Tropospheric or Ground-level Ozone

Context: Researchers have evaluated the near surface ozone in the Brahmaputra River Valley (BRV) and found relatively low concentration of Ozone over Guwahati compared to the other urban locations in India.


  • Tropospheric, or ground-level ozone, is created by chemical reactions between oxides of nitrogen (NOx) and volatile organic compounds (VOC).
  • It usually increases when pollutants emitted by cars, power plants, industrial boilers, refineries, chemical plants, and other sources chemically react in the presence of sunlight, impacting human health.

Ground-level ozone (O3)

  • Ozone is primarily a “sunny weather problem” in India, that otherwise remains highly variable during the year.
  • It is a highly reactive gas; even short-term exposure of an hour is dangerous for those with respiratory conditions and asthma.
  • That’s why an eight-hour average is considered for ozone instead of the 24-hour average for other pollutants.
  • Ozone is not directly emitted by any source but is formed by photochemical reactions between oxides of nitrogen (NOx) and other volatile organic compounds (VOCs) and gases in the air under the influence of sunlight and heat.
  • It can be curtailed only if gases from all sources are controlled.

Ozone builds up in the cleaner areas

  • Ozone pollution is the highest in areas with the lowest NO2 pollution – ozone levels build up in the greenest parts of the city where the NO2 levels are very low.
  • This is because ozone is formed when NOx, VOCs and gases react with each other under the influence of sunlight and temperature. A high NOx level can again react with ozone and mop it up.
  • The ozone that escapes to cleaner areas has no NOx to further cannibalise it – and as a result, ozone concentration builds up in these areas. 
  • This is because of chemistry. NOx consists of NO and NO2. NO eats ozone and NO2 makes ozone. Transport is responsible for more than 50% of the NOx emissions in the city. So, with a limited amount of NO present, ozone accumulates.
  • Ground-level ozone (O3), unlike other primary pollutants, is not emitted directly into the atmosphere but is a secondary pollutant produced by the reaction between nitrogen dioxide (NO2), hydrocarbons and sunlight.
  • Ozone can irritate the eyes and air passages causing breathing difficulties and may increase susceptibility to infection.
  • It is a highly reactive chemical, capable of attacking surfaces, fabrics and rubber materials. Ozone is also toxic to some crops, vegetation and trees.
  • Whereas nitrogen dioxide (NO2) participates in the formation of ozone, nitrogen oxide (NO) destroys ozone to form oxygen (O2) and nitrogen dioxide (NO2).
  • For this reason, ozone levels are not as high in urban areas (where high levels of NO are emitted from vehicles) as in rural areas.
  • As the nitrogen oxides and hydrocarbons are transported out of urban areas, the ozone-destroying NO is oxidised to NO2, which participates in ozone formation.

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