Asian Pollution Rides the Monsoon

Like a great smokestack, Asia's summer monsoon is blowing high into the atmosphere a climate-altering cocktail of industrial pollutants generated by the burgeoning economies of China, India and Indonesia, scientists report.

For years now, sensors at ground-based monitoring stations have been detecting low-level pollution clouds riding the westerly winds from Asia to North America, but this is the first research that shows how the monsoon winds carry industrial chemicals into the stratosphere.

 
Pollutants such as black carbon, sulfur dioxide and nitrogen oxides are lofted 20 to 25 miles into the atmosphere by a vigorous natural circulation that is part of the annual monsoon pattern.

"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," said William Randel, a scientist at the National Center for Atmospheric Research in Boulder, CO, and lead author of the study published today in the online journal Science Express.  "As a result, the monsoon provides a pathway for transporting pollutants up to the stratosphere."

(Click on this satellite image of a hydrogen cyanide pollution plume over Asia and watch a video of the daily evolution of the monsoon anticyclone at an altitude of 10 miles during the summer of 2003.)

The scientists traced the land-based pollution plume by measuring the concentration of hydrogen cyanide, a chemical formed by burning of trees and vegetation, a byproduct of the clearing of Asian agricultural land during summer. Other circulation patterns originating in the Tropics contain only small amounts of the chemical.

While scientists know that the pollutants spend several years wafting around the globe in the stratosphere, Randel said more research will be needed to fully understand the atmospheric effects of the monsoon-borne pollutants.  For example, sulfur aerosols impact the ozone layer that blocks harmful ultraviolet radiation from reaching the ground.  Aerosol pollutants also interact with other greenhouse gases, such as water vapor, that influence the amount of solar heat reaching Earth.

IMAGE: Courtesy Science/AAAS