Rain no respite, pollutants return to Delhi's air in 5 days: Study

Rain no respite, pollutants return to Delhi's air in 5 days: Study

New Delhi, Rains may wash away pollution from Delhi's air, but polluting particles relentlessly return within five days, indicating the need for sustained emission reduction measures in the national capital, a study spanning 22 years showed.

Rain no respite, pollutants return to Delhi's air in 5 days: Study

The study, which analysed 612 rainfall events between 2003 and 2024, found that pollution particles returned to the pre-rainfall levels within five days in 310 cases, accounting for more than 50 per cent of the cases.

Researchers said the recovery of pollution levels is largely influenced by continued emissions. Vehicles, industries, construction activities and other sources continue to release pollutants into the air, while new particles can also form through chemical reactions in the atmosphere.

"Our study shows that rainfall provides only temporary relief from Delhi's aerosol pollution. This highlights the importance of sustained emission reduction measures rather than relying on rainfall for lasting improvements in air quality," Dr Satheendran, the corresponding author, told PTI.

Researchers also found that heavier rain did not necessarily provide longer-lasting relief, as the amount of rainfall had no significant link to how quickly pollution particles returned to their earlier levels.

Experts said the time taken for return to pre-rainfall pollution levels depended on continued emissions from human activities, pollutants already present in the atmosphere and the formation of new particles. It could vary depending on the intensity of these emissions.

Researchers said the time taken for pollution levels to return to their pre-rainfall levels also varied across seasons. It was six days during the monsoon, five days in summer and winter, and three days during the post-monsoon period.

These are median figures, which means they represent the middle recovery time recorded across rainfall events in each season.

To conduct the research, scientists combined satellite observations of pollution particles with rainfall and weather data in Delhi over 22 years from 2003 to 2024. They identified rainfall events in which daily precipitation exceeded 5 mm and tracked pollution particle levels for up to 30 days after each event.

The researchers used a satellite-based measurement called Aerosol Optical Depth, which measures how much light is blocked or scattered by pollution particles in the atmosphere. However, this does not directly measure pollution at ground level or the Air Quality Index.

The study found no significant change in the time taken for pollution particle levels to return to their pre-rainfall levels over the 22-year period.

Titled "Why rainfall alone cannot mitigate urban pollution: Two decades of satellite observation over Delhi", the study was conducted by Gayathri Balachandran and Dr Sumith Satheendran S of the Amrita-Natural Resource Monitoring Laboratory, Amrita Vishwa Vidyapeetham, Kerala.

It was published in the Journal of Atmospheric and Solar-Terrestrial Physics.

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