On the contrary, Mumbai recorded the highest deterioration with a 30 percent increase in PM2.5 levels, followed by other peninsular Indian cities like Coimbatore (28 percent), Bengaluru (20 percent), Chennai (12 percent), etc.

Many North Indian cities almost reached the 5-year-long target set under the National Clean Air Programme in no time.

What caused this had been a puzzle, the researchers said.

“The winter of 2022-23 coincided with the last phase of an unusual triple-dip La Niña event, the first in the 21st century.

This phenomenon, influenced by climate change, impacted the large-scale wind pattern, playing a decisive role in preventing stagnation conditions in north Indian cities and thus improving air quality,” said R H Kripalani, a climate scientist at the Indian Institute of Tropical Meteorology and co-author of the report.

In contrast, it led to calmer conditions in peninsular Indian cities, accelerating transboundary pollution and significantly deteriorating air quality, he added.

“The dominance of higher northerly winds at the transport level forced an influx, along with relatively slower winds near the surface, trapping pollutants in peninsular India and increasing PM2.5 concentration.

Conversely, feeble western disturbances, unique wind patterns, and the absence of rain, clouds, and faster ventilation led to a significant improvement in air quality in the north,” Kripalani said.