Ambient air quality: an approximate tolerable limit for nitrogen oxides (as NOx, expressed as NO2) in air is closest to which value (ppm)?

Introduction / Context:
Nitrogen oxides (NO and NO2, collectively NOx) are key urban air pollutants that contribute to ozone formation, smog, and respiratory irritation. Regulatory limits are typically set in parts per million (ppm) or micrograms per cubic meter for different averaging periods.

Given Data / Assumptions:

• We seek an approximate, representative “tolerable” ambient concentration.
• Context is environmental air (not occupational exposure limits).
• Conventional textbook/entrance-exam benchmarks are referenced.

Concept / Approach:
Ambient standards around the world generally cap NO2 in the vicinity of tenths of a ppm for short-term averages, and much lower for annual means. Values like 1, 5, or 25 ppm are far above typical ambient targets and align more with short-term occupational exposure ceilings, not public air-quality criteria.

Step-by-Step Solution:
Recognize that 0.1 ppm corresponds to about 188–200 µg/m^3 for NO2, consistent with short-term ambient guidelines used historically in several jurisdictions.Compare options: 1 ppm (≈ 1,880 µg/m^3) and higher are excessive for ambient air.Select the closest acceptable ambient reference: 0.1 ppm.

Verification / Alternative check:
While exact standards vary by country and by averaging period, educational references often cite 0.1 ppm as a representative order-of-magnitude limit for NO2 in ambient air on a short-term basis.

Why Other Options Are Wrong:
1, 5, and 25 ppm are much higher than typical ambient limits and would represent unhealthy air quality.

Common Pitfalls:
Confusing occupational exposure limits with ambient public-health standards; ignoring averaging time when comparing ppm values.

Final Answer:
0.1