Pollution levels in Delhi fell slightly on Monday, moving from "severe" to "very poor" category, with an increase in wind speed which reduced the noxious haze that obscured Delhi's skies for over a week.
A day after pollution levels peaked to a three-year high (494), the city's air quality index at 4 pm on Monday read 407. It dipped further and at 8.30 pm the AQI stood at 370, which falls in the "very poor" category.
The relief came as the number of vehicles on Delhi roads came down significantly with the AAP government rolling out its car rationing scheme.
"Though there is a significant, visible change in pollution levels, the national capital's air quality index remained in the "severe" category till 4 pm. This is because the AQI reading at any given time is an average of AQIs recorded in the previous 24 hours," Central Pollution Control Board's former air lab chief Dipankar Saha said.
With winds gusting up to 20 kilometres per hour dispersing pollutants faster, the visibility level improved to 2,000 metres.
The India Meteorological Department said Cyclone Maha and a western disturbance will cause rainfall in parts of the northern plains, covering Rajasthan, Punjab, Haryana, western Uttar Pradesh and Delhi-NCR, on Wednesday and Thursday which is likely to improve the situation.
Weather experts said a significant increase in wind speed dispersed the pollutants faster. The cloud cover over Delhi and the neighbouring areas also dissipated, revealing the sun that caused the air close to the ground to rise up and flush out pollutants.
The Delhi government also shared data that showed a drastic improvement in PM10 and PM2.5 levels in the city.
According to the data, the levels of PM2.5 -- tiny particulate matter less than 2.5 microns in diameter that can enter the lungs and even the bloodstream, reduced from 562 micrograms per cubic metre at 6am to 94 micrograms per cubic meter at 6 pm.
PM10 levels reduced from 710 micrograms per cubic meter at 6 am to 216 micrograms per cubic meter at 6 pm.
Kuldeep Srivastava, a senior scientist at the IMD, said, "The two main reasons for the improvement in air quality are increased wind speed and no cloud cover. The situation will improve on Tuesday with wind speed increasing further."
Mahesh Palawat of Skymet Weather, a private forecaster, said rainfall is likely in Delhi and neighbouring areas on Wednesday and Thursday under the influence of Cyclone Maha.
"A western disturbance will also increase wind speed... Strong easterly winds due to Cyclone Bulbul will reduce the impact of smoke from stubble burning in (Haryana and Punjab)," he said.
The Ministry of Earth Sciences' air quality monitor, SAFAR, said, "High surface and boundary layer winds are expected on Tuesday also. A fresh western disturbance, scattered rainfall and change in wind direction are likely to positively influence the AQI in the region."
In the National Capital Region (NCR), Noida (391), Ghaziabad (392) and Greater Noida (392), Gurgaon (355), Faridabad (378), also recorded improvement in air quality.
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''. Above 500 falls in the ''severe plus'' category.