According to a recent report, Delhi-NCR topped a list of 10 cities across the country that were found to have high concentrations of ground-level ozone (O3).
The study was released by the Centre for Science and Environment (CSE). CSE said that although ground-level ozone should ideally become negligible in the night air, the 10 metropolitan areas have been witnessing a rare phenomenon, where ozone levels remain elevated for hours after sunset.
Ozone drifts and gathers in areas where comparatively lesser amount of gaseous pollutants are present to further mop it up. Thus, relatively cleaner areas, including urban peripheries, can have comparatively higher ozone concentrations.
While ozone is created in polluted areas with nitrogen oxide being the catalyst, it also gets mopped up in high NO2 areas as it further reacts. But the ozone that escapes to cleaner areas with less NO2 builds up faster, as unavailability of NO2 hampers its dissipation.
Ground level 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) in the presence of sunlight.
This happens when pollutants emitted by cars, power plants, industrial boilers, refineries, chemical plants, and other sources chemically react in the presence of sunlight.
Once ozone is formed, it poses severe health risks, particularly for those with respiratory conditions, children and the elderly.
Ground-level ozone is not just a seasonal problem. While summer months see spikes, ozone pollution persists throughout the year.
Night-time exceedances are particularly troubling, with cities like Mumbai-MMR, Delhi-NCR and Pune recording high ozone levels well into the night.
Ozone can also be transported long distances by wind, so even rural areas can experience high ozone levels.
According to CPCB, the one-hour standard for ozone is 180µg/m3, and the eight-hourly standard is 100µg/m3.
