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South Asian Monsoons Remove Smog, Spread Pollution


FILE - Two men use a plastic sheet to protect themselves from heavy monsoon rain as they stand by the sea in Colombo, Sri Lanka, May 17, 2018.

There’s good news and bad news about the Indian summer monsoon. According to a new study, South Asian monsoons both help clean the atmosphere and spread pollution as far away as Tibet.

As South Asia burns fossil fuels, researchers from Germany say the clockwise-spinning storm pulls the emissions high into the troposphere. There, lightning-fueled chemical reactions transform the pollutants into more stable forms, which fall to earth as harmless rain.

However, not all the emissions reach those heights, and that is the pollution researchers found far from South Asia.

Jos Lelieveld of the Max Planck Institute for Chemistry and his fellow researchers believed that the circulating air currents carry pollutants upwards and gradually scatter them across a greater area.

To test this, the scientists made seventeen high-altitude flights over the seasonal monsoons to collect air samples, which they measured for chemical signatures of pollution specifically from South Asia.

A HALO (High Altitude Long Operation) plane is used to collect air samples from the Western edge of the South Asian monsoon. (Courtesy - Jos Lelieveld)
A HALO (High Altitude Long Operation) plane is used to collect air samples from the Western edge of the South Asian monsoon. (Courtesy - Jos Lelieveld)

By combining these measurements with computer models of air circulation, the researchers tracked the path of the contaminants and how they changed as they reached higher altitudes.

As expected, at lower altitudes, the South Asian monsoon did disperse pollutants over a wide region, including distant areas like Tibet. However, the scientists also found that the monsoon pulled the polluted air from the atmosphere into the much higher troposphere, where with the assistance of the storm’s lightning, it reacted with other gases, and could be washed out by the rain.

While this cycle of extracting pollution does help clean the local atmosphere, the air flow towards the troposphere high above earth is an incredibly slow process that can take weeks. Researchers say that it can take more than a month for the pollution to collect and get chemically processed into a non-harmful form.

Unfortunately, Lelieveld told VOA, “the monsoon is weakening, which can reduce the cleaning mechanism. We also believe that the air pollution contributes to a weakening of the monsoon.” He added, “Intuitively, if the monsoon weakens, the pollution will stay more near the ground rather than being transported upward.”

Although the South Asian monsoon has managed to keep pace with emissions thus far, the scientists say the continuing escalation of air pollution may be more than the storm can handle, leading to a breakdown of the system.

The research is published in the journal Science.

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    Sadie Witkowski

    Sadie Witkowski is a PhD candidate in Psychology at Northwestern University where she studies sleep and memory. She is also the 2018 AAAS Mass Media Fellow for Voice of America and is reporting this summer on science, medicine, and technology. She received her undergraduate degree from the University of Texas in Austin.

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