With winter setting in, calm wind conditions, shallow mixing layer height and low temperature have caused accumulation of pollutants. The minimum temperature was recorded at 14oC on Tuesday and low temperature slows the dispersion of pollution.
Before this, the highest AQI of the season was recorded at 298 in the “poor” category on October 17.
Though it had touched the “very poor” range that day for some hours, the 24-hour average AQI improved later in the day after rain.
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Dipankar Saha, former head at the Central Pollution Control Board’s air laboratory, said, “The wind speed has been fluctuating between 2 m/sec to 4 m/sec in Delhi. The lowest wind speed recorded in the city at 2 m/s is categorised as calm wind. The low local wind speed has led to accumulation of pollutants. The situation will remain the same for the next few days and pollutants may start dispersing after the festival.”
The System of Air Quality and Weather Forecasting And Research (SAFAR), the forecasting body under the Union ministry of earth sciences, said, “Delhi’s AQI is in the ‘very poor’ category and likely to remain so for the next two days. The impact of stubble burning is expected to be low due to unfavourable winds coming from south-west and east direction. Share of crop residue burning emissions in PM2.5 is about 6% (effective fire count 1,795). Prevailing shallow mixing layer height reduces dispersion of pollutants.” SAFAR added that during November 4-6 (Diwali and the next two days), the share of stubble burning in PM2.5 is expected to be in the range of 20-38% as the wind direction is likely to change to north-westerly.
Anumita Roychowdhury, executive director (research and advocacy), Centre for Science and Environment, said, “We have hit the first ‘very poor day’ of the season. Air quality is expected to worsen with winter conditions hardening and trapping pollution. We need a clear strategy in Delhi and across the region to prevent a full-blown smog episode.”