Share of pollutants from farm fires in Delhi’s air increased to an estimated 40% on Sunday, the highest so far this year.
Delhi’s air quality index (AQI) remained in the high end of the ‘very poor’ category on Sunday due to emissions from stubble burning fires in Punjab, Haryana, Uttar Pradesh and Uttarakhand, as per the Centre’s System of Air Quality and Weather Forecasting and Research (SAFAR).
“In spite of improved wind conditions, air quality has not progressed as expected due to high stubble-related intrusion and trapping of pollutants during night time due to low wintertime boundary layer height,” a bulletin from SAFAR said on Sunday.
Boundary layer height or the mixing depth is the height available in the atmosphere for the pollutants to disperse. On days with low temperatures, calm wind speed and low sunlight, this height reduces, causing higher concentration of pollutants close to the ground.
The city on Sunday recorded the season’s lowest minimum temperature of 11.4 degrees Celsius —five degrees below normal — at the IMD’s Safdarjung observatory which is representative of Delhi.
Over the past month, wind speed at night has also reduced to 3 kmph or lower until early morning hours, which is characteristic of post-monsoon period, hindering dispersion of pollutants, as per IMD officials.
However, higher wind speed is being recorded during day time over the last two days, which helps in dispersion, as per SAFAR. But the direction of this wind is from the northwest, which is favourable for transporting pollutants from stubble burning states.
“Highly favourable conditions for fire-related intrusion in Delhi is expected to continue… AQI is likely to improve only marginally (on Monday) and is now becoming dependent on stubble fire episodes. It is set to marginally deteriorate on November 3,” the SAFAR bulletin said.
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