Lowering temperatures and reduction in wind speed are likely to push Delhi’s air quality into the ‘poor’ category by Sunday, according to a forecast by the Centre’s System of Air Quality and Weather Forecasting and Research (SAFAR).
The 24-hour average air quality index (AQI) of the city has been rising for over a week, touching 180 in the ‘moderate’ range on Friday, as per Central Pollution Control Board (CPCB) data.
The SAFAR’s forecast shows the lead pollutant in the city’s air as of Friday was coarse dust particles, or particulate matter of 10 micrometres (PM10).
“Delhi AQI to marginally deteriorate and reach the higher end of the moderate category tomorrow. Further deterioration to the higher end of moderate or lower end of the ‘poor’ category is likely by October 4,” the forecast added.
The average AQI on September 23 was 76 in the ‘satisfactory’ range, and this was also when day and night time temperatures had peaked in the city after a steady rise over the last month. The maximum temperature on September 23 was 37.1 degrees Celsius and minimum was 27.6 degrees Celsius.
Mercury began falling after this date and the night time temperature recorded on Friday was 20.6 degrees and that during day was 36.4 degrees. It is forecast to fall further by a degree in the coming week.
Meanwhile, concentration of particulate matter pollutants — fine particles of 2.5 and 10 micrometres (PM2.5, PM10) that remain suspended in the air and enter the bloodstream through breathing — has also been rising steadily.
CPCB data shows PM10 level on September 1 was around 40 micrograms per cubic metre air (µg/m3), but it gradually rose to 196.3 µg/m3 as of 5 pm on Friday.
PM2.5 — caused from combustion activities including biomass and waste burning, vehicular emissions — also rose from below 20 µg/m3 on September 1 to 68.6 µg/m3 on Friday.
On days with strong wind speed, pollutants in the air are easily dispersed. However, after the withdrawal of monsoon last week, wind speeds have reduced in the city to 10-14 kmph and also dropping further to being ‘calm’ during parts of the day.
For the next five days, calm wind speed is forecast in the city in the morning hours until October 7, as per the India Meteorological Department (IMD).
A reason for accumulation of pollutants in the air is the reduction of the atmospheric mixing layer, or the vertical height in which pollutants are suspended.
On cloudy days, with lower temperatures and calm wind speed, this height is reduced, thereby leaving pollutants suspended closer to the surface. Days with good amount of sunlight, high wind speed and high temperatures disperse the pollutants.
As per IMD, wind direction over the city is forecast to remain northwesterly till October 7, which is favourable for transport of pollutants from stubble burning states Punjab and Haryana.
“However, the farm fires are low or marginal at present. Their impact would be felt after the second week of October,” as per a SAFAR forecast.
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