Study Finds Asian Monsoon Carries Pollution Into the Stratosphere

Time average mixing ratio (ppbv) of hydrogen cyanide (HCN) near 13.5 km during boreal summer (June- August) derived from satellite instrument observations. Arrows denote winds at this level derived from meteorological analysis, showing that the HCN maximum is linked with the upper tropospheric Asian monsoon anticyclone. Source: Randel et al., Science. Click to enlarge.

The Asian monsoon circulation provides an effective pathway for pollution from Asia, India, and Indonesia to enter the global stratosphere, according to a new international study led by scientists at the National Center for Atmospheric Research (NCAR) in Boulder, Colo. The finding, published online 25 March in the journal Science, provides additional evidence of the global nature of air pollution and its effects far above Earth’s surface.

Using satellite observations (from the Atmospheric Chemistry Experiment Fourier
Transform Spectrometer, ACE-FTS) and computer models, the research team determined that vigorous summertime circulation patterns associated with the Asian monsoon rapidly transport air upward from the Earth’s surface. Those vertical movements provide a pathway for black carbon, sulfur dioxide, nitrogen oxides, and other pollutants to ascend into the stratosphere, about 20-25 miles above the Earth’s surface.

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