Which of the following concentrations of Ozone in ambient air is considered good?
Show Answer & Explanation
Correct Answer: (A)
Explanation:
According to standard Air Quality Index (AQI) parameters (such as those by the US EPA), an 8-hour average ambient ozone concentration between 0.000 to 0.054 ppm is categorized as “Good” (AQI 0-50). Therefore, the closest threshold for “Good” among the options is Upto 0.06 ppm. Concentrations above this level begin to trigger “Moderate” or “Unhealthy for Sensitive Groups” warnings.
📚 Additional Info: Ozone as a Pollutant
1. “Good Up High, Bad Nearby”
- Stratospheric Ozone: The “good” ozone layer located high up in the atmosphere that protects the Earth from the sun’s harmful Ultraviolet (UV) radiation.
- Tropospheric (Ground-Level) Ozone: The “bad” ozone located near the ground. It is a harmful air pollutant that damages lung tissue and vegetation.
2. Formation of Ground-Level Ozone
- It is a Secondary Pollutant, meaning it is not emitted directly into the air.
- It is created by chemical reactions between Oxides of Nitrogen ($NO_x$) and Volatile Organic Compounds ($VOCs$) in the presence of sunlight.
- This is why ozone levels are typically highest during hot, sunny summer days.
3. Indian NAAQS (National Ambient Air Quality Standards)
In India, the Central Pollution Control Board (CPCB) sets the standards for ambient air quality (often measured in $\mu g/m^3$ rather than ppm):
| Pollutant | Time Weighted Average | Industrial, Residential, Rural Area Limit |
|---|---|---|
| Ozone ($O_3$) | 8 hours | $100 \mu g/m^3$ (approx 0.05 ppm) |
| Ozone ($O_3$) | 1 hour | $180 \mu g/m^3$ (approx 0.09 ppm) |