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Insights into Editorial: The much-neglected pollutant

Insights into Editorial: The much-neglected pollutant


As a blanket of pollutants descends on large parts of north India, it is important to remember that killer pollutants go beyond particulate matter. A much-neglected pollutant in discussions about pollution is surface ozone (O3). A recent study shows that O3 levels will continue to rise drastically, particularly in north India.

An analysis report, “Impact of regional climate change and future emission scenarios on surface O3 and PM2.5 over India”, on prevalence of O3 was recently published.

Good Ozone

Called stratospheric ozone, good ozone occurs naturally in the upper atmosphere, where it forms a protective layer that shields us from the sun’s harmful ultraviolet rays. This beneficial ozone has been partially destroyed by manmade chemicals, causing what is sometimes called a “hole in the ozone.” 

Bad Ozone

Tropospheric, or ground level ozone, is not emitted directly into the air, but is created by chemical reactions between oxides of nitrogen (NOx) and volatile organic compounds (VOC). This happens when pollutants emitted by cars, power plants, industrial boilers, refineries, chemical plants, and other sources chemically react in the presence of sunlight. Ozone at ground level is a harmful air pollutant, because of its effects on people and the environment, and it is the main ingredient in “smog.”

Ozone can also be transported long distances by wind, so even rural areas can experience high ozone levels. 

Ground-Level Ozone

Ozone (O3) is a colourless, reactive oxidant gas that is a major constituent of atmospheric smog. Many factors impact ground-level ozone development, including temperature, wind speed and direction, time of day, and driving patterns.

  • Ground level ozone is formed by the reaction of NOx and VOCs under the influence of sunlight hundreds of kilometers from the source of emissions.
  • Ozone concentrations are influenced by the intensity of solar radiation, the absolute concentrations of NOx and VOCs, and the ratio of NOx and VOCs.
  • Peak ground-level ozone concentrations are measured in the afternoon.
  • Both natural and anthropogenic sources contribute to the emission of ground-level ozone precursors, and the composition of emissions sources may show large variations across locations.
  • Anaerobic biological processes, lightning, and volcanic activity are the main natural contributors to atmospheric NOx, occasionally accounting for as much as 90% of all NOx emissions.
  • Motor vehicles are the main anthropogenic sources of ground-level ozone precursors. Other anthropogenic sources of VOCs include emissions from the chemical and petroleum industries and from organic solvents in small stationary sources such as dry cleaners.

Impacts of Ozone

  • The main health concern of exposure to ambient ground-level ozone is its effect on the respiratory system, especially on lung function.
  • Elevated ground-level ozone exposures affect agricultural crops and trees, especially slow growing crops and long-lived trees.
  • Ozone damages the leaves and needles of sensitive plants, causing visible alterations such as defoliation and change of leaf colour.

Ground level ozone’s Toll in India

Ozone pollution in India is damaging millions of tons of the country’s major crops.

  • Despite air quality standards passed in the 1980s that were designed to curb industrial and vehicle emissions, pollution remains a major challenge for India.
  • Long-term measurements of surface ozone across India—measured on the ground or by aircraft—are not available, making it difficult to get a clear picture of how levels of the pollutant have changed.
  • But satellite-based studies show ozone has increased in the last two decades.
  • The Centre for Science and Environment (CSE) noted that due to high pollution levels and growing heat stress owing to climate change, the ozone level is frequently exceeding the standards and rising to poor and very poor levels — as classified by the National Air Quality Index (NAQI).
  • Delhi-NCR is in the grip of multi-pollutant crisis. Even before the health risk from particulate matter could be addressed, deadly ozone has raised its ugly head in Delhi and NCR.

Way Forward

Climate change will adversely impact soil moisture, rains, or vegetation density, which will further impact the absorption of ozone. The scenario is scarier when tabulating man-made sources such as vehicles, power plants, or any machinery that uses fossil fuels, where the O3 component will increase by up to 45% in parts of north India.

The increase, albeit in varying degrees, will be seen in much of the subcontinent save for a few spots close to the Western Ghats — perhaps a lesson on how to contain pollutants — where O3 will actually decrease by 4%.

Surface ozone not only damages health but also destroys crops. In a country where food insecurity is high, this should be reason enough to act.