Which one of the following is included while estimating Air Quality Index (AQI)?

The correct option is NO₂ Nitrogen dioxide.

What is the Air Quality Index (AQI)?
The Air Quality Index (AQI) is a standard indicator used by environmental agencies to report daily air quality. It informs the public about how clean or polluted the air is, and what associated health effects might be of concern, especially for sensitive groups.

Key Pollutants Tracked by the AQI:
The AQI is calculated based on the measurement of major air pollutants regulated under clean air standards. These primary pollutants include:
1. Ground-level Ozone (O₃)
2. Particulate Matter (such as PM₁₀ and PM_2.5)
3. Carbon Monoxide (CO)
4. Sulfur Dioxide (SO₂)
5. Nitrogen Dioxide (NO₂)

Why Nitrogen Dioxide (NO₂) is Monitored:
Nitrogen dioxide is a highly reactive gas formed primarily from emissions from motor vehicles, power plants, and industrial facilities. It is a critical component of air pollution because it contributes to the formation of photochemical smog and acid rain, and it can cause severe respiratory issues in humans. Consequently, it is directly measured and included in the estimation of the AQI.

Other Oxides of Nitrogen:
Although other oxides of nitrogen exist—such as Nitrous oxide (N₂O), Dinitrogen trioxide (N₂O₃), and Dinitrogen pentoxide (N₂O₅)—they are not used as standard criteria pollutants for calculating the immediate Air Quality Index.