• Mon. Jul 15th, 2024

Centre for Science and Environment (CSE), releases the latest analysis of air quality data for Delhi-NCR, as winter season sets in

As the winter season sets, Centre for Science and Environment (CSE) has analysed air quality trends so far in Delhi and National Capital Region (NCR), besides the larger Indo-Gangetic Plain (IGP) region. The results of the analysis have been released today.

Says Anumita Roychowdhury, executive director, research and advocacy, CSE: “The objective has been to understand the starting line of the onset of the winter pollution season or pre-winter levels in this region, and also to understand the longer term trends in seasonal variations.”

She adds: “The 2021 winter is starting with a cleaner threshold compared to the previous years, largely due to the intense and prolonged effect of the monsoons. While winter pollution cannot be predicted at this moment, the evidence of rising summer pollution in 2021 despite the lockdown and the evidence of a synchronised effect of winter pollution across the Indo-Gangetic Plain add to our concern. How soon and intensely the winter pollution will hit us will depend on the scale and speed of action across the region and leveraging it for more sustained air quality gains.”

This is an assessment of annual and seasonal trends in PM2.5 concentration for the period January 1, 2018 to October 15, 2021. It captures three successive winter seasons, pre-winter trends and the pre-pandemic and pandemic era periods, including stages of lockdown in Delhi and NCR. This analysis is based on real-time data available from the currently operational air quality monitoring stations in Delhi-NCR and the larger IGP.

For this analysis, a huge volume of data points have been cleaned and data gaps addressed, based on the United States Environmental Protection Agency (USEPA) methodologies. The analysis covers 156 continuous ambient air quality monitoring stations (CAAQMS) spread across 67 cities in Punjab, Chandigarh, Haryana, Delhi, Rajasthan, Uttar Pradesh, Bihar and West Bengal.

“Distinct trends are emerging over the years. The number of days with severe concentration of PM2.5has declined and duration of smog episodes are shorter, as is evident during the 2020-21 winter. Moreover, pollution hotspots are showing more volatile trends with some recording higher PM2.5 levels compared to the city as a whole and the regional averages even during monsoons,” says Avikal Somvanshi, programme manager at the Urban Data Analytic Lab at CSE.

“This winter we may get a much severe peak in smog, as farm stubble fire counts may get more concentrated due to the delayed effect of the rains – to counter this, action must be scaled up right away,” Somvanshi adds.

Trends in PM2.5 concentration: Key highlights

Cleanest monsoon season in the last four years in Delhi: The extraordinarily wet monsoon season has also translated into the cleanest monsoon season for Delhi in the last four years. The city-wide average for the monsoon this year stood at 41 microgramme per cubic metre (ug/m3), with 96 days meeting the 24-hour standard for PM2.5. There was a progressive increase in the number of cleaner days with an average 6 percent annual improvement since 2018. In Delhi, Dr Karni Singh Shooting Range recorded the lowest seasonal average of 33 ug/m3, while the National Stadium had 100 days that met the daily standard. Anand Vihar, with a seasonal average of 61 ug/m3 had a lesser number of days – 54 — meeting the standard and was the worst hotspot in the city.  

The start of the monsoon’s retreat date almost perfectly coincides with the start of bad air days in Delhi (when the daily PM2.5 average breaches the standard and mostly remains above for the rest of the season).

Summer pollution in Delhi – rising: The advantage of improved air quality during the summer of 2020 due to the lockdown was lost in the 2021 summer, with the seasonal average climbing to 79 ug/m3 and the number of days meeting the standard plummeting to 51 (from 90 in 2020). In fact, PM2.5 levels this summer have almost returned to the 2019 levels despite the partial lockdowns. 

Farm stubble fire and early winter smog in Delhi: Normally, the first phase of winter smog is often triggered by large-scale stubble burning in the region. The contribution of farm stubble fire this year started roughly around October 10, the same time as in 2018 and 2020. Peak contribution this year so far was registered on October 16 when 14 per cent of Delhi’s PM2.5 was attributed to stubble fire smoke (according to SAFAR). Heavy rains in the following days washed out the first build-up of the season. The smoke season generally peaks around the cusp of October and November, with contribution spiking to over 40 per cent on the worst days, says Somvanshi.

In 2020, during the stubble burning season October 1-November 29, an average 12.2 per cent per day of Delhi’s PM2.5 pollution came from farm stubble fire smoke. This was considerably higher than previous years – in 2019, contributions stood at 8.9 per cent per day, while it was 10.9 per cent per day in 2018. Somvanshi points out that the average contribution of stubble fire to Delhi’s daily PM2.5 concentration increased by 36 per cent from 2020 and by 12 per cent from 2018.

The farm fire count during the pre-winter season of 2021 has been less than half of the number recorded last year. The lower count, as reported by satellite imagery, can also be due to increased cloud cover which hinders spotting of fires by satellites. Nevertheless, lower fire count during pre-winter correlates well with the lower pollution level in Delhi-NCR.

Decline in number of days with severe concentration of PM2.5 and duration of smog episodes during 2020-21 winter: During the winter of 2020-21, 23 days had city-wide average of PM2.5concentration in ‘severe’ or ‘worse’ AQI sub-categories — down from 25 such days in 2019-20 winter and 34 such days in 2018-19 winter. Technically, a smog episode is defined for the purpose of implementing emergency action under the Graded Response Action Plan (GRAP) when the levels of PM2.5 remain in ‘severe’ category for three consecutive days. From this perspective, there were two continuous smog episodes in this period. The first episode was of longer duration, starting November 3, 2020 and lasting for seven days. The second started on December 22, 2020 and lasted for three days. Thus, the continuous smog episodes were fewer and shorter compared to previous winters.

PM2.5 winter trends in NCR: On an average, the 2020-21 winter season was 5 per cent worse among the major NCR cities and Delhi compared to the 2019-20 season; but the peak pollution, on an average, was 13 per cent lower. Given the experience of 2019-20, it can be expected that smog episode this year might have a higher peak pollution if special steps are not taken to reduce pollution from sources.

Even though the average level during the earlier winter of 2019-20 was lesser than in other winters, the peak daily pollution was considerably higher across all major NCR cities. This can be attributed to the September-October rains that pushed and concentrated the farm stubble burning towards the end of October and start of November when winter conditions were turning more adverse.

City hotspots record higher PM2.5 levels compared to the city and regional averages even during monsoons: There is a broad classification of hotspots. Originally, hotspots were defined as those with annual average levels higher than the mean value of the city – that is, any case which is worse than the national ambient air quality standards. Eleven of 18 recognised hotspots registered higher seasonal averages than the city and the region. But only two — Anand Vihar and Punjabi Bagh — show worsening of air quality during the 2021 monsoon compared to the 2020 monsoon.

Among the 14 new locations in NCR identified last winter by CSE as emerging hotspots, 50 per cent registered lower levels this monsoon compared to city and regional averages. Three more locations have emerged this monsoon as potential hotspots: Bhiwadi (Rajasthan), Manesar (Haryana), and Sector 11 Faridabad (Haryana). These locations show significantly higher season averages compared to the rest of the region. In fact, with 64 ug/m3, Sector 11 Faridabad had the worst air during this monsoon in the NCR. From this list of 35 recognised and emerging hotspots, only three locations (Mayapuri, Okhla Phase 2, and Sanjay Nagar) had their summer averages below the regional average of 75 ug/m3. Earlier in the winter, 16 of the 18 recognised hotspots registered a worsening of air. Two hotspots that showed improvement were Wazirpur and Sahibabad.

Indo Gangetic Plain (IGP)-wide phenomena: Tracking all the 67 monitored cities in the region makes it very clear that air quality dips to ‘poor’ and ‘worse’ categories as the monsoon retreats. For most of the northern plains from Punjab to central Uttar Pradesh (UP), the start of bad air quality days is almost perfectly synchronised. Eastern plains witness the onset of pollution almost three-four weeks later. It is interesting to note that air cleans up in Punjab much earlier than in the rest of IGP, while pollution lingers on longer in NCR and adjoining western UP.  In the lower IGP (Bihar and West Bengal), cleaning of air also starts earlier than in NCR. General observation from the data is that northern plains (up till central UP) are severely impacted by farm stubble fire smoke during the start of winter season, but the high levels seen later in the season are due to inversion and local pollution. Meanwhile, winter build-up in the eastern plains is driven almost exclusively by inversion and local pollution with limited impact of smoke from farm stubble fires.

AQI during pre-winter months and changing pattern of lead pollutants: Normally, particulate pollution dominates the daily pollution in Delhi. However, as is evident from the composite AQI, other gaseous pollutants like ozone and carbon monoxide have often become the lead pollutants along with particulate pollution of the day due to the wash-out effect of rains. Also, the months of September and October show that the number of days in the ‘satisfactory’ category have increased from 43 per cent in 2018 to 90 per cent in 2021. In 2020, however, this came down to 27 per cent. A similar pattern is seen in the ‘moderately polluted’ category. In October 2021, the number of days in ‘poor’ category has reduced from 67 per cent in 2018 to zero per cent in 2021.

Says Anumita Roychowdhury: “Delhi and the larger region will require urgent action to prevent severe smog episodes as well as speed up deeper reforms to sustain the gains. Reduce traffic volume, eliminate waste burning, eliminate dirty industrial fuels, and implement stringent dust control measures — especially in the construction sector. Act now, and act decisively.”

( Inputs from Pratyusha Mukherjee)

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