Sulfate aerosols are airborne particles of pollution with sulfur as part of their structure. They reflect more solar energy than they absorb in the
atmosphere.
They play a key role in the debate about global warming. Before the early 1990's, computer models of the earth's climate showed a much greater warming than the real world when scientists simulated the amount of carbon dioxide and other global warming gases in the real world. Most scientists now believe this is because they did not take into account the temporary cooling effect of sulfate aerosols, and have found their models much closer to the real world after modification, and believe the concerns they raise are real.
Some scientists are not convinced that this was the real problem with the model. One scientist, Peter Hobbs, actually measured the effect on the temperature of the pollution over Washington D.C. It appears in some places sulfate aerosols are accomplanied by soot and carbon in the air, which have the opposite effect. In at least some places, the effect of the mixture may have little effect on climate - and perhaps may not explain why more warming has not occured previously.
Peter Hobbs himself is quite cautious, thinking only there is material for further study, and the cooling effect might exist but be less than previously predicted.
Sources:
http://www.ucar.edu/communications/quarterly/fall97/sciencebits.html
The Satanic Gases by Robert Balling and Patrick Michaels.