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Ground-Level Ozone Exceeding The National Standards

Ground-Level Ozone Exceeding The National Standards:

According to a new analysis by the Centre for Science and Environment (CSE), parts of the Delhi-NCR region witnessed ground-level ozone exceeding the national standards on 87 out of 92 days between March and May in 2023.

  • The analysis, based on data from the Central Pollution Control Board (CPCB), highlights the duration and geographical spread of Ozone Pollution, its impact during different seasons, and the underlying causes.


  • While ground-level ozone pollution in Delhi-NCR was lower in 2023 compared to the past five years, the duration of its exceedance has increased.
  • This phenomenon is of concern as elevated ozone levels persist even hours after sunset, contrary to expectations.
  • This summer, at the stations which reported exceedance the rolling 8-hr average stayed above standard for 4.9 hours on average, which is up from 4.6 hours observed last summer.
  • The WHO Air Quality Guidelines for ambient (outdoor) ozone is 100 μg/m3 (~50 ppb) measured as 8-hr maximum moving average within a day.
  • Ozone pollution is not limited to specific seasons. Even during winter months, when cold and foggy conditions inhibit ground level ozone formation, Delhi-NCR experienced excess ozone levels on multiple days in January 2023.
  • Ozone levels exceeded the standard at multiple stations on 26 days in January 2023.
  • New Delhi and South Delhi areas were the most affected by ground-level ozone pollution.

Ground-Level Ozone:

  • Ground-level ozone, also known as Tropospheric ozone, is a colorless and highly irritating gas that forms near the Earth’s surface, typically within two miles above the ground.
  • Ground-level ozone is not directly emitted from any specific source.
  • It is formed through complex interactions between Nitrogen Oxides (NOx), Volatile Organic Compounds (VOCs), and carbon monoxide emitted from vehicles, power plants, factories, and other combustion sources.
  • These compounds undergo cyclic reactions in the presence of sunlight to generate ground-level ozone.