Question

Which of the following gas is both a green house gas and a criteria air pollutants?

• A. Nitrous oxide
• B. Carbon dioxide
• C. Ozone
• D. Sulphur hexafluoride

Hint:

Tropospheric Ozone Dual Identity:
- Criteria Pollutant $\rightarrow$ Strong oxidant, causes respiratory illness and crop necrosis.
- Greenhouse Gas $\rightarrow$ Absorbs infrared radiation at $9.6\,\mu\text{m}$.
"Good up high (stratosphere), bad nearby (troposphere)."

Answer 1

The Correct Option is C

Concept:
Air pollutants can be classified as criteria air pollutants or greenhouse gases (GHGs) based on their environmental impacts:
- Criteria pollutants are regulated based on direct human health toxicity and ambient exposure limits.
- Greenhouse gases trap outgoing terrestrial longwave infrared radiation, contributing to planetary warming.

Step 1: Evaluation of Candidate Gases:
1. Nitrous Oxide ($\text{N}_2\text{O}$): A potent greenhouse gas with a global warming potential nearly 300 times that of $\text{CO}_2$. However, it is non-toxic at ambient concentrations and is not classified as a criteria air pollutant.
2. Carbon Dioxide ($\text{CO}_2$): The principal anthropogenic greenhouse gas driving global climate change. However, under the National Ambient Air Quality Standards (NAAQS) of India, it is not designated as a criteria air pollutant.
3. Sulphur Hexafluoride ($\text{SF}_6$): A synthetic dielectric greenhouse gas with a high global warming potential ($\sim 23,500$), but not a criteria air pollutant.
4. Tropospheric Ozone ($\text{O}_3$):
- As a criteria air pollutant: Ground-level ozone is a powerful respiratory irritant that causes coughing, lung damage, and reduces crop yields. It is monitored under both Indian NAAQS and the US EPA.
- As a greenhouse gas: Ozone absorbs longwave infrared radiation near $9.6\,\mu\text{m}$, making it the third most important greenhouse gas in the troposphere after carbon dioxide and methane.

Step 2: Conclusion:
Ground-level ozone is the only gas among the choices that belongs to both regulatory categories.
Final Answer:
The gas that is both a greenhouse gas and a criteria air pollutant is Ozone, corresponding to option (C).