Rising moisture, low wind speed trap pollutants in Delhi's air

The National Capital Region may see a smog episode in the first week of November, predict scientists

By Shagun
Published: Wednesday 16 October 2019

Winter hasn’t even begun yet and Delhi’s air quality has already dipped to ‘very poor’ level, according to the Air Quality Index maintained by Centre-run System of Air Quality and Weather Forecasting. The AQI dropped to 309 on October 16, 2019.

What’s to blame? Answer: meteorological factors like wind speed, moisture level and boundary layer height. These become unfavourable in northern India as temperatures start to fall.

Rise in moisture content along with low wind speed are the main reasons why pollutants are trapped in Delhi’s air, thus leading to smog episodes.

“When we have high moisture, the aerosols in air start absorbing water vapours and swell. This leads to low visibility and that is how smog episodes are created,” said Sachin Ghude, scientist, Indian Institute of Tropical Meteorology (IITM), which operates SAFAR.

IITM experts analysing and forecasting the meteorological conditions believe the Capital may see a smog episode in the first week of November. This may be because of high moisture and increase in stubble burning activity in neighbouring states.

The wind direction is westerly and north westerly, which means winds carrying toxic smoke from crop residue burning are coming from Punjab and Haryana to Delhi. “Till around 10-15 days ago we had easterly monsoon winds but the direction has now changed to westerly,” said Kuldeep Srivastava, senior scientist at India Meteorological Department (IMD).

On October 15, the humidity level was at 90 per cent in the morning and 48 per cent in the evening, added Srivastava.

A western disturbance may hit Punjab, Haryana, Jammu and Kashmir, and Uttarakhand on October 18, 2019 and can bring with itself light showers, predicted the IMD. If this happens, the wind speed will get better and help disperse pollutants.

However, insufficient rain may just make matters worse since further increase in moisture will trap pollutants. The wind speed, which was around 10 kmph some days ago, has now come down to 4 kmph.

The atmospheric mixing height (where the lower atmosphere meets the upper atmosphere, allowing dispersion) is also gradually falling and combined with low wind speed, the air quality will deteriorate further.

Further deterioration of Delhi’s AQI may start from the fourth week of October, according to SAFAR extended outlook.

“Late monsoon withdrawal is not good for air quality in north India as the time progresses towards winter. During the fourth week of October, the temperature will also start to drop. The anticyclone is expected to re-strengthen only by mid-fourth week and associated clear skies and sinking motion will make the atmosphere very stable with calm surface winds,” it said.

“Both will lead to stagnant weather conditions (eg, low wind speeds, descending air, and compressed boundary layer), which favour rapid fine particulate matter formation and accumulation of pollutants,” predicted SAFAR.

Moreover, the height of the boundary layer, which carries pollutants away from the ground and mixes them with cleaner air in the upper layers of the atmosphere, also reduces as temperature starts to dip.

“There should be a balance between boundary layer height and wind speed, both of which form the ventilation index. If the height is good and wind speed is better, it helps in dispersion of pollutants,” said Ghude.

The crisis deepens if it is encountered with any additional internal (like firecrackers) or external (stubble burning) sources of emission. In a landlocked city like Delhi, this leads to rapid accumulation of pollutants and triggers high pollution events.

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