Factors like reduced contribution from stubble burning, November rains and milder winters were undone by low wind speed
The gradual, long-term improvement in the annual levels of ultrafine particulate matter (PM) 2.5 in the Delhi-National Capital Region (NCR) air since 2015-17 halted in winter 2023, an analysis by Delhi-based think tank Centre for Science and Environment (CSE) found. Meteorological conditions, combined with high local pollution, were at fault.
The summer and monsoon seasons were cleaner than usual, the CSE analysis said. The ingress of smoke from farm stubble fires in the northern states was also lower this time. But unusually low surface wind speeds in winter meant that local pollution, which was already quite high, was trapped, leading to a spike in pollution levels.
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The impact of the unusual winter levels caused the overall annual levels to plateau and even worsen, undoing the gains of the long-term downward trend, the analysis showed.
“This winter is a lasting reminder of the fact that Delhi, despite succeeding to bend the long-term pollution curve downward over the last few years, has not achieved deeper cuts in emissions to sustain a more aggressive reduction for meeting air quality standards and prevent winter spikes,” said Anumita Roychowdhury, executive director, research and advocacy, CSE.
Delhi’s PM2.5 annual average for 2023 (through December 29, 2023) stood at 100.9 microgrammes 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 due to lockdowns imposed over the COVID-19 pandemic.
However, the levels were not as high as they were previously — the average of the 2018-22 winters is higher than the winter of 2023.
The long-term three-year average trend among the five oldest Continuous Ambient Air Quality Monitoring Stations (CAAQMS) stations of the city (ITO, IHBAS, Mandir Marg, Punjabi Bagh and RK Puram) also showed a similar pattern. This 2021-23 average is nearly 3 per cent higher than its 2020-22 counterpart.
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CSE also discovered that winter conditions influenced pollution trends in 2023, and that summers are becoming cleaner. Summer months (March to June) in 2023 were significantly less polluted (14-36 per cent) than the same months in 2022. However, the winter months of January, November, and December were significantly more polluted in 2023 (12-34 per cent) than in 2022.
The number of days with ‘very poor’ and ‘severe’ PM2.5 concentrations increased, CSE found. However, peak pollution was down: Between October 1 and December 29, there were only three days of ‘satisfactory’ air quality and zero days of ‘good’ air quality. There were more days in these categories in the same period in the previous two years.
Source: CSE
The peak intensity of ‘severe+’ days was lower, even though the number of days was the same as in the winters of 2019, 2020 and 2021. The 2023 peak was 349 g/m3 and occurred on November 13 (the day after Diwali). This is significantly lower than previous years’ peaks: in 2022, the peak was 401 g/m3, while in 2019, it increased to 546 g/m3.
Delhi also witnessed six smog episodes (at least three or more minimum continuous days with air quality in the ‘severe’ category) in the season, when it usually witnesses around two such episodes. This was the highest in a single season in the previous six winters.
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Farm stubble fire smoke is widely regarded as the primary source of elevated PM2.5 levels during the first half of the winter season. However, the number of fires in Punjab and Haryana in 2023 was nearly identical to the number of incidents in 2022.
The contribution of fires to PM2.5 in November 2023 was the lowest in the last six years; 2023 winter also recorded the maximum rainfall in the last six years, and the cold conditions were less severe than usual. The lack of surface winds undid the hopes of any clean winter.
“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,” said Avikal Somvanshi, senior programme manager, Urban Lab, CSE.
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