Delhi’s air remains in ‘severe’ zone

Nov 18, 2023 05:16 AM IST

The factors causing the crisis remained more or less the same: calm winds, low temperatures and smoke from farm fires

The air quality index (AQI) in the Capital remained in the severe category, at 405, on Friday making it the 12th of the 17 days this month when people in city have had to breathe in pollution at severe or near-severe (AQI 390-plus) levels. However, the pollution levels started to ease in the evening due to favourable weather conditions.

Connaught Place engulfed in dense smog on Friday morning. (Raj K Raj//HT photo)
Connaught Place engulfed in dense smog on Friday morning. (Raj K Raj//HT photo)

The factors causing the crisis remained more or less the same: calm winds, low temperatures and smoke from farm fires, which combined to form a blanket of haze that appeared to thin late on Friday afternoon, when the wind direction changed and began to remove some of the effects from farm fires.

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“Visibility was down to around 300 metres at Safdarjung at 7am and around 500 metres at Palam, but we saw a significant improvement during the day, when the wind direction changed. The wind direction, which had been northwesterly till 12 pm, then became easterly and by 4 pm, visibility had improved to around 3,000 metres, the highest we have seen since the spell of rain seen last week,” said Kuldeep Srivastava, a scientist at IMD.

But these conditions are unlikely to help the air quality to improve to levels under “very poor” (the fourth highest classification on a five-point scale) since winds are expected to remain weak until Tuesday. Calm conditions limit the dispersal of pollution, which accumulate more quickly and linger on longer in cold weather.

The Met department has forecast easterly winds to remain dominant over the next three days, but said significant relief is only expected from November 21, when strong winds of up to 15 km/h are expected.

The AQI 405 on Friday, according to the Central Pollution Control Board (CPCB)’s 4pm bulletin, was a marginal improvement from Thursday’s 4pm reading of 419 (severe). By 9pm, this improved to 379 (very poor).

The improvement, however, follows what may be the worst November that Capital has seen in terms of air quality yet. Data analysed by HT showed that if AQI 390 is taken as a cut-off for a near-severe threshold, the city spent 12 of the first 17 days of the month with such toxic air quality.

In contrast, the highest number of AQI 390-plus days prior to this November was in 2021 when the month had nine such days.

Srivastava added that next week’s possible improvement in wind speeds will be due to the effects of a western disturbance. “A western disturbance will begin to influence Jammu and Kashmir from November 19, with strong winds expected in Delhi-NCR on November 21 and 22,” Srivastava said.

As per Delhi government’s real-time source apportionment on Friday, vehicles accounted for 44% of the city’s PM 2.5 pollution, followed by 22% from biomass burning, or farm fires. Secondary inorganic aerosols — particles formed in the air as a result of gases reacting with each other from combustion sources —contributed by 20% to PM 2.5 formation, it said.

Biomass burning had contributed 28% on Thursday and the contribution from vehicles was 26%.

The impact of stubble burning is expected to reduce over Delhi in the next three days now, firstly due to the change in wind direction, as well as a gradual drop in farm fire numbers. Data from the Indian Agricultural Research Institute (IARI) showed Punjab recorded 1,150 fires on Friday and Haryana 45. The two states had 1,271 and 46 fires respectively on Thursday and 2,544 and 62 fires respectively on Wednesday.

Experts said even with stubble contribution reducing marginally, Delhi still had its local sources to deal with, which will keep background emissions high. “A change in wind direction can only lead to slight improvement, as vehicles and other sources will continue to emit pollutants. Depending on the wind direction, pollution will also come from neighbouring National Capital Region (NCR),” said Sunil Dahiya, analyst at the Centre for Research on Energy and Clean Air (CREA), adding that Delhi either needed rain or strong winds to bring about a significant change in AQI. “It should still be in the upper end of the very poor category, until we see strong winds.”

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