City breathless under shroud of smoke
Parts of Delhi received traces of rain on Saturday evening. Although it failed to wash away pollutants, it pushed up moisture levels, making the air heavy with particulate matter
NEWDELHI: Pollutants enveloped the Capital, concentrating into a blanket of smoke, dust and other fine particulate matter upto a height of 50 metres on Sunday, turning it into a gas chamber, said experts. This happened because of high moisture levels in the air and very slow winds that failed to flush out the pollutants.
“Imagine yourself in a room with high levels of pollution. What would happen if you lower the height of the ceiling? The pollution would instantly increase as it would get concentrated. That’s exactly what happened in Delhi on Sunday,” said D Saha, former head of the air quality lab of the Central Pollution Control Board (CPCB).
The trigger was Saturday’s scanty rain. Only parts of Delhi received traces of rain in the evening. Though this rain failed to clear the pollution, it pushed up moisture levels in the air, thus making the air heavy.
“The unexpected light drizzle and calm winds had an adverse effect on Delhi’s air. The air holding capacity of the air increased and the mixing layer height (the height up to which pollutants can reach when air warms up on a sunny day) dropped to season’s lowest of only around 50m. This aggravated the situation,” said G Beig, a scientist who heads System of Air Quality and Weather Forecasting And Research (Safar), the forecasting unit under the ministry of earth sciences.
Experts said that on a sunny day, the ground is warmed, causing the air above it to rise, dispersing pollutants.
The level up to which the pollutants can move up vertically is called the mixing height.
The higher the mixing height, the faster is the dissipation of pollutants. In summer, it can go up to a few thousand metres.
“The heavy doses of moisture in the air served as a platform for the pollutants to settle down on them (the water particles). They became heavier and failed to move up vertically beyond 50 metres. As a result the entire pollution load got trapped within 50 metres from the ground,” said a CPCB official.
While the day’s average AQI at 4 pm was 494, the average level of PM2.5 (particulate matter of less than 2.5 microns in diameter) had breached the emergency levels and shot up to 556μg/m³, at least nine times above the safe limits. The hourly concentration of PM2.5 in some stations such as Vivek Vihar and Anand Vihar shot up to at least 25 times above the daily safe limits.
What aggravated matters was slow wind. In the early hours of Sunday there was no wind over Delhi. The wind speed had dropped to zero kmph around 2:30 am on Sunday.
“On Saturday the wind speed picked up after 2:30 pm and by 5:30 pm it had reached 16 kmph flushing out some pollutants. This helped to bring down Saturday’s average AQI value at 4 pm to around 399 from 484 on Friday. But from Saturday evening the speed dropped and by 2:30 am on Sunday the wind speed fell to zero,” said an IMD official.
According to experts from the Indian Institute of Tropical Meteorology, Delhi needs a minimum wind speed of 10 kmph to flush out pollutants.
“This slow wind further failed to flush out the pollutants and it all got concentrated within 50 metres from the ground, thus choking the city,” said a CPCB official.
FORECAST
Experts said that the air quality is likely to remain in the severe zone on November 4 and is likely to come down to very poor levels only on November 5.
Scientists at the India Meteorological Department (IMD) said that the weather conditions had started improving from Sunday afternoon, with the maximum wind speed touching 22kmph.
“The AQI is likely to improve from Monday morning, but remain in the ‘severe’ category. By (Monday) night things will improve further, as the wind speed is expected to be high on Monday,” said V.K Soni, senior scientist at the India Meteorological Department .