Even as Diwali is just a day away, air quality in Delhi remained in 'very poor' category on Tuesday with the winds blowing from the stubble burning areas of neighbouring states.

According to the Central Pollution Control Board (CPCB) data, the overall air quality index (AQI) was recorded at 320, which falls in the 'very poor' category.

Situation saw an improvement as AQI on Monday was 434, the highest of the season, which falls in 'severe' category.

On Tuesday, 25 areas in Delhi recorded 'very poor' air quality while it was 'poor' in eight areas.

In NCR, Ghaziabad, Faridabad, Noida, Greater Noida and Gurgaon also recorded 'very poor' air quality.

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'.

Centre-run System of Air Quality Forecasting and Research (SAFAR) has predicted a significant decline in pollution till Wednesday afternoon.

"Although upper air wind direction (transport height) is still from stubble regions but the speed is sufficiently high to cross Delhi without descending and as lifetime of PM2.5 (particles in the air with a diameter of less than 2.5 micrometres) is short, it will diminish prematurely," a SAFAR official said.

The trend this year in Delhi shows that dispersion of pollutants is much faster compared to last year's winter as they have more free path, he said.

In Delhi-NCR, the PM2.5 level was recorded at 186 and PM10 was recorded at 319, the CPCB data said.

Stubble burning contributed 9 per cent to the PM2.5 levels, the SAFAR data showed.

Authorities have predicted calm winds and shallow fog on the morning after Diwali with an increase in a horizontal impact of stubble fire, which is likely to increase the pollution levels.

"Even if 50 per cent of the total load of toxic fire crackers, as compared to Diwali 2017, is added, the prevailing weather conditions will aggravate the high smoke level and make air quality to persist in severe range for at least two days on November 8 and November 9," the SAFAR said in a report.

The PM10 and PM2.5 levels are expected to reach 575 and 378 respectively a day after Diwali, recording the worst air quality of the year, it said. 

(With PTI inputs)