarified practice variant — no response statistics for this version.
Which gas is a major component of photochemical smog, formed near ground level through sunlight-driven reactions involving nitrogen oxides and volatile organic compounds?
This is self-practice; your selection here is not submitted to a server.
Reveal answer and explanation
D — O₃ (ozone). Near ground level, ozone forms through atmospheric reactions involving nitrogen oxides and volatile organic compounds in sunlight. It is a major component of photochemical smog. This question specifically describes ground-level ozone; it does not mean that CO or CO₂ can never form through atmospheric reactions.
Ground-level ozone is an air pollutant. Distinguish it from stratospheric ozone, which helps shield Earth from harmful ultraviolet radiation.
EPA: Ground-level Ozone Basics · EPA: Overview of Carbon Monoxide Air Quality, July 11, 2025
Work through this question, then continue with today’s set.
Historical record — original question only
Original question
Which of the following is a secondary pollutant?
A: SO2 · B: CO2 · C: CO · D: O3
On the original question, 25 of 27 nonstaff accounts that answered chose a different option from the stored key D; 2 chose D, and 2 additional accounts explicitly skipped. Choices: A14, B2, C9, D2. The original wording was broad, and its explanation overstated the primary/secondary distinction. These counts do not establish 25 student errors or question difficulty. They do not apply to the clarified practice variant. Five unknown-form records are excluded; response language was not recorded.
Our explanation needed correcting
The intended answer is D, O₃ (ozone), in the ground-level photochemical-smog context. Ozone forms through sunlight-driven atmospheric reactions involving nitrogen oxides and volatile organic compounds. The original explanation overstates the distinction: CO can also be formed secondarily, and CO₂ can arise in atmospheric oxidation. Therefore, do not say that A–C can never form secondarily or that only ozone is a secondary pollutant under every circumstance. The broad original question needs this context. Option choices alone do not establish why anyone answered incorrectly.
EPA: Ground-level Ozone Basics · EPA: Overview of Carbon Monoxide Air Quality, July 11, 2025
These records concern the original daily question on 14 September 2026 (IST). The first retained answer per account was used; completing the whole set was not required. Accounts are not verified unique people. Evidence checked on 15 September at 05:36 IST; late or missing records remain possible. We do not claim that the app question or historical scores have been changed.
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