In the cities of our country, which among the following atmospheric gases are normally considered in calculating the value of Air Quality Index? 1. Carbon dioxide 2. Carbon monoxide 3. Nitrogen dioxide 4. Sulfur dioxide 5. Methane Select the correct answer using the codes given below:

The correct answer is B (2, 3 and 4 only). The Air Quality Index used in Indian cities, developed by CPCB, tracks gaseous pollutants that are regulated and measured at urban monitoring stations: carbon monoxide, nitrogen dioxide and sulphur dioxide, along with particulate matter, ozone and others.

• Statement 1 (Carbon dioxide) is wrong. CO2 is a greenhouse gas tracked for climate purposes. It is not a criteria pollutant in the AQI and has no national ambient air quality standard for urban air quality.
• Statement 2 (Carbon monoxide) is right. It is one of the gaseous pollutants in the index.
• Statement 3 (Nitrogen dioxide) is right. It is a key traffic-related pollutant in the index.
• Statement 4 (Sulphur dioxide) is right. It comes mainly from coal combustion and is in the index.
• Statement 5 (Methane) is wrong. It is a greenhouse gas, not an urban air quality parameter.

The core concept is the difference between criteria pollutants, which affect health and are covered by the National Ambient Air Quality Standards under the Air (Prevention and Control of Pollution) Act, 1981, and greenhouse gases, which belong to climate policy. For Mains GS-3 (environment and pollution), this links directly to vehicular emissions, the NCAP and the sources of urban pollution.

Note that the CPCB index has since been revised to include ammonia and lead as well. That does not change the answer here, because CO2 and methane are still not part of it.

Trap: Aspirants often pick D because CO2 and methane are familiar as pollutants. Separate greenhouse gases from the criteria pollutants that the AQI actually uses.