Higher winter air pollution in the Mumbai region after unlocking of the economy, says latest regional assessment by CSE

Despite lower PM_2.5 levels due to prolonged lockdown this year and overall lower levels compared to those in the Indo-Gangetic Plain, air pollution in Maharashtra’s Mumbai region has gone up with the onset of winter and the unlocking of the economy 

• PM₅ levels in winter rose beyond the standard in Greater Mumbai Region and rest of Maharashtra.
• This year, the winter trend has set in almost two weeks earlier in the season and the average PM₅ levels in October and November have been 25-30 per cent higher in Greater Mumbai Region compared to the corresponding period previous year
• From the respective cleanest week, the weekly average of PM₅ rose dramatically to the most polluted week in Mumbai by 10 times, in Navi Mumbai by 16 times, in Kalyan by eight times, and in Pune by five times.
• Winter air gets more toxic as the share of tinier PM₅ in overall PM₁₀ concentration increases to 40 per cent during October and averaging 46 per cent all through November. The share of PM_2.5 in PM₁₀ had reached 60 per cent on Diwali day this year.
• Worli, Vile Parle, Kurla, and CSIA stations’ have recorded “very poor” category on multiple days this November even though citywide average remained moderately polluted.
• There are lesser number of good air quality days this winter
• Winter pollution and overall toxic exposure during the year can be controlled only with scale and speed of action on transport, clean fuels in industry, clean power plants, waste management and biomass burning across the region, says CSE 

See graphs related to this press release here:
https://cdn.cseindia.org/userfiles/Note-CSE-analysis-of-winter-pollution-Mahrashtra.pdf 

New Delhi, December 11, 2020: New analysis of winter pollution until the first week of December this year, done by New Delhi-based Centre for Science and Environment (CSE), shows how clean air gains of the lockdown and monsoon period were lost with the reopening of the economy and hostile winter weather. While this was expected, the analysis of the real-time data from monitoring stations across Greater Mumbai Region as well as other major cities in Maharashtra show the changing pattern in winter pollution this year. 

Says Anumita Roychowdhury, executive director-research and advocacy, CSE: “Even though trapping of winter pollution in Greater Mumbai region is not as high as that of the Indo-Gangetic Plain due to its proximity to sea and better ventilation, the levels increased during winter despite the geographical advantages and favourable meteorology. This indicates high local pollution and strong regional influences.” 

The CSE analysis shows that even though the overall average level of PM_2.5 for the 11 months in 2020 is considerably lower than the previous year due to the pandemic related to summer lockdown, the PM_2.5 levels in winter rose beyond the standard in Greater Mumbai Region and rest of Maharashtra. 

Says Roychowdhury: “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, this trend has set in almost two weeks earlier in the season and the average PM_2.5 levels in October and November have been 25-30 per cent higher in Greater Mumbai Region compared to previous October and November. It is clear that this region cannot rely only on its locational advantage of proximity to the sea.” 

Avikal Somvanshi, programme manager in CSE’s Urban Lab team of Sustainable Cities programme, says that “this detailed data analysis points to the fact that the air pollution is a more pervasive problem in the Mumbai region and beyond and this requires quicker reforms and action in key sectors of pollution – vehicles, industry, power plants and waste management to control winter pollution and further bend the annual air pollution curve.”  

Data used in the analysis: The 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. Ten cities – Mumbai, Navi Mumbai, Thane, Kalyan, Pune, Nagpur, Nashik, Aurangabad, Solapur, and Chandrapur -- have been selected for this analysis because real time data is available for these cities. 

Analysis has been done of data recorded by 10 air quality monitoring stations in Mumbai; three in Navi Mumbai; two in Chandrapur; and one station each in Thane, Kalyan, Pune, Nagpur, Nashik, Aurangabad, and Solapur under the Continuous Ambient Air Quality Monitoring System (CAAQMS) of CPCB. Recently, new stations owned and operated by the Indian Institute of Tropical Meteorology in Pune have been added to CAAQMS network, but they do not have adequate data needed for this analysis – therefore, they have not been used. Weather data for Mumbai has been sourced from the Santa Cruz weather station of the India Meteorological Department (IMD). 

Key highlights of the CSE analysis 

Average levels of PM_2.5 have been lower during this year due to the lockdown, but that could not prevent the winter spike.
The overall PM_2.5 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 made PM_2.5 levels rise starting October. 

From the respective cleanest week, the weekly average of PM_2.5 in Mumbai rose 10 times; in Navi Mumbai 16 times; Kalyan eight times; Pune, Aurangabad and Nagpur five times; Nashik six times; Chandrapur eight times; and in Solapur nine times to the dirtiest week. The cleanest week for Mumbai, Navi Mumbai, Kalyan and Solapur was the week ending on 5 July, 2020. Nagpur, Nashik, Pune and Solapur had their cleanest week in August 2020. Aurangabad had its cleanest week on the week ending 25 May 2020. 

The most polluted weeks this winter so far are the week ending on 15 November 2020 in Mumbai and Kalyan; week ending 29 November 2020 in Navi Mumbai, Nagpur and Nashik; the week ending 8 November in Pune; week ending 1 November in Chandrapur; week ending 22 November in Aurangabad; and the week ending 6 December in Solapur. 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. 

Average October-November PM_2.5 levels have been considerably higher this year.
October this year was dirtier across all cities in Maharashtra and was the worst in the Greater Mumbai Region. The PM_2.5 average this October was 25 per cent higher in Mumbai, 26 per cent in Navi Mumbai and 28 per cent in Kalyan compared to the corresponding time in 2019. Thane does not have a working PM_2.5 monitor. November was also dirtier with the monthly average higher by 7 per cent in Mumbai, 21 per cent in Navi Mumbai, and 31 per cent in Kalyan. Cities outside the Greater Mumbai Region had similar or lower November average as last year. 

Air quality gets more toxic with the onset of winter -- share of tinier PM_2.5 in PM₁₀ increases.
The share of tinier and finer particles in the overall coarser PM₁₀ concentration determines the toxicity of air. When the overall share of tinier PM_2.5 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 the health risk. Interestingly, during lockdown, when the overall suspended coarser particles had settled down reducing the PM₁₀ levels, the PM_2.5 had also come down. But its share was 36 per cent – higher than it is usually noted during summer (below 30 per cent). But with the onset of winter the overall levels of both have gone up, along with the percentage share of PM_2.5 in the overall PM_10. This rose to the high 40s during October and remained high through November averaging at 46 per cent. The share of PM_2.5 in PM₁₀ is generally highest on Diwali and it reached 60 per cent this year. 

Dirtier Diwali in Mumbai this year.
The average PM_2.5 level on Diwali day in Mumbai was 76 μg/m³ -- up from 53 μg/m³ recorded in 2019. This year there was about 75 per cent higher rise in hourly PM_2.5 concentration between afternoon and night of Diwali that is mostly caused due to firecracker bursting. The change in hourly PM_2.5 concentration between afternoon and night of 2020 Diwali was 145 μg/m³, up from 83 μg/m³ in 2019. Diwali also occurred later in November than the previous year. 

In Mumbai, November 2020 was dirtier but the peak was lower.
The rolling weekly average rose over the 24-hour standard or 60 μg/m³ on November 2, while it had done so on November 16 last year. 

Variations within Mumbai show some locations are highly polluted.
Worli, Vile Parle, Kurla and CSIA stations’ 24-hour average slipped into the “very poor” category on multiple days this November, even though city-wide average remained in the ‘moderately polluted’ category. This is quite different compared to previous November when no station registered a “very poor” day even when there was no significant change in the city’s average temperature or rainfall in the second half of the month. There is a wider variation in PM_2.5 levels within the city; standard deviation among the city’s 10 stations is 80 per cent higher this November on an average, and the difference between upper and lower bounds has increased to 53 μg/m³ from 33 μg/m³ last November. 

Air quality in the city is usually at its worst around Christmas and New Year: last year it registered a “very poor” category across all stations. Going by this trend, similar if not worse can be expected this year. Kurla and Worli have the highest November averages in the city, while Bandra, CSIA and Colaba have the lowest. Nerul in Navi Mumbai is the most polluted in the Greater Mumbai Region. CSIA, despite one of the lowest monthly concentrations, has the highest daily spikes.  

In Mumbai, city-wide number of days with PM_2.5 concentration in ‘good’ category was considerably lower this winter, but there were no ‘poor’ days.
There have been seven days of “good” air days this winter compared to 21 recorded last year. But the “poor” days have come down to 0 days from four last year. This is evident in the distribution of days classified according to the PM_2.5 concentration based on the categories of Air Quality Index in Mumbai in winter (1 Oct – 8 Dec, 2019 and 2020). 

In Mumbai, cyclical ups and down of pollution this winter are less volatile – showing slower rise and fall than in previous winter.
This inelastic behavior of PM_2.5 levels in Mumbai is in contrast to what is seen in Delhi-NCR and Kolkata-Howrah, where the trend has been more volatile during winter with frequent and quicker rise and drop. This can be the impact of changed meteorology, but more investigation is needed to understand the reasons for this. 

Even with comparatively cleaner air during this year, Greater Mumbai cities recorded daily spikes similar to those observed in 2019.
CSE has compared the annual averages and peak 24-hour averages in these cities of Maharashtra between 2019 and 2020. This shows that Greater Mumbai cities even with much lower annual average levels of PM_2.5 have experienced almost same or higher maximum daily levels during winter when the entire region got air-locked. Cities outside the Greater Mumbai region have registered much lower daily spikes. 

Also, a review of PM₁₀ levels in Greater Mumbai region (until December 8) shows high level in Thane (which does not have a working PM_2.5 monitor). Other cities have similar curvature in the trend in their PM₁₀ concentrations as noted in PM_2.5 levels. Thane’s annual average for 2020 (up till December 8) is 85 μg/m³ which is already higher than the 2019 average of 84 μg/m³. This increase in PM₁₀ levels from 2019 in Thane is driven by the extraordinary dirtier October and November this year.  

Need deep cuts
To avoid winter pollution peaks all cities of Maharashtra will have to reduce the annual average level of pollution across all cities to meet the national ambient air quality standards and even bring it further down to be closer to the health-based guidelines of the World Health Organization to protect public health.  

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 has to take forward its wins so far and raise the level of ambition to drive action across all key sectors of pollution and in the entire region. Enforce power plant standards across the state, minimise use of coal and other dirt fuels in the industry while improving emissions control, scale up public transport and vehicle restraint measures and manage waste to have a zero waste and zero landfill strategy.” 

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.