New Delhi: Delhi’s air quality dipped on December 20 due to a drastic fall in wind speed, triggering rapid accumulation of pollutants and a grey haze, which affected visibility across the city. The Central Pollution Control Board (CPCB) recorded an air quality index (AQI) of 359, marking the return of ‘very poor’ air after eight days. The AQI had turned ‘moderate’ on December 12 and since then had remained ‘poor’. CPCB scientist Dipankar Saha said the situation is likely to improve soon as winds blowing from the north will bring down temperature resulting in precipitation of fog.
We are witnessing calm conditions but good news is that wind is likely to come from north and we can expect drop in temperature, and fog, scientist Dipankar Saha said. Due to precipitation of fog (liquid droplets turning into vapour), the level of particulates also fall.
Vivek Chattopadhyay, a researcher with the Centre for Science and Environment (CSE), said conditions are considered calm when wind speed hovers around 0.3 metres per second. “It makes the air denser. Moreover, due to drop in temperature, the boundary layer, the layer where particulates and air mix, also comes near the surface,” he said.
The concentration of PM10 was 408.8 micrograms per cubic meter (ug/m3) at 8 pm after rising through the day on December 20. PM2.5, which are finer and deadlier particulates, was recorded at 256.2 micrograms per cubic meter (ug/m3) as per the Central Control Room for Air Quality Management of the CPCB.