The sulfate, emitted by burning of fossil fuels such as coal and oil, is thought to act as a sort of grimy shield by reflecting solar radiation back into space. It's theorized that the sulfate particles are offsetting the effect of the blanket of carbon dioxide and other greenhouse gases that could cause the Earth's surface to overheat. That could explain why the global temperature is not rising as quickly as once estimated.
But the "sulfate paradigm" appears to be flawed. Peter Hobbs, professor of atmospheric sciences at the University of Washington, and his colleagues have made the first direct measurements of atmospheric particles over the highly polluted U.S. East Coast and have found that sulfates are outnumbered by carbon particles. And carbon doesn't reflect solar radiation, it absorbs it. That, in theory at least, could increase global warming, not lessen it.
"Sulfate certainly plays an important role in the atmosphere, but so does carbon," says Hobbs. That means, he says, that the greenhouse warming puzzle is now deeper than before because atmospheric particles as a factor offsetting greenhouse warming may not be as great as previously thought.
Hobbs and his colleagues in the research, Dean Hegg, research professor of atmospheric sciences at the UW, and T. Novakov at Lawrence Berkely National Laboratory, gave the first report on the surprising findings at the spring meeting of the American Geophysical Union in Baltimore. The team was one of the research groups in the Tropospheric Aerosol Radiative Forcing Observational Experiment, or TARFOX, which last year set out to measure the effect of atmospheric particles on climate.
The UW's research, which was funded by the National Science Foundation, used the university's Convair 131 research aircraft to study atmospheric particles over a 200 mile-long urban corridor centered on Washington, D.C.
The researchers used data from satellites to locate an urban "plume" of atmospheric particles extending hundreds of miles from the coast into the Atlantic. In 14 flights on the UW aircraft, researchers sampled the constituents of a coastal section of the plume: their chemical properties, their size and their optical properties -- that is, whether they reflect or absorb solar radiation.
The high amount of carbon they found in the atmosphere is partly elemental carbon, or soot, from the burning of forests and from the inefficent use of fossil fuels. Other carbon sources are organic carbon, some from industrial emissions, but some from natural sources, such as ocean gases or forest-generated chemicals called turpines.
The new evidence that the atmospheric particles have less of a cooling effect than thought, conforms with another research project described by Hobbs in Science magazine earlier this year. It had been assumed that smoke from the burning of Amazon forests in Brazil had a large cooling effect worldwide, because smoke-generated particles would reflect solar radiation back into space. But airborne measurements of the smoke over the Brazilian forests revealed the smoke was strongly absorbing solar radiation, as well as reflecting it. "Consequently we found it had quite a bit less cooling effect than previously estimated, " says Hobbs. "That offset the radiation scattering properties that most people thought were dominant."
The next stage of the TARFOX project is to use the sampling data to calculate the magnitude of the effect of atmospheric particles on offsetting global warming by greenhouse gases. Says Hobbs: "You have to know what your particles are and what their optical properties are before you can calculate the relative magnitudes of their scattering, or cooling effect, compared with their absorbing, or warming effect, to find out the net effect. "
Once the net effect is known, scientists can calculate the implications for greenhouse warming. Hobbs guesses that the balance will still indicate a cooling effect, but significantly reduced from previous estimates.
How then to explain the fact that the Earth is not warming as fast as the greenhouse theory would have it? The answer, says Hobbs, might lie with the phenomenon of cloud condensation nuclei. These are microscopic particles in the atmosphere around which water vapor has condensed to form a cloud droplet. If a cloud contains more of these nuclei, it can increase cooling by becoming a far more efficient reflector of solar radiation back into space. However, no measurements this so-called "indirect effect" have yet been made. Indeed, the TARFOX team made their flights on cloudless days in order to avoid the presence of these droplets.
"Cloud cover is the most sensitive part of regulating the temperature of the Earth," says Hobbs. "That is why the indirect effect of these cloud particles could be potentially all important."
To contact Hobbs, call Judith Opacki at (206) 543-6026 or 543-6027