Research Briefs

Stratospheric sulfates have human cause

Scientists link coal, oil, and biomass to a layer of sulfates high above Asia

Pollution above Asia: Layer in stratosphere may originate from global sources

Factories line the shores of the lower Yangtze River in China. Although large amounts of sulfate aerosols are emitted by China and India, a layer of sulfur pollution that forms high above Asia each summer may include emissions from more distant sources. (©UCAR. Photo by Willliam Bradford.)

February 19, 2014 | Sulfur pollution from human activities is likely the cause of a large layer of sulfate particles, or aerosols, that form over Asia during the summer months, researchers have found. But it’s not just pollution from Asia that’s responsible.

In a paper recently published in the Journal of Geophysical Research, NCAR Advanced Study Program postdoctoral fellow Ryan Neely and his colleagues concluded that the layer of sulfate aerosols is most likely due to emissions from burning coal, oil, and biomass. About 30 percent of the emissions that contribute to the layer come from China and India. The rest of the sulfates come from more distant sources, including other parts of Asia, the Middle East, and North America.

“We thought the layer would be all due to emissions from China and India, since they’ve really ramped up their coal power production since the late 1990s,” Neely said. “But we found the monsoon concentrates pollution from other areas across Asia, like Saudi Arabia and Indonesia. We were able to show it’s from a larger, perhaps global source.”

The layer was first detected by satellite-based measurements in 1998, when there was a lull in colossal volcanic activity that appears to have masked the phenomenon previously. Known as the Asian Tropopause Aerosol Layer (ATAL), it forms between about 8 and 11 miles above Earth (near or above the height of commercial aircraft flights) between June and August, and extends from the eastern Mediterranean Sea to eastern China.

Researchers linked the formation of the seasonal layer to the monsoons that swirl across Asia each summer, trapping sulfur pollution and sending it high into the atmosphere where it straddles the tropopause, which is the dividing line between the troposphere and lower stratosphere.

Neely and his co-authors determined how the extent to which emissions from China, India, and other regions contribute to the ATAL by running two NCAR-based computer models: the NCAR-based Whole Atmosphere Community Climate Model and the Community Aerosol and Radiation Model for Atmospheres. They used the models to simulate the atmosphere with and without emissions from various regions and compared the results.

A layer similar to the ATAL also forms above North America (the North American Tropopause Aerosol Layer, or NATAL), but it is much weaker, Neely said. Researchers think that’s due to the less active monsoon dynamics in that region, as well as reductions in sulfur emissions in developed countries over the past four decades.

The ATAL and the NATAL do not have a significant climate impact because they only occur during three months of the year and cause a minimal amount of cooling. However, more research is needed to understand the extent the impact of increased sulfate aerosols on climate and air quality throughout the whole atmosphere, Neely said.

The study, “The Contribution of Anthropogenic SO2 Emissions to the Asian Tropopause Aerosol Layer,” was co-authored by researchers from the Cooperative Institute for Research in Environmental Sciences; the University of Colorado, Boulder; the National Oceanic and Atmospheric Administration; and the Massachusetts Institute of Technology.

Neely III, R. R., P. Yu, K. H. Rosenlof, O. B. Toon, J. S. Daniel, S. Solomon, and H. L. Miller (2014),The contribution of anthropogenic SO2 emissions to the Asian tropopause aerosol layer, Journal of Geophysical Research - Atmospheres, 119, doi:10.1002/2013JD020578.