High winter air pollution in Kolkata-Howrah, says latest assessment by CSE

Despite lower PM2.5 levels due to prolonged lockdown this year, pollution has gone up with the onset of winter and unlocking of the economy: CSE 

  • In Kolkata, weekly average level of PM2.5 jumped 13 times from the cleanest week of August to most polluted week of December. In Howrah, it rose 11 times
  • Winter air turned dramatically toxic as the share of PM2.5 in overall PM10 increased significantly
  • Kolkata has experienced its worst week and peak in last two years so far with PM2.5 level exceeding "severe" threshold at the city station at Rabindra Bharati University on December 4. Rest of the city remained in very poor category
  • 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
  • This calls for scale and speed of action on transport, clean fuels in industry, clean power plants, waste management and biomass burning across the region, says CSE

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New Delhi, December 8, 2020: New analysis of winter air pollution (till the first week of December this year), done by Centre for Science and Environment (CSE), shows how clean air gains of the lockdown and monsoon period were lost in West Bengal with the reopening of the economy and hostile winter weather. While this was expected, the analysis of real-time data from monitoring stations across Kolkata-Howrah as well as from Asansol and Siliguri show the changing patterns in winter pollution this year. 

Even though the overall average level of PM2.5 for the 11 months in 2020 is considerably lower than the previous year, the levels in winter have spiked to “very poor” category in the twin cities of Kolkata and Howrah. Says Anumita Roychowdhury, CSE executive director-research and advocacy, says: “This is a typical and predictable winter trend when continuous emissions from local sources including vehicles, industry, construction, and episodic pollution from biomass burning get trapped due to meteorological changes. But this year, there is higher pollution peak and substantial jump after the reopening of the economy.”  

Avikal Somvanshi, programme manager in CSE’s Urban Lab team of Sustainable Cities programme, says that this data analysis points to the fact that the “Reforms and action in key sectors of pollution – vehicles, industry, power plants and waste management – will have to be scaled up at speed across the region to further bend the annual air pollution curve.”  

Data and analysis: Somvanshi says the CSE analysis is based on publicly available granular real-time data (15-minute averages) from the Central Pollution Control Board’s (CPCB) official online portal Central Control Room for Air Quality Management. 

He adds: “Four cities – Kolkata, Howrah, Asansol and Siliguri -- have been selected for this analysis because real-time data is available for these cities. CSE has 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 (CAAQMS). Weather data has been sourced from the Dum Dum weather station of India Meteorological Department (IMD).” 

Key highlights of CSE’s analysis 

Average level of PM2.5 has been lower this year due to the lockdown, but this could not prevent the winter spike.
The overall PM2.5 average this year (until the first week of December) has been predictably lower compared to the previous year, largely because of the unprecedented economic disruption during the summer lockdown and monsoon. But reopening of the economy, coinciding with the onset of the winter trapping pollution, has forced PM2.5 levels 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 13 times, in Howrah eleven times, Asansol seven times and in Siliguri 11 times. Says Roychowdhury: “The transient change of the lockdown phases could not be sustained without the systemic changes needed to control pollution from vehicles, industry, power plants, and waste.” 

Air quality gets more toxic with the onset of winter -- share of tinier PM2.5 in PM10 increases.
Somvanshi says that “the share of tinier and finer particles in the overall coarser PM10 concentration determines the toxicity of air. 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.” The CSE analysis points 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 is usually noted during summer. With the onset of winter, the overall level of both have gone up; the percentage share of PM2.5 in the overall PM10 has also increased to over 60 per cent during the high pollution episode following Diwali in mid-November, and has 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. 

Cleaner Diwali in Kolkata this year.
This year, Diwali pollution from firecrackers was considerably lower. The average PM2.5 level on Diwali day in Kolkata was 89 μg/m3 -- down from 123 μg/m3 recorded in 2019, even though Diwali happened much later in the winter. This year, pollution nevertheless didn’t dissipate and the daily PM2.5 level increased to 110 μg/m3 on November 17, 2020 due to meteorological factors. Also, unlike in previous years, there was about 80 per cent lesser change in hourly PM2.5 concentration between the afternoon and the night of Diwali (this is mostly caused due to firecracker bursting). The change in hourly PM2.5 concentration between afternoon and night of 2020 Diwali was 136 μg/m3, down from 642 μg/m3 in 2019 and 751 μg/m3 in 2018. 

In Kolkata, November 2020 was cleaner but the first week of December has been more polluted.
The rolling weekly average rose to “very poor” or 120 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. This is quite different compared to the previous winter even when there was no significant change in the city’s average temperature. 

In Kolkata, the number of days with PM2.5 concentration in the ‘good’ and ‘satisfactory’ categories have been higher this winter, but ‘very poor’ days have also increased a little.
There have been 16 days of “good” air days this winter (October 1-December 6) compared to just six recorded last year. But the “very poor” days have climbed from four days to six this year. 

In Kolkata, the cyclical ups and down of pollution this winter is more volatile, showing quicker rise and fall than in previous winter.
This could also be a reflection of changes in local pollution patterns and overall lower pollution load in the air-shed while aided by meteorology. 

Even with comparatively cleaner air during this year, four cities have recorded daily spikes even higher than those observed in 2019.
CSE has compared the annual averages and peak 24-hour averages in these cities of West Bengal between 2019 and 2020. This shows that these cities -- even with much lower annual average levels of PM2.5 -- have experienced almost same or higher maximum daily and weekly levels during winter when the entire region got air-locked. 

Kolkata’s air is generally cleaner than Delhi’s, but comes close and even exceeds on a few days.
CSE has 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 have 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 seems 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. 

Need deep cuts
It may be noted that all the four cities require deep cuts in the average PM2.5 levels.  Kolkata has to reduce by 41 per cent from the base level of three year average, Howrah by 42 per cent, Asansol by 32 per cent and Siliguri by 28 per cent to meet the national ambient air quality standards. 

Says Roychowdhury: “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. 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.” 

For CSE’s analysis of winter air pollution in Delhi-NCR, please see: https://www.cseindia.org/winter-air-pollution-and-smog-is-showing-new-patterns-says-latest-assessment-10528

For more on this, please contact Sukanya Nair of The CSE Media Resource Centre at sukanya.nair@cseindia.org, 8816818864.