What’s causing Delhi’s air quality swings?

Over the weekend, Delhi experienced easterly and south-easterly winds, which are comparatively cleaner from the northwesterly winds. This made Sunday the city’s cleanest November-day since 2015.

On Sunday, Delhi breathed the cleanest air in November in the last three years and within 24 hours, the pollution levels had peaked to the season’s worst. To find out the reasons for this unprecedented fluctuation, HT spoke to scientists monitoring weather and pollution. Here is how they explained it:

Man-made factors such as stubble burning in Punjab and Haryana and local emissions inside Delhi compound the problem. (AFP)

The cause

The primary factor is meteorology — rapidly changing wind directions and wind speed. Man-made factors such as stubble burning in Punjab and Haryana and local emissions inside Delhi compound the problem.

Role of meteorological conditions

Over the weekend, Delhi experienced easterly and south-easterly winds, which are comparatively cleaner from the northwesterly winds. This made Sunday the city’s cleanest November-day since 2015. But soon after Sunday midnight, upper-level (500m and above) north-westerly winds started blowing, which brought in toxic particles from Punjab and Haryana. They started precipitating over Delhi and other NCR towns. The surface level winds were comparatively slower and failed to clear this pollution.

What happened on Monday

If the upper-level winds are too slow, they fail to transport pollutants. If they are too strong, the pollutants are transported beyond Delhi. But on an average, if they are between the range 15km/hr and 30 km/hr, they bring pollutants to nearby destinations like Delhi. On Monday, these upper-level winds were at around 29km/hr. The lower-level winds, which were blowing at around 5 km/hr, failed to disperse these pollutants that had been brought to Delhi.

How it cleared on Tuesday

Primarily, the wind speed picked up. As the upper winds gathered strength, they were able to carry the pollutants beyond Delhi. On Tuesday, the upper-level winds were blowing at a rate of 35km/ hr-40km/hr. The lower-level winds also picked up speed from 5 km/hr (on Monday) to 15 km/hr on Tuesday. The strong lower winds dispersed the txins, which had accumulated on Monday.

How wind speed is determined

Wind speed and direction are governed by atmospheric systems that trigger pressure fluctuations. These systems could be anything from a cyclonic circulation over as close to Rajasthan or Haryana, a western disturbance approaching from Pakistan or even a low pressure that is developing thousands of kilometres away in south central Bay of Bengal off the Sri Lankan coast.

Why did wind pick up speed?

Soon after the western disturbance that had triggered snow in the hills left northwest India, there was a huge pressure fluctuation and winds started changing direction after Sunday midnight and even picked up speed. A low pressure that is developing over the Bay of Bengal and a cyclonic circulation over Rajasthan are also adding force to the winds and they got stronger.

Is this normal?

Yes, such phenomenon is normal during this time of the year when monsoon has withdrawn and winter is approaching. If winds are strong and get clear passage, pollution remains low. If it slows down, pollution accumulates.