The year 2023’s winter months were a period when the region of Delhi-NCR slipped back into the abyss of high pollution levels — despite having seen a gradual long-term improvement in annual PM 2.5 concentrations since 2015-17.

Particles are defined by their diametre for air quality regulatory purposes. Those with a diametre of 10 microns or less (PM10) are inhalable into the lungs and can induce adverse health effects. Fine particulate matter is defined as particles that are 2.5 microns or less in diametre (PM2. 5).

The forward march towards cleaner air and clearer skies came to a grinding halt in 2023, says a new analysis by Centre for Science and Environment (CSE), partly due to meteorological factors.

The region witnessed cleaner-than-usual summer and monsoon seasons, and much lower ingress of smoke from farm stubble fires in the northern states.

But unusually low surface wind speed in winter meant that local pollution, which was already quite high, was trapped. This led to the spike in pollution levels.

Says Anumita Roychowdhury, executive director, research and advocacy, CSE: “Due to the impact of the unusual winter levels, the overall annual concentrations plateaued and even worsened, undoing the gains of the long-term downward trend.

"With the contribution of stubble burning to Delhi’s pollution going down this year, with more rains in November and less severe cold winter conditions, the annual level should have improved further. Instead, the worsening of meteorological conditions — lowering of wind speed — tilted the scale adversely simply because the local pollution was still very high. Delhi needs more aggressive emissions reduction steps to meet the national ambient air quality standard.”

Avikal Somvanshi, senior programme manager, Urban Lab, CSE points out: “Interestingly, the summer and monsoon months in 2023 were unusually cleaner compared to the months in the previous years. But winter levels turned out to be one of the worst due to very slow surface wind speed. This impeded the dispersal of pollution and led to higher levels.

"This trapping of local pollution from all sides made this winter exceptionally bad despite lesser smoke from farm fires. The meteorology also explains lower pollution peaks and relatively uniform bad air quality with minimal fluctuations in the PM2.5 concentration throughout the season.”

The winter pollution analysis was carried out by CSE’s Urban Lab. The objective, says Somvanshi, has been “to understand the trend and the starting line of the onset of the winter pollution season in this region.”

Key Highlights Of The CSE Analysis

2023’s annual average PM2.5 levels worse than that of 2022 — but not as high as it used to be: Delhi has been witnessing a gradual yet consistent decline in its annual PM2.5 levels since 2015-17, with 2020 being the only outlier due to massive disruption caused by pandemic lockdowns.

But this downward movement stopped in 2023. Delhi’s PM2.5 annual average for 2023 (up till 29 December 2023) stood at 100.9 microgram per cubic metre (µg/m3).

This was an increase of 2 per cent compared to the 2022 annual level, and was 6 per cent higher than an exceptionally clean 2020.

However, the levels were not as high as they used to be earlier — the average of 2018-22 winters is higher than that of the winter of 2023.

Winter condition has affected the 2023 trend — summer is getting progressively cleaner: In 2023, about 151 days met the National Ambient Air Quality Standard.

This was similar to the 2021 trend and second only to 2020 (a lockdown year) when 174 days had met the standards. In 2022, only 117 days met the standard. Almost all of these cleaner days were observed during summer and monsoon seasons.

There was a significant fall in the number of ‘good’ air quality days (when levels are 50 per cent below the standard) in 2023; this stood at 24 days compared to 41 in 2022.

Meanwhile, in 2023, the number of days with PM2.5 concentration in the ‘very poor’ or ‘worse’ categories (as per the air quality index classification) stood at 107 days, including 24 with ‘severe’ levels (until 30 December 2023).

In 2022, there were 106 ‘very poor’ or ‘worse’ days, but ‘severe’ days were just nine in number — this was less than half of that of 2023.

Summer is cleaner, while winter is becoming worse: In 2023, the summer months (from March to June) were significantly less polluted (14-36 per cent) than the corresponding months in 2022.

But the winter months of January, November and December were much more polluted in 2023 (12-34 per cent) compared to the months in 2022 (see corresponding graph in the complete report).

The transition period including February, July, September and October does not show much change, which might be due to the fact that the monsoon months were already very clean.

What Went Wrong In The Winter Of 2023?

Though the number of days with ‘very poor’ and ‘severe’ PM2.5 concentration increased, peak pollution was down: Between 1 October and 29 December, there were only three days of ‘satisfactory’ air quality and zero days of ‘good’ air quality.

There were more days of these categories in the same period in the previous two years.

The number of ‘severe+’ days was the same as in the winters of 2019, 2020 and 2021, but the peak intensity was lower. The peak in 2023 was 349 µg/m3 and happened on 13 November (the day after Diwali).

This is much lower than the peaks recorded in the previous years: in 2022, the peak was 401 µg/m3, while in 2019 it went up to 546 µg/m3.

Highest number of smog episodes observed this season in last six winters: Delhi usually experiences two smog episodes (a smog episode is defined as at least three or more minimum continuous days with air quality in the ‘severe’ category) in the months of November and December.

In 2023, there were three episodes by 24 December; on 30 December, Delhi was reeling under its third day of continuous ‘severe’ air quality, which was turning into the city’s fourth smog episode.

This was the highest number of smog episodes observed in a season in last six winters.

Overall, 2023 winter seems to have had the most consistently bad air compared to the last six winters, with minimal fluctuations in daily PM2.5 levels. Data indicates that it was the longest spell of ‘very poor’ or ‘worse’ air in last six years.

Slow surface winds seem to be the main contributory factor to the winter trend: Farm stubble fire smoke is generally perceived to be the main contributor to heightened PM2.5 levels during the first half of the winter season.

But in 2023, the number of fires in Punjab and Haryana were nearly identical to 2022’s number of incidences.

In November 2023, the average surface wind speed in Delhi was 9.8 m/s, which was the lowest in the last six years. It was 21 per cent less than the wind speed in November of 2022.

Somvanshi explains: “Low wind speed means that local pollution dispersion is horizontally restricted, while vertical dispersion is already restricted because of the winter inversion phenomenon. This trapping of local pollution from all sides has made this winter exceptionally bad despite lesser smoke from farm stubble fires.

"This also explains low peaks and relatively uniform bad air quality with minimal fluctuations in PM2.5 concentration throughout the season. It must also be noted that heavy rains in November had insignificant impact on air quality.”

Way Ahead

Steps are required for clean fuel transition in all segments of industries, with stringent compliance with emissions targets and replacement of solid fuels in households and eateries.

Action towards elimination of waste burning, with a comprehensive waste management system ensuring 100 per cent segregated collection, material recovery and remediation of legacy waste and stringent measures in the construction sector.

Delhi-NCR pollution

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