New Delhi: Delhi did not record a single “clean air” day in the winter months, even though annual pollution levels showed signs of improvement, according to a report by Climate Trends based on Central Pollution Control Board’s (CPCB) air quality monitoring data for 2024–25.The study warns that current evaluation metrics under the National Clean Air Programme (NCAP) may be overshadowing the true health burden on residents by focusing on annual averages rather than seasonal pollution spikes. Researchers argue that winter pollution episodes remain extremely severe due to unfavourable weather conditions.According to the analysis, meteorological factors such as low wind speeds and high humidity — conditions that lead to atmospheric stagnation — can influence pollution levels by up to 40% even without changes in emissions. In cities like Delhi, these weather patterns significantly worsen particulate matter concentration.“Even a 20-30% reduction in annual PM2.5 does not translate into winter air quality compliance in stagnation-prone cities like Delhi,” said Aarti Khosla, founder and director of Climate Trends. She added that over 70% of days in Delhi witness low winds and high humidity, which trap pollutants close to the ground.Khosla said the next phase of NCAP must include season-specific targets, weather-triggered interventions and airshed-level planning to deliver meaningful improvements in public health.Delhi continues to face the country’s most severe pollution crisis, the report noted, with the highest annual average PM2.5 levels and the longest stretches of severe air quality days among major Indian cities. Pollution in the capital is driven by a combination of local emissions — including transport, construction dust and waste burning — as well as regional sources across the Indo-Gangetic Plain.Scientists involved in the analysis emphasised that weather plays a critical role in determining how severely these emissions affect air quality.“The persistence of PM2.5 exceedances is strongly linked to sub-1 metre-per-second wind speeds and high humidity across northern cities,” said Sagnik Dey, head of the Centre for Atmospheric Sciences at Indian Institute of Technology Delhi. “Ventilation efficiency becomes the dominant factor explaining differences between cities.”Dey said that current NCAP assessments largely evaluate changes in observed pollution levels without adjusting for weather conditions, which could distort the perceived effectiveness of policy interventions.The report recommends that NCAP Phase III — due this year — adopt a more sophisticated framework that integrates meteorological analysis into air quality management. Suggested reforms include separate winter pollution targets, dynamic action plans triggered by weather conditions, and integrated airshed-level strategies.Researchers estimate that a shift to well-ventilated atmospheric conditions alone could reduce PM2.5 levels by 35–40%, highlighting how strongly pollution episodes depend on seasonal meteorology.
