Sunday, Feb 05, 2023

Delhi Pollution: Not just PM, benzene, nitrogen dioxide also up

Like every year, the onset of winter has moved the topic of pollution to headlines and dinner table conversations.

Delhi Pollution: Not just PM, benzene, nitrogen dioxide also up Combustion of petrol and diesel are key contributing factors. (Express photo by Gajendra Yadav)

A light morning haze and a yellow-tinted film that rises in the evening as traffic peaks — for over two decades, both have become a defining feature of Delhi’s autumn.

“I remember a time when the colour of the sky in Delhi was no longer blue. This was before we got CNG buses. Between November and February, Delhi’s skies remind me of that time,” said Rajiv Sehgal, a former government official born in the capital in 1948, who now lives in Defence Colony.

Like every year, the onset of winter has moved the topic of pollution to headlines and dinner table conversations. But while the toll taken by particulate matter — PM 10, 2.5 and 1 — has been the focus of measures to combat pollution, another alarming aspect, experts said, has been ignored: the spike in nitrogen dioxide and benzene levels.

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Nitrogen dioxide, on reacting with sunlight, creates photochemical smog, which lends the yellowish tinge to the city haze. Its levels increase with increasing traffic, and data shows that the peaks of the gaseous compound coincide with peak traffic hours in the morning and evening. The most common source of nitrogen dioxide is combustion of petrol and diesel.

Levels of benzene, a carcinogen, have also started increasing over the past week. With a dip in temperature, concentration of benzene — even short exposure to which is harmful — increases. Its peak concentration is usually seen at night.

On October 31 at India Gate, NO2 levels in the air reached a peak of 215.8 µg/m3 — over two-and-a-half times more than the acceptable limit of 80 µg/m3. On subsequent days, peak concentration remained around twice the acceptable limit — it was the worst from 9 am-10 am and 5 pm-7 pm.


The concentration of benzene, too, was worrying. At the same station on October 31, the highest level was recorded at 11 pm at 23 µg/m3 against the acceptable limit of 5 µg/m3.

Also Read | Delhi’s air quality deteriorates to ‘hazardous’ category, PM 10 breaches 700 mark at Mandir Marg

Benzene, according to a Central Pollution Control Board study started a week before Diwali in 2016, violated the set standards on three days. “Data shows that Dhanteras is the worst as traffic movement gets maximum sluggishness. There is a sharp increase in the number of vehicles plying and duration of idling increases highly, causing more emission of benzene. Among the three stations (assessed), ITO (was) the worst affected on Dhanteras. Deepawali was comparatively better and it may be attributed that vehicular emission is more responsible and bursting of crackers may not contribute to benzene emission too much,” the study said.


According to a source apportionment study carried out by scientists at the National Environmental Engineering Research Institute for Delhi, diesel engine exhaust contributes to 26%-58% of total benzene; 14%-23% comes from vehicle exhaust; 10-18% from evaporative exhaust; and the rest from auto repair, degreasing and natural gas.

The National Green Tribunal Thursday ruled that no extension would be given to oil marketing companies to install Vapour Recovery Systems (VRS) in fuel stations across Delhi-NCR. These systems are meant to stem the release of volatile organic compounds (benzene, toluene and xylene) during transfer of fuel. All stations in Delhi-NCR will now have to have the systems in place by February 1.

The spike in pollution has also brought the odd-even road rationing scheme back on the table for consideration, with Environment Pollution (Prevention and Control) Authority chairperson Bhure Lal last week stating that restrictions on private vehicles will also be placed, if needed.

First published on: 05-11-2018 at 03:01 IST
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