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Stubble burning in villages surrounding Delhi and the National Capital Region, blamed for the region’s intense air pollution every winter, may actually have a much smaller role to play, said a new report.
Over 80 per cent of the daily average concentration of particulate matter 2.5 (PM2.5) was attributed to non-stubble sources during October and November — the major crop-burning months, the report by the Centre for Science and Environment, a Delhi-based non-profit, showed.
Smog episodes (PM2.5 concentration over 250 micrograms per cubic metre) during late December occur every winter even without the influence of stubble smoke, the analysts noted in the report.
Local pollutants, entrapped close to ground because of low temperature and low winds, led to 5 per cent higher PM2.5 levels in the months following the stubble-burning season this year than during.
The average PM2.5 concentration due to stubble-burning during the 52 days in Oct-Dec this year was 28 ug / m3 per day — less than a quarter of that due to other sources. “This is consistent with the trend noted during the previous winter,” the report said.
Delhi’s air this winter was slightly cleaner than previous years, but still extremely unhealthy, the researchers said. The improvement was marginal despite the city recording its wettest winter in a century.
The city-wide winter average for Delhi stood at 172 μg/m3 — identical to the seasonal average of the winter of 2019-20 but 9 per cent lower than the seasonal average of 2020-21 winter.
The analysis is based on real-time (15-minute averages) air pollution data collected from 81 air quality monitoring stations by the Central Pollution Control Board.
Farm stubble fire data has been sourced from System of Air Quality and Weather Forecasting and Research (SAFAR) and weather data from the Safdarjang weather station of Indian Meteorological Department (IMD).
The capital recorded more days (25) with ‘severe’ or ‘worse’ air quality index than the previous winter. The figure is equal to that recorded in 2019-20, indicating the intensity and duration of pollution has fallen back to pre-pandemic levels.
Clean air days were also higher this winter, the researchers observed: There were two days of ‘good’ air and seven days of ‘satisfactory’ air, while there were none last winter.
This high variability in air quality can be attributed to increased number of heavy rainfall days and colder-than-usual weather, the report said.
The winter smog episodes this year was less intense but more prolonged in the region, it added.