Delhi breathed easier on Wednesday as pollution levels dropped further due to clearer skies and moderate wind speed though weather experts feared a cloud cover at night and light drizzle could lead to deterioration of air quality.
The government's air quality monitoring and forecasting service, SAFAR, said though Punjab recorded the season's highest farm fire count of 6,668 on Tuesday, its impact on Delhi's air quality will be negligible due to a change in the wind direction.
According to senior IMD scientist Kuldeep Srivastava, clear sky or absence of clouds allows sun rays to reach the ground, warming up the air close to the ground which lifts up and clears the pollutants.
"However, a cloud cover, very light rains, clam winds are expected at night, which may accelerate accumulation of pollutants and air quality may deteriorate slightly," the IMD official said.
The situation will improve again on Thursday due to strong winds and rains. Delhi is expected to breathe comparatively cleaner air till November 10, he said.
At 1.30 pm, the city's overall air quality index read 216, which falls in the 'poor' category. At many places, the pollution levels dropped to 'moderate' category.
The levels of PM2.5 -- tiny particulate matter less than 2.5 microns in diameter that can enter into the lungs and event the bloodstream -- dropped to 107 micrograms per cubic metre (ug/m3) in the Delhi-NCR region.
The PM10 concentration was 226 ug/m3 at 1.30 pm.
The safe levels of PM2.5 and PM10 are 60 and 100 ug/m3, respectively.
SAFAR said, "Delhi's overall AQI has improved significantly, more rapidly than predicted, and further improvement during sunlight hours is expected."
It said a change in the wind direction led to a very low biomass plume intrusion despite the farm fire count remaining very high on Tuesday.
SAFAR (System of Air Quality and Weather Forecasting) said the share of smoke from stubble burning in Delhi's PM2.5 pollution will be just 3 per cent on Wednesday and 2 per cent on Thursday.
"Western disturbances have brought cleaner air to the region. Isolated thundershowers are expected for the next two days and wind direction is likely to be southeasterly on November 7. So, no biomass intrusion," it said.
The Supreme Court-mandated Environment Pollution (Prevention and Control) Authority had on Monday extended the restrictions on hot mix plants, stone crushers and dirty fuel-based industries in Delhi-NCR till the morning of November 8 expecting a repeat of Sunday's episode.
The apex court had also banned construction activities in Delhi-NCR till further orders.
On Sunday, pollution level in Delhi peaked to a three-year high as high humidity due to light rains on Saturday led to formation of more potent secondary particles.
Secondary particles are product 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.
Examples of secondary particles include sulphates, nitrates, ozone and organic aerosols.
An AQI between 0-50 is considered 'good', 51-100 'satisfactory', 101-200 'moderate', 201-300 'poor', 301-400 'very poor' and 401-500 'severe'.
An AQI above 500 falls in the severe plus category.