Asian Monsoons Spread Pollutants

The economic growth in Asia in recent years has meant more pollution coming from the continent, and, according to a new study, that pollution is being wafted up into higher layers of the atmosphere during the Asian monsoons, which makes it longer-lived in the air.

This finding, detailed in the March 25 online issue of the journal Science, suggests that the impact of Asian pollutants on the stratosphere could increase in coming decades because of the growing industrial activity in China and other rapidly developing nations.

William Randel, of the National Center for Atmospheric Research (NCAR), and his colleagues suspected that the seasonal Asian monsoons might funnel air from the lower layer of the atmosphere, called the troposphere, to the layer above it, called the stratosphere. Such a mechanism could explain satellite measurements showing anomalous levels of stratospheric ozone, water vapor, and other chemicals over Asia during the summer. (A monsoon is a seasonal shift in the wind that tends to bring some rainy, violent weather with it.)

The team used a chemical called hydrogen cyanide — produced largely as a result of the burning of trees and other vegetation — as a way to trace the movement of parcels of air during the Northern Hemisphere summer months.

The team examined satellite measurements, which detected significant amounts of hydrogen cyanide throughout the lower atmosphere and up into the stratosphere over the monsoon region. Furthermore, satellite records from 2004 to 2009 showed a pattern of increases in the chemical's presence in the stratosphere each summer, correlating with the timing of the monsoon.

The observations also showed hydrogen cyanide, which can last in the atmosphere for several years before breaking down into other chemicals, moving over the tropics with other pollutants, including ones that contribute to acid rain and impact ozone levels, and then circulating globally.

The researchers then used computer modeling to simulate the movement of hydrogen cyanide and pollutants, such as black carbon, sulfur dioxides, and nitrous oxides, from other sources, including industrial activity. The model indicated that emissions of pollutants over a broad region of Asia, from India to China and Indonesia, were becoming stuck in the monsoon circulation and transported into the lower part of the stratosphere.

Once in the stratosphere, the pollutants circulate around the globe for several years. Some eventually descend back into the lower atmosphere, while others break down.

"The monsoon is one of the most powerful atmospheric circulation systems on the planet, and it happens to form right over a heavily polluted region," Randel, the lead author of the study, said. "As a result, the monsoon provides a pathway for transporting pollutants up to the stratosphere."

The research was funded by the National Science Foundation, NCAR's sponsor, together with NASA and the Canadian Space Agency.

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