Delhi's air quality went south and once again entered the 'emergency' category on Saturday evening, dashing hopes of recovery generated during the morning hours when level of pollutants showed a steady drop.
The dramatic reversal in the situation, hours after the government announced that levels of ultrafine particulates PM2.5 and PM10 had seen a reduction, caught people and weather scientists unaware.
Centre-run air monitoring agency SAFAR's project director Gufran Beig said that the sharp drop of the boundary layer where pollutants remained trapped for being unable to escape into the upper layer of the atmosphere.
"There is no likelihood of last week's repeat as there is no fresh influx of pollutants from external sources such as stubble burning or dust storm in the larger region. But recovery will get delayed by at least one more day," Beig said.
The hourly graph of the Central Control Room for Air Quality Management run by the Central Pollution Control Board also captured the sudden change in circumstances.
The CPCB's air quality index for the day (based on pollution levels till 5 pm) had a score of 403, as against Friday's 468.
The concentration of PM2.5 and PM10 hovered around 490 and 290 micrograms per cubic metre during the morning hours, marginally below the emergency limit of 500 and 300.
But by 6 pm the readings had changed to 522 and 332. In fact, the gains made started diminishing from around 2 pm itself.
Beig said the drop in both the minimum and maximum temperatures led to the coming down of the boundary layer from around 1600 metres from the surface at 11 am to 50 metres at 5 pm.
"Otherwise our monitors recorded steady improvement over the last one day. The air had turned very poor from severe. In fact, the measures implemented under the Graded Response Action Plan yielded good results. Pollution levels came down by 15-20 per cent due to these measures alone," he said.
A 'very poor' AQI comes with the warning that people may develop respiratory illness on prolonged exposure while exposure to 'severe' air affects healthy people and seriously impacts those with existing respiratory or cardiovascular diseases.
IMAGE: People commute during the foggy day in New Delhi. Photograph: Atul Yadav/PTI Photo