Delhi's Air Quality Improves Due to High Wind Speed, But Still 'Poor' as Farm Fires Continue to Rage

Pollution levels in the national capital dipped on Monday with high wind speed aiding dispersion of pollutants, even as farm fires continued to rage in neighbouring states. The city recorded an air quality index (AQI) of 286, which falls in the ”poor” category.

The 24-hour average AQI was 364 on Sunday, with stubble burning contributing 40 percent to Delhi’s pollution and negating the effect of better ventilation. It was 367 on Saturday, 374 on Friday, 395 on Thursday, 297 on Wednesday, 312 on Tuesday, and 353 on Monday.

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 Air Quality Early Warning System for Delhi said a significantly large number of fires were observed over Punjab, Haryana, Uttar Pradesh and Madhya Pradesh on Sunday which is likely to impact the air quality in Delhi-NCR and northwest India.

Higher wind speed and better ventilation index will be favourable for dispersion of pollutants on Monday, it said. According to the Ministry of Earth Sciences’ air quality monitor, SAFAR, the share of stubble burning in Delhi’s pollution rose to 40 percent on Sunday, the maximum so far this season.

It was 32 percent on Saturday, 19 percent on Friday and 36 percent on Thursday, the second highest this season so far. Last year, the stubble contribution to Delhi’s pollution had peaked to 44 percent on November 1, according to SAFAR data.

NASA’s satellite imagery showed a large, dense cluster of fire dots covering Punjab and parts of Haryana and Uttar Pradesh. According to the India Meteorological Department, the predominant wind direction was westerly-northwesterly and the maximum wind speed was 18 kilometers per hour on Monday. The city recorded a minimum temperature of 10.8 degrees Celsius, the lowest in the season so far.

Calm winds and low temperatures trap pollutants close to the ground, while favourable wind speed helps in their dispersion. According to the central government’s Air Quality Early Warning System for Delhi, the city’s ventilation index a product of mixing depth and average wind speed was around 12,000 meter square per second on Monday favourable for dispersion of pollutants.

Mixing depth is the vertical height in which pollutants are suspended in the air. It reduces on cold days with calm wind speed. A ventilation index lower than 6,000 sqm/second, with the average wind speed less than 10 kmph, is unfavourable for dispersal of pollutants.

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