New Delhi, Nov 15 (IANS): The air quality of the national capital and areas around it markedly improved on Thursday from "very poor" to "poor" category as pollutants got washed away due to sporadic rains and unexpected good surface winds.

Air quality is likely to slightly improve further over the next few days and temperatures are expected to fall but dry north-westerly winds are blowing over Delhi.

The wind speeds are likely to catch up from Friday, extending till Monday, experts said.

The Air Quality Index (AQI) of Delhi on Thursday was 217 or "poor", against 339 or "very poor" on Wednesday and 409 or "severe" on Tuesday.

The AQI is likely to remain in "poor" category on Friday, but slip back to "very poor" thereafter, says the forecast from System of Air Quality and Weather Forecasting and Research (SAFAR).

"Air quality improved significantly due to sufficient rainfall earlier in the night due to the wash away effect. Surface wind speed moderately increased but pollution level declined faster than expected," said SAFAR.

The best effect of the weather was on drop in the major pollutant PM2.5 or particles with diameter less than 2.5 mm.

With average presence of PM2.5 currently 98 microgrammes per cubic meters in Delhi and 95 in entire NCR, the pollutant leading to major health impact in Delhi now stands in "moderate" category against "severe" since the beginning of November, with few exceptional days of it getting in the "very poor" category.

On Wednesday, the average presence of PM2.5 was 160 microgrammes per cubic metres which on Tuesday was 271 units.

Known to impact health severely by entering the bloodstream, the safe limit for PM2.5 is 25 units by international standards and 60 units by national standards.

"Air quality is likely to improve till Sunday as the western disturbance has passed and north-westerly wind speeds will catch up from current pace of 10-12 kmph to over 20 kmph, which will disperse the local pollution," said Mahesh Palawat, Director, private weather forecast group Skymet.