The air quality in Delhi turned "poor" today as wind direction changed to northwesterly, increasing the contribution of smoke from stubble burning in neighbouring states to the pollution in the city.
Westerly and northwesterly winds bring dust from western regions and smoke caused by burning of crop residue in the neighbouring Punjab and Haryana to Delhi-NCR.
After fluctuating between "poor" and "very poor" for over a week, the city's air quality improved to "moderate" category on Saturday, mainly due to a squall in Delhi that dispersed pollutants and rainfall in neighbouring areas that subdued the effect of stubble burning.
However, pollution levels in Delhi shot up by around 100 points at air quality index (AQI) today, the Central Pollution Control Board data showed.
The AQI, which was 161 on Saturday, rose to 258 today by afternoon.
An AQI between 0 and 50 is considered "good", 51 and 100 "satisfactory", 101 and 200 "moderate", 201 and 300 "poor", 301 and 400 "very poor", and 401 and 500 "severe".
The Ministry of Earth Sciences' air quality forecast and research service, SAFAR, said: "The overall air quality of Delhi is back in the poor category. The surface and boundary layer wind direction is northwesterly now."
"An increase in stubble burning in the northern part of India has been observed. The circulation pattern is supportive for efficient biomass plume transport to Delhi," it said in a report.
The System of Air Quality and Weather Forecasting and Research (SAFAR) said the share of smoke from stubble burning in Delhi's PM2.5 concentration was 13 per cent today, the highest in over a week. It was zero on Saturday and is likely to increase to 19 per cent on Monday.
"A considerable increase in biomass share in pollution contribution is expected over the next two-three days. AQI is forecast to deteriorate to very poor category by Monday," SAFAR said.
Kuldeep Srivastava, a senior scientist at the India Meteorological Department (IMD), said strong surface winds clocking 15-20 kilometers per hour blew across Delhi, which is favourable for dispersion of pollutants.
"But the direction will completely change to northwesterly October 21 onwards," he said.
According to CPCB data, more than 3,000 stubble burning incidents have occurred in Punjab and Haryana since the harvesting of paddy crop started around September 25. During the corresponding period last year, the count was around 2,600.
The period between October 15 and November 15 is considered critical as maximum stubble burning takes place during this time in Punjab and adjoining states and is one of the main reasons for the alarming spike in pollution in Delhi-NCR.
Despite a ban on stubble burning in Punjab and Haryana, farmers continue to defy it amid lack of financial incentives.
"Air quality in Delhi has been good or moderate for the last 7-8 months, but they (pollution levels) have starkly gone up now... Clearly, the sudden spike in pollution is a result of smoke coming from outside," Chief Minister Arvind Kejriwal had said earlier.
The Delhi government has also tied up with Washington University and set up a centre near India Gate to collect data, with the help of which it will be able to tell the degree of air pollution due to specific sources from April next year.