delhi

Updated: Nov 02, 2019 16:54 IST

Delhi has become prey to a vicious circle of cloud cover, diffused sunlight and smoke coming from the stubble-burning regions of Punjab and Haryana — and this is what has pushed up pollution levels in the national capital into the emergency zone, experts say.

Delhi’s air quality started deteriorating from Tuesday afternoon, when concentration of pollutants in the air breached the severe category. The spike continued and by Thursday late night, the city encountered the worst form of pollution, categorised as ‘severe+’ level. Scientists and meteorologists said it started with a cloud cover, which initially helped to cut off the sun’s rays from reaching Delhi on Tuesday and Wednesday.

According to the India Meteorological Department, the day temperature dropped to 29 degrees Celsius on October 29 (Tuesday), which was at least three degrees lower than what it was on November 1 (Friday). “There was significant cloud cover on Tuesday and Wednesday and it helped to bring down the day temperature,” said Kuldeep Srivastava, a scientist with the India Meteorological Department’s regional forecasting office in New Delhi.

With diffused sunlight on Tuesday and Wednesday, because of the cloud cover in the city sky, the ground failed to heat properly.“This in turn stopped the vertical movement of the air, which could have helped in dispersing the post-Diwali pollution to a large extent. As soon as this mixing height got disturbed, pollution started accumulating in the air. Smoke from stubble burning was an added input,” said MP George, a scientist with DPCC.

What made matters worse was the wind speed, which has slowed down because of an approaching western disturbance and smoke from the stubble-burning regions of Punjab and Haryana.

At 5:30 pm, the IMD said wind speed in Delhi was almost zero. The smoke layer further helped to cut down the sun’s rays.

Aerosols in the air and the clouds seeded by them have been known to reflect about a quarter of the Sun’s energy back to space. Different aerosols scatter or absorb sunlight to varying degrees, depending on their physical properties. “As pollutants started accumulating in Delhi’s air, it formed a haze and further helped to cut off the sunlight. The ground failed to heat up again. With smoke from stubble-burning regions increasing constantly and Delhi emitting from its own sources, more pollutants were being added up to the already foul air. This formed a vicious circle. The more the pollution, the lesser the sun’s ray and lesser is the heat. The lesser the heat the more would be the pollution as pollutants gets easily trapped.,” said a senior official of the CPCB.

Safar, the union government’s pollution forecasting wing, said contribution from stubble burning to Delhi’s air has been constantly increasing since Tuesday. It shot up from 15% to 46% over the last four days. Even though Safar evaluated that 46% of Delhi’s PM2.5 load was coming from stubble burning, the contribution of other local sources within the city could not be ascertained as there was no real-time source apportionment.

Since Tuesday, while the surface level winds, which help to disperse pollutants, were low, the upper level winds, which help to transport pollutants, were strong. This resulted in increased stubble burning contribution.

So what’s the solution now? How can Delhi come out of this circle that has turned the city into a gas chamber forcing authorities to declare it as a health emergency? Meteorologists say – sunshine and strong winds.

“A western disturbance is approaching and expected to trigger some strong surface winds, which would help to flush out pollutants. Significant improvement is expected from Sunday ,” said a senior official of Safar.