Delhi remained blanketed by a thick smog for the fourth consecutive day on Friday as unfavourable weather hampered dispersion of pollutants, with improvement in air quality expected only by Sunday.
The air quality index in the national capital stood at 463 at 4 pm, with Dwarka Sector 8 being the most-polluted area with an AQI of 495. Most of the air quality monitoring stations recorded an AQI above 450.
Neighbouring Faridabad (450), Ghaziabad (475), Greater Noida (445), Gurgaon (461) and Noida (474) too continued to reel under severe air quality.
An AQI between 201 and 300 is considered 'poor', 301-400 'very poor' and 401-500 'severe'.
The government's air quality monitoring and forecasting service SAFAR said drizzle on Thursday night proved counter-productive and led to formation of secondary particles -- a situation Delhi witnessed in the first week of November.
The situation may worsen due to a drizzle on Friday night, it said.
Secondary particles, such as sulphates, nitrates, ozone and organic aerosols, are products of complicated atmospheric reactions between primary particles, such as particulate matter, sulphur dioxide and nitrogen dioxide directly emitted by stubble burning and vehicles, in the presence of other factors such as sunlight and moisture.
A cloud cover and slow wind speed due to a western disturbance have been unconducive for dispersion of pollutants, experts said.
The System of Air Quality and Weather Forecasting and Research (SAFAR) said only two farm fires could be detected on November 14 and the count could be more as cloudy conditions affect the satellites' capability to detect stubble burning. It said the share of farm fires in Delhi's pollution is expected to be only 10 per cent.
Gufran Beig, project director, SAFAR, said though the impact of stubble burning is expected to reduce on November 16, high humidity due to isolated light rains may worsen the situation.
"Drizzle (last night) delayed the recovery and hence, improvement in ar quality is now expected only by November 17," SAFAR said.
The Delhi government, meanwhile, said on Friday that a call on extending the odd-even road rationing scheme will be taken on Monday morning as air quality is expected to improve over the next two-three days.