The Capital’s air quality worsened from ‘poor’ to ‘very poor’ on Thursday mainly due to light showers witnessed during the day.
The city’s Air Quality Index (AQI) on Thursday was 309, up from 214 (poor) on Wednesday, according to the Central Pollution Control Board (CPCB).
The air quality is expected to improve on Friday if the Capital gets more rainfall as predicted.
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“It only drizzled in Delhi on Thursday and this increased the humidity. Due to free water molecules in the air, chemical reactions took place leading to the formation of secondary aerosols, which increased the PM2.5 [deadly respirable particles] level in the air,” said an official of the government-run monitoring agency SAFAR (System of Air Quality and Weather Forecasting and Research).
“If there had been good rainfall, then it would have washed away the pollutants. But the drizzling increased the pollution,” he added.
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Meanwhile, the contribution of stubble burning in neighbouring States to the city’s air pollution was only 2% on Thursday and is expected to be 3% on Friday, said SAFAR. The contribution of stubble burning was as high as 44% earlier this season.
The air quality is expected to deteriorate on Saturday as the wind direction will change, bringing in smoke from stubble burning, it said, adding: “The effective biomass fire counts observed [in Haryana and Punjab] on November 6 were high [3,534] but the wind direction was not favourable for any stubble fire plume intrusion [into Delhi].”
Farm fires
The monitoring agency said PM2.5 level in Delhi does not increase just because the number of farm fires increases in neighbouring States, as it depends on other factors too.
On November 3, the PM2.5 level in the city hit 625 ug/m3 — the highest this season, though the number of farm fires was around 3,000.
On November 6, the PM2.5 level hovered around 100 ug/m3 even though the number of farm fires increased to 3,534, said SAFAR. The safe limit for PM2.5 is 60 ug/m3.