Winter air pollution and smog is showing new patterns, says latest assessment by Centre for Science and Environment

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

  • Daily PM2.5 levels increased by about 8-11 times between the cleanest August to dirtiest November across Delhi-NCR
  • Winter air turned dramatically toxic as the share of PM2.5 in overall PM10 increased significantly
  • One smog episode (in terms of consistent severe level for more than three consecutive days) occurred this winter (as compared to the same time last year). Severe pollution on Diwali did not lead to another smog episode as the meteorology changed.
  • The number of days with higher contribution of smoke from crop burning -- more than 30 per cent -- was higher this year (more than double that in 2019)
  • Pollution levels in several locations in Delhi show quicker dissipation; range of variation in pollution concentration shows comparatively more lower bound levels than last year
  • This calls for scale and speed of action on transport, clean fuels in industry, clean power plants and waste management across the region, says CSE

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New Delhi, December 2, 2020: New analysis of winter pollution until November this year, done by 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 Delhi-NCR exposes the changing pattern 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 PM2.5 levels in winter went up to make the air “very poor” to “severe” levels across Delhi-NCR. Says Anumita Roychowdhury, CSE executive director-research and advocacy: “This is a typical and predictable winter trend when continuous emissions from local sources and episodic pollution from biomass burning get trapped due to meteorological changes. But this year, there is a change in the pattern that shows up in lesser number of smog episodes compared to last year, wider variation on location-wise concentration with more lower bound ranges compared to last year, higher number of days with greater contribution from the stubble burning among others. There are also days when pollution levels have dropped to moderate level even without rains but better wind conditions.” 

Avikal Somvanshi, programme manager in CSE’s Sustainable Cities team, says that this 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”. 

CSE’s analysis – the key highlights 

Lower average level of PM2.5 throughout the year due to the lockdown could not prevent the winter spike. The overall PM2.5 average this year (until November) 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 PM2.5 levels spiral during October and November. 

From the cleanest weeks of August (the cleanest month on record so far) the levels rose dramatically to one of the dirtiest Novembers in recent years. This rise varied from 9.5 times increase in Delhi to 11 times in Ghaziabad; followed by Noida (9.2 times), Gurugram (6.4 times) and Faridabad (6.2 times). 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. 

Even with comparatively cleaner air round the year, other towns in NCR have recorded spikes as high as those observed in Delhi and the big four NCR cities. CSE has compared the annual averages of the cities and towns of the larger NCR region with that of Delhi and the big four (Gurugram, Faridabad, Noida and Ghaziabad). This shows that even with much lower annual average level of PM2.5, other smaller cities and towns in NCR experience almost same maximum levels during winter when the entire region is in an airlock. In fact, even Delhi -- that has in recent times witnessed a decline in annual average levels on a year-on-year basis – has experienced high pollution build-up during winter. This brings out the deadly combination of regional influences with local pollution when meteorology is adverse.    

Air quality gets more toxic with the onset of winter -- share of tinier PM2.5 in the PM10 increases. The share of tinier and finer particles in the overall coarser PM10 concentration determines the toxicity of the air. Interestingly, during the lockdown, when the overall suspended coarser particles had settled down reducing the PM10 levels, the PM2.5 had also come down. But its share was 47 per cent – higher than it is usually noted during summer. 

But with the onset of winter the percentage share of PM2.5 in the overall PM10 rose to over 70 per cent during the smog episodes in early November, and remained high at 50-60 per cent during most of November. The share of PM2.5 in PM10 was highest on Diwali, reaching over 80 per cent at many locations. Tinier particles are more dangerous as they can penetrate deep inside the lungs and through blood barriers, thus increasing the health risk. 

Pattern of smog episode is different this year. Technically, a smog episode is defined for the purpose of emergency action under the Graded Response Action Plan when the levels of PM2.5 remain in “severe” category for three consecutive days. By that logic, the region has experienced one severe smog episode this year (November 7-10, 2020) compared to two episodes last year until November (October 31-November 3, 2019 and November 12-15, 2019). This year, the episode preceded Diwali -- that too much later in November compared to end-of-October last year. Says Roychowdhury: “In fact, last year, Diwali had catalysed the first big smog episode of the season.” 

But this year, Diwali pollution from firecrackers, combined with smoke from stubble burning and local pollution, led to a build-up of severe-plus and extremely hazardous level on the day of Diwali itself (November 14, 2020). But this did not last long to become another smog episode as meteorology along with a short rain spell helped to dissipate it quickly. 

The average PM2.5 level on Diwali day in Delhi was 404 ug/m3; it dropped to 308 ug/m3 the next day in contrast to the trend in the previous years when the levels increased the next day. This year, pollution was able to disperse faster due to meteorology. But like previous years, there was a dramatic change in hourly PM2.5 concentration between afternoon and night of Diwali due to a spurt in firecracker bursting. 

Levels are more volatile, recording quicker ups and downs and dispersal aided by an overall downward movement in annual trends and meteorology. The cyclical ups and down of pollution this winter are more volatile, showing quicker rise and fall than in the previous winter. This could also be a reflection of changes in local pollution pattern and overall downward trend while aided by meteorology. 

Firstly, an interesting observation is that there are days in November 2020 when the air quality improved substantially without rains but with overall improvement in wind patterns. There are three days during the second half of November in 2020 when the air quality improved to “moderately polluted” AQI category in Delhi. This is the same as 2019, but the clean-up in 2019 was induced by rains. In 2018, the city-wide average never dropped below “poor” AQI category in November. This winter the daily city-wide average dropped down even to 76 μg/m3 on November 27, without any rain to help aid in the cleaning process. In fact, five stations in Delhi met the 24-hour standard with Shadipur registering 41 μg/m3. Gurugram (68 μg/m3), Faridabad (58 μg/m3), and Noida (73 μg/m3) had even lower city-wide averages than Delhi; in fact Faridabad met the standard. Ghaziabad had a relatively higher 84 μg/m3. This is quite different compared to the previous winter.

Secondly, this year local pollution across the monitoring locations of Delhi shows wider variations between the lower and upper ranges of pollution. This is in contrast to the range of variations noted last year, which was more upper bound. Even on the peak smog day in November this year, the PM2.5 levels in several stations varied from a lower bound 108 μg/mat NSIT to 699 μg/mat Mundka. But last year the variation was noted at a higher range – between 351 μg/mat Shadipur to 725 μg/mat Alipur – while the overall level stayed above 374 μg/m3. In fact, the standard deviation among the 36 stations of Delhi this November on an average is 60 per cent higher compared to last year. This indicates somewhat clearing up of local pollution, though meteorology plays a part. 

Even the rolling weekly average rose slower this year compared to last year. It took 37 days to reach the “severe” category (250 μg/m3) from the week that last met the standard of 60 μg/m3 in early October (week ending on October 2, 2020). This is considerably slower rise compared to 2019 when it took 23 days. In 2018, it took 40 days but weather was also about 3oC warmer compared to this November. This November is also among the coldest in recent years. 

Contribution of crop burning to the region’s pollution was volatile and a higher number of days recorded a higher share. CSE has analysed the data provided by SAFAR on daily percentage contribution of stubble burning to PM2.5 concentrations in Delhi-NCR depending on the direction and speed of the wind. This shows that the smoke from crop stubble fire started impacting Delhi more discerningly from October 10, 2020 onwards. This was a week earlier than last year when it started on October 16, 2019. CSE analysed classified days (up till 30 November) based on daily percentage contributions – less than 10 per cent, 10-20 per cent, 20-30 per cent and above 30 per cent. This shows there were 7 days this year when the contribution of smoke to Delhi's PM2.5 concentration exceeded 30 per cent in contrast to three days in 2019 and 2018. There were six days when contribution was between 20-30 per cent, (up from four days in 2019) and 16 days of 10-20 per cent contribution (up from 15 days in 2019) and 23 days with less than 10 per cent contribution (down from 30 days in 2019). This year Diwali pollution was also compounded by the heightened contribution of smoke from crop stubble fire as the contribution increased to 32 per cent. 

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, eliminate coal from the industry, scale up public transport and vehicle restraint measures and manage waste to have a zero waste and zero landfill strategy.”

For any additional information, please contact Sukanya Nair of the CSE Media Resource Centre at, 8816818864.




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