New Delhi: All mega cities in India, irrespective of their locations in different geo-climatic zones, faced the challenge of worsening PM2.5 levels during the winter season of 2022-23. While levels in Delhi have been the highest, the remaining cities have also experienced very poor to worsening trends.

This has emerged from an analysis of real-time PM2.5 data done by the Centre for Science and Environment in Delhi, Kolkata-Howrah, Mumbai, Hyderabad, Bengaluru and Chennai for the winter period of October 1, 2022, to February 28, 2023. The objective of this analysis has been to assess the peer mega cities to understand the longer-term seasonal variations and annual trends in particulate pollution.

“While Delhi’s winter air quality hogs all the eyeballs, the rising winter air pollution in other megacities including Kolkata, Mumbai, Hyderabad, Bengaluru and Chennai, do not get adequate attention. While Delhi has bent its seasonal pollution curve, winter air pollution is high or on the rise in most other megacities. These cities located outside the northern plains may have more favourable meteorological conditions to contain the peaking of pollution during winters, but their overall city average and levels across locations can cause very high exposures,” says Anumita Roychowdhury, executive director-research and advocacy, CSE.

“Winter season presents a serious challenge in all mega cities despite them being located in different geo-climatic zones with varying meteorological and topographical conditions. The PM2.5 levels remain elevated and peak during winter in all megacities. This winter, several of these cities, excluding Delhi, recorded higher seasonal PM2.5 averages compared to their previous winter. This clearly indicates that the overall emissions are high or may be rising in those cities,” says Avikal Somvanshi, senior programme manager at the Centre for Science and Environment.

The key highlights

• Compared to the winter of 2021-22, only Delhi has shown an improvement in air quality -- its current winter air was 9 per cent less polluted. Winter averages of PM2.5 increased in the remaining five megacities.
• When PM2.5 levels in the current winter are compared with the average for the previous three winters, Bengaluru and Chennai’s performance emerges as the worst - their current winter air was 15 per cent more polluted than the average of their previous three winters.
• Mumbai’s winter air was 14 per cent and Hyderabad’s 3 per cent more polluted.
• Kolkata’s overall winter average of PM2.5 has improved compared to the previous three years but is stagnating since last year. Kolkata’s winter air was 8 per cent less polluted compared to the average of the previous three winters, but this winter’s pollution level is identical to that of last winter, showing a stagnant trend.
• On January 27, 2023, the daily PM2.5 level in Bengaluru hit 152 µg/m³ -- the highest 24-hour PM2.5 average recorded in the city since 2019. Similarly, Hyderabad registered its highest 24-hour PM2.5 average since 2019 this winter when on February 23, 2023, its daily average reached 97 µg/m³.
• Peak daily values this winter for Kolkata, Mumbai and Chennai were not as high as their previous winter peaks but still in the “very poor” AQI category. Kolkata’s winter peak stood at 162 µg/m³ (registered on January 21, 2023); for Mumbai, it was 148 µg/m³ (registered on January 18, 2023); and for Chennai, it stood at 139 µg/m³ (registered on October 24, 2022). Delhi’s peak pollution this winter stood at 401 µg/m³ (registered on November 3, 2022).
• Delhi, Kolkata and Mumbai had lower peaks compared to the average of the previous three winter peaks. Mumbai’s winter peak was 7 per cent lower, Kolkata’s 11 per cent lower and Delhi’s 23 per cent lower.
• The days with bad air quality occurred in clusters during the winter season in megacities. The clustering of bad air days was longer in Delhi, Kolkata, Mumbai and Hyderabad, but of shorter duration in Bengaluru and Chennai. The intensity and duration of these bad air days were long enough in Delhi to get classified as a smog episode. Other than Delhi, compared to the previous winter, the number of bad air days was more in other megacities.