Toxic winter so far for Kolkata, 3 other Bengal cities: CSE

Action in key sectors of pollution will have to be scaled up across region to further bend annual air pollution curve, says non-profit

By DTE Staff
Published: Tuesday 08 December 2020
Toxic winter so far for Kolkata, three other Bengal cities: CSE. Photo: Swagata Dey / CSE
A smoggy day on the Hooghly in Kolkata. Photo: Swagata Dey / CSE A smoggy day on the Hooghly in Kolkata. Photo: Swagata Dey / CSE

Winter has been toxic for four major West Bengal cities, with unlocking of the economy reducing gains made during the novel coronavirus disease lockdown, an analysis by Centre for Science and Environment (CSE) said December 8, 2020.

Researchers from the Delhi-based think tank selected four cities of the state — Kolkata, Howrah, Asansol and Siliguri, as real-time data was available for these cities, a statement from CSE said.

They collected publicly available granular real-time data (15-minute averages) from the Central Pollution Control Board’s official online portal Central Control Room for Air Quality Management.

They analysed data recorded by seven air quality monitoring stations in Kolkata, three stations in Howrah and one station each in Asansol and Siliguri under the Continuous Ambient Air Quality Monitoring System.

Weather data was sourced from the Dum Dum weather station of the India Meteorological Department.

The results were pretty shocking.

The overall Particulate Matter (PM) 2.5 average this year (until the first week of December) was predictably lower compared to the previous year, largely because of the unprecedented economic disruption during the summer lockdown and monsoon.

But the reopening of the economy, coinciding with the onset of the winter trapping pollution, forced PM2.5 levels to spiral upward starting November, hitting a record high in early December.

From the respective cleanest week, the weekly average of PM2.5 in Kolkata rose 14 times, in Howrah nine times, Asansol seven times and in Siliguri 11 times.

Second, winter air turned dramatically toxic as the share of PM2.5 in overall PM10 increased significantly.

“The share of tinier and finer particles in the overall coarser PM10 concentration determines the toxicity of air,” Avikal Somvanshi, programme manager in CSE’s Urban Lab team of Sustainable Cities programme, said.

“When the overall share of tinier PM2.5 in the overall coarser PM10 is higher, the air is more toxic as the tiny particles penetrate deep inside the lungs and cut through the blood barrier, increasing our health risk,” he added.

The CSE analysis pointed out that during the lockdown, when the overall suspended coarser particles had settled down reducing the PM10 levels, the PM2.5 level had also come down.

But its share was 50 per cent higher than what was usually noted during summer. With the onset of winter, the overall level of both went up; the percentage share of PM2.5 in the overall PM10 also increased to over 60 per cent during the high pollution episode following Diwali in mid-November and had remained high at over 50 per cent since then.

The share of PM2.5 in PM10 is generally highest on Diwali (it had reached over 80 per cent in 2019), but due to lesser bursting of firecrackers this year, it remained in mid 50s.

Third, in Kolkata, November 2020 was cleaner but the first week of December had been more polluted.

The rolling weekly average rose to ‘very poor’ or 120 microgramme per cubic metre (μg / m3) on December 4, but it never did so last year. In fact, the Rabindra Bharati University station’s 24-hour average slipped into the ‘severe’ category on December 4.

An Air Quality Index of 0-50 is considered ‘good’, 51-100 ‘satisfactory’, 101-200 ‘moderate’, 201-300 ‘poor’, 301-400 ‘very poor’, and 401-500 ‘severe’. Above 500 is the ‘severe-plus or emergency’ category.

This is quite different compared to the previous winter even when there was no significant change in the city’s average temperature.

Fourth, there were three days this winter in Howrah and one day in Kolkata when the PM2.5 levels were higher than Delhi as Delhi’s pollution subsided during those days.

CSE compared Kolkata’s winter pollution so far (until December 6) with that of Delhi and found both cities to have a similar pollution build-up pattern — but Kolkata and Howrah had considerably lower absolute concentrations.

As the weather gets colder and more adverse, the average pollution in Kolkata continues to climb when Delhi’s pollution levels seem to have plateaued. 

This year on November 16, 17 and 18, Kolkata and Howrah had higher PM2.5 levels than Delhi. On those days, winter peak pollution in Delhi had got lower.

“How the pollution level will play out during the rest of the winter remains to be seen. But it is clear that the region cannot afford to lose the wins already made and at the same time, raise the level of ambition to drive action across all key sectors of pollution and the entire region,” Anumita Roychowdhury, CSE executive director-research and advocacy, was quoted as saying in the statement.

“Enforce power plant standards in the larger region, provide clean fuels to the industry, scale up public transport and vehicle restraint measures and manage waste to have a zero waste and zero landfill strategy. But the peak winter pollution also shows that cities need graded response action plan for emergency response during smog episodes,” she added. 

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