Delhi’s air quality likely to improve to ‘very poor’ zone on Tuesday as sun, winds come to rescue
There was some relief for Delhi on Monday when the sun finally shone and air quality is likely to improve further if steady wind persists on Tuesday.Updated: Nov 05, 2019 06:44 IST
The sun finally shone on Monday, pulling Delhi out of its thick blanket of smog and gloom. As the clouds cleared and winds picked up pace, the haze began clearing — much to the relief of residents who were struggling to breathe in the noxious air.
Though the air quality index (AQI) was still in the ‘severe’ zone, it was a marked improvement from Sunday when Delhi turned into a gas chamber with a toxic haze and smoke.
Air quality is likely to improve to ‘very poor’ zone on Tuesday, Central Pollution Control Board (CPCB) officials said. The 24-hour AQI as per CPCB’s 4 pm bulletin on Monday was 407. It was 494 on Sunday; the worst since November 6, 2016 when it plunged to 497. Light rainfall is expected during November 6-8.
Visibility improved to 500 metres on Monday. On Sunday, it was less than 200 metres. It was also the first day of the implementation of the odd-even road rationing scheme, aimed at bringing down pollution levels.
The concentration of PM 2.5 – the most harmful aerosols in Delhi’s air -- still remained above the ‘emergency’ mark of 300ug/m3 till at least 6 pm. It started to fall marginally to 290ug/m3 around 7 pm .
According to Indian parameters, the standard levels for PM 2.5 is 60ug/m3.On Sunday, the PM 2.5 levels had shot up nine times above the safe limit to 550ug/m3 as people complaining of burning eyes, sore throat and shortness of breath.
The levels of PM 10 – relatively less harmful than PM 2.5 --- were out of emergency mark of 500ug/m3 during the day. It was recorded at 412 at 5 pm and fell to 398 by 7 pm.
The sharp spike in PM 2.5 levels was mainly because of increased formation of ‘secondary particles’, which continued to show its affect on Monday. ‘Traces’ of rain were observed at Safdarjung Observatory, representative of Delhi’s weather.
Experts say secondary particles are more toxic than particulate matter (PM). Scanty rainfall on Sunday resulted in an increase in humidity, triggering the massive pollution spike. The air trapped pollutants, which reacted with gases and resulted in an increased formation of such particles. The ratio of PM2.5 and PM10 usually remains 50:50 in the air. The levels of PM 2.5 had shot up to 75% on Sunday.
“Very light rainfall occurred in Delhi-NCR on Sunday night. Until midnight, the formation of secondary particles was on while the share of PM 2.5 was still at 75%. Even though increased wind speed started flushing out pollutants during the day, the average PM concentration was high and took time to recede, keeping the air quality in ‘severe’ zone. It is likely to improve to ‘very poor’ on Tuesday,” said G Beig, head, System of Air Quality and Weather Forecasting and Research (Safar), the pollution forecasting wing under the union ministry of earth sciences.
Beig said the boundary layer (mixing height), rose to a height of around 400 metres, which had reduced to 50 metres on Sunday.
“With the sun coming out, there was a rise in the boundary layer, which helped in lifting of dust and finer particles that had accumulated up to a height of 50 metres, too close to the ground. The lowering of the layer had aggravated the PM concentration,” said Beig.
The mixing height is like the ceiling in the air, which lowers during winter because of cold and heavy air. During summer when the sun is bright, the boundary layer is up to 2-3km, he added.
Safar’s bulletin said the share of crop residue burning from Punjab and Haryana in the overall air quality of Delhi fell to 14%, down from 46% on October 31, the season’s highest.
According to scientists in the India Meteorological Department (IMD), a clear sky, sunlight and good wind speed helped bring air quality out of the emergency zone. The average wind speed was 15-20 kmph during the day and is expected to increase further up to 25 kmph on Tuesday.
“It was after long, maybe after post-Diwali day, that the sky was clear with sunlight able to penetrate to the ground. The winds picked up pace, helping pollution levels to improve. Though in ‘severe’ zone, the toxicity in the air reduced by noon. It is expected to get better on November 5. Air quality may dip again due to cloud formation and rainfall on November 6, but pollution levels will most likely dissipate by the evening,” said Kuldeep Srivastava, head, regional weather forecasting centre, IMD.
The day was warmer with the maximum temperature recorded at 29.6 degrees Celsius against 27.5 degrees C on Sunday, which helped bring the boundary layer up, he added.