Delhi’s pollution level that peaked to approximately three times the severe limit prolonged for nearly a week due to abundance of bigger dust particles in air that reduced the wind speed and prevented its clearance, according to a new government report.
The report, titled ‘Pollution Emergency of Summer in Delhi’, by the Centre-run System of Air Quality and Weather Forecasting and Research (SAFAR) explained the unusual summer-time pollution which rose to ‘severe plus’ level in the past seven days.
The reason behind the rise in pollution was due to a spike in PM10 particles (presence of particles with diameter less than 10 mm).
“Because high abundance of bigger particles of dust (much bigger than even PM10) tends to reduce the wind speed towards stagnation and opposes the flush out which aggravated the situation further and made to hang five-six days before getting flushed out,” the report said.
The PM10 level had climbed to 778 in Delhi-NCR area and 824 in Delhi on Wednesday. The PM10 between 0-100 is considered “good”, 101-250 “moderate ”, 251-350 “poor”, 351-430 very poor and 431-550 “severe”.
The PM2.5 level (presence of particles with diameter less than 2.5 mm) had also deteriorated from “very poor” to “severe” last week.
“The severity of the dust over Delhi can be traced out from the fact that the air quality has worsened and has reached at severe plus levels with 24 hour rolling average PM10 recorded remained consistently around 1000 µg/m3 during the crisis period,” the report said.
This dust cover also restricted the escape of the outgoing long-wave radiations, which is getting trapped in the atmosphere, and increased the night temperature, it said.
The report also displayed a satellite image that showed a haze or dust cloud over north-west India on June 13, the day which saw an alarming peak of over 800 in the PM10 level.
“Summer time stagnant wind condition is quite rare. However, when temperature sours and thunderstorm activity develops, it will push wind speed up and stagnation cannot last longer,” the report said.
“The moisture content in air increased on June 17 that also helped in settling down the dust. Hence, 2-way cleansing process: Flushing out due to moderate winds and settling down due to moisture content made rapid improvement in air quality,” it added.