Delhi odd-even policy: Key air pollutant, surface-level ozone lower than last April

The national ambient safe standard for ozone is 100 µg/m3 (around 50 ppb) for 8-hour average and 180 µg/m3 for 1 hour average, says the Centre for Science and Environment (CSE).

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In fact, the volume of ozone saw a sharp dip in the first three days of odd-even scheme, but since then, it has been rising, although the spike has not been as steep as last year around the same time.

The first spike was seen between April 17 and April 19 when it crossed over to the ‘moderate’ category from ‘good’.

After tapering off for a while, it steadily started rising, as sunlight scorched the city.

Ground-level ozone, as opposed to stratospheric ozone which shields the earth from ultra-violet rays, is a product of chemical reaction between oxides of nitrogen (NOx) and carbon monoxide (CO) among others in the presence of strong sunlight.

In one of its studies, CSE explained that ozone is a highly reactive and variable gas and that it builds up during
the day when sun shines and temperatures are high.

“Volatile hydrocarbons and other gases from vehicles and other combustion sources react photochemically in the air to create ozone. Nitrogen oxide, that largely comes from vehicles especially diesel vehicles, is to a great extent responsible for catalyzing the reaction,” the study said.

CSE Director General Sunita Narain recently said while odd-even cannot be a permanent solution to the city’s pollution woes, it can be invoked as an emergency measure.

She said during the first phase of odd-even, enforced between January 1 and 15, pollution did not come down but did not go up as much as it could have, meaning the peaks were lower than December.