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This is an archive article published on January 5, 2024
This winter, Delhi saw highest number of smog episodes in 6 years
The average surface wind speed in Delhi in November was 9.8 m/s, the lowest in the last six years, going by the analysis, which identifies this as the “driving factor” for the poor air quality this winter.
The analysis has used data on wind speed from the India Meteorological Department.
Despite lower levels of smoke from farm fires impacting Delhi, this winter saw the highest number of smog episodes in the national capital in six years with “unusually low surface wind speeds” that trapped local pollution levels that were already high, according to an analysis by the Centre for Science and Environment (CSE).
The average surface wind speed in Delhi in November was 9.8 m/s, the lowest in the last six years, going by the analysis, which identifies this as the “driving factor” for the poor air quality this winter. This is because low wind speed prevents pollutants from dispersing horizontally even as the inversion phenomenon in winter already prevents vertical dispersion of pollutants, according to the analysis. Inversion in winter is when cold air close to the surface of the earth remains trapped by a lid of warm air above it. In contrast, the average wind speed in November last year was 12.4 m/s, while it was 11 m/s in November 2021. The analysis has used data on wind speed from the India Meteorological Department.
“We are not really sure why wind speed is lower than usual, but it has remained low all of November and December. With both vertical and horizontal movement stopping, pollutants have remained stuck. So, even with little smoke from farm fires coming in and winter temperatures not being as cold as they were earlier, we still saw high pollution levels this winter. Much smoke from farm fires has not reached Delhi because of the winds not being favourable. Data from NASA shows that farm fires reported this year were similar to what was reported last year,” said Avikal Somvanshi from CSE, who was part of the team that carried out the analysis.
The trapping of local pollution “has made this winter exceptionally bad despite lesser smoke from farm stubble fires,” the analysis noted. It estimates the contribution of farm fires to PM2.5 levels in Delhi in November to be 1.48 metric tonnes, which was found to be the lowest in the past six years. This contribution was 3 metric tonnes last year and 5.68 metric tonnes in November 2021.
The lower contribution from farm fires notwithstanding, this year, the national capital recorded its most polluted winter in six years in terms of smog episodes – when the air quality has remained in the ‘severe’ category in terms of PM2.5 levels for at least three consecutive days. This year, there have been three such episodes by December 24, and a fourth one at the end of December, going by the analysis.
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