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. As a result, the monsoon provides a pathway for transporting pollutants up to the stratosphere.

—NCAR scientist William Randel, the lead author

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

Scientists have long known that air over the tropics moves upward between the lower atmosphere and the stratosphere, part of a large-scale pattern known as the Brewer-Dobson circulation. But Randel and his colleagues suspected that the monsoon might also transport air into the stratosphere during the Northern Hemisphere’s summer months. This could explain satellite measurements showing high levels of stratospheric ozone, water vapor and other chemicals over Asia during summer.

To isolate the role of the monsoon on the stratosphere, the researchers focused on a chemical, hydrogen cyanide, which is produced primarily as a result of biomass and biofuel burning, and is often used as a tracer of pollution originating from fires.

The parcels of air over the tropical ocean that are lifted to the stratosphere by the Brewer-Dobson circulation contain low amounts of hydrogen cyanide, which breaks up over the ocean. But air over land that gets lifted up by the monsoon contains high levels of the chemical, especially during times of year when Asia has widespread fires, many set to clear land for agriculture.

When the scientists examined satellite measurements, they detected significant amounts of hydrogen cyanide throughout the lower atmosphere and up into the stratosphere over the monsoon region. 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 up, moving over the tropics with other pollutants and then circulating globally.

The researchers used computer modeling to simulate the movement of hydrogen cyanide and pollutants 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 entrained in the monsoon circulation and transported into the lower stratosphere.

The study suggests that the impact of Asian pollutants on the stratosphere may increase in coming decades because of the growing industrial activity in China and other rapidly developing nations.

In addition, climate change could alter the Asian monsoon, although it remains uncertain whether the result would be to strengthen or weaken vertical movements of air that transport pollutants into the stratosphere.

Randel says more research is needed into the possible effects of the pollutants. When sulfur rises into the stratosphere, it can lead to the creation of small particles called aerosols that are known to influence the ozone layer.

The monsoon transport pathway may also have effects on other gases in the stratosphere, such as water vapor, that affect global climate by influencing the amount of solar heat that reaches Earth.

This is a vivid example of pollutants altering our atmosphere in subtle and far-reaching ways.

—William Randel

In addition to the NCAR researchers, the team included scientists from the Universities of Waterloo and Toronto in Canada, the University of York in England, and the University of Edinburgh in Scotland. The research was funded by the National Science Foundation (NSF), NCAR’s sponsor, along with NASA and the Canadian Space Agency.

Resources

• William J. Randel, Mijeong Park, Louisa Emmons, Doug Kinnison, Peter Bernath, Kaley A. Walker, Chris Boone, Hugh Pumphrey (2010) Asian Monsoon Transport of Pollution to the Stratosphere. Science doi: 10.1126/science.1182274