More than three-quarters of the particulate pollution known as black carbon transported at high altitudes over the West Coast of the United States during spring is from Asian sources, according to a research team led by Professor V. Ramanathan at Scripps Institution of Oceanography, UC San Diego in the United States.

Black carbon concentrations diminish as they move farther away from their sources in cities and farmlands in countries such as China and India. However, over the Pacific Ocean, the particles are in sufficient concentration to have a heating effect on the upper atmosphere. At the same time, the radiation-absorbing particles dim skies at the surface.

Soot and other particulate pollution from Asian sources make up more than 75% of black carbon transported at high altitudes, according to the Scripps Institution of Oceanography, UC San Diego-led study

Though the transported black carbon, most of which is soot, is an extremely small component of air pollution at land surface levels, the phenomenon has a significant heating effect on the atmosphere at altitudes above two kilometers. As the soot heats the atmosphere, however, it also dims the surface of the ocean by absorbing solar radiation, said Ramanathan, a climate scientist at Scripps, and Odelle Hadley, a graduate student at the Center for Atmospheric Sciences at Scripps. The two are lead authors of a research paper appearing in the March 14 issue of the Journal of Geophysical Research.

The dual effect carries consequences for the Pacific Ocean region that drives much of Earth's climate.

"The soot heating of the atmosphere exceeds the surface dimming and as a result the long range transported soot amplifies the global warming due to increase in carbon dioxide," said Ramanathan. "We have to find out if this amplification is just restricted to spring time or is happening throughout the year."

The researchers found that transported black carbon from Asian sources is equal to 77% of North American black carbon emissions in the troposphere during the spring. In a follow-on study funded by the California Energy Commission, Hadley, Ramanathan and fellow Scripps climate scientist Craig Corrigan are now studying how much carbon might be incorporated into precipitation and what the effects on melt rates of Sierra Nevada snow pack could be.

Results from Hadley's study of black carbon's snow pack effects are expected by the end of this year.

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