Has soot been unfairly blamed?

But recent field tests near cities in California indicate that such chemicals may only boost the warming effect of black carbon by about 6%.

"Our measurements indicate the condensation of material onto black-carbon particles influences their warming ability to a significantly lesser extent than models predict and laboratory experiments had suggested," Chris Cappa of the University of California, Davis, US, told environmentalresearchweb. " This in turn indicates that global climate models may be overstating the warming ability of black-carbon particles."

Cappa believes that the observations have important implications for understanding the effects of the particles – and the role of technology changes and regulations – in changing regional and global climate.

"In the atmosphere, these black-carbon particles undergo significant changes as material that is formed from photochemical activity condenses onto them," he said. "The addition of this condensed material to the black-carbon particles can influence the mass, size, shape and chemical composition of the particles, all of which may affect the efficiency with which the black-carbon component absorbs solar radiation, and thus their warming ability."

Cappa and colleagues used a photoacoustic spectrometer developed jointly by the University of California Davis and the US National Oceanic and Atmospheric Administration (NOAA) to analyse the particles' optical properties, and a novel soot-particle aerosol mass spectrometer (SP-AMS) developed by Aerodyne Research to measure their chemical and physical properties.

The team compared the absorption of ambient black-carbon-containing particles with that of particles after thermal heating to remove the condensed material, which is volatile. "This technique provided a direct measure of the influence of the condensed material on the warming ability of black-carbon particles, in contrast to past studies that relied on studying changes in the absorption properties over time," said Cappa.

Now the team plans to carry out field testing in other parts of the world, and measure black-carbon particles emitted into the atmosphere by "other combustion sources, in addition to the urban environments in northern–southern California sampled in this study". This is likely to include black carbon produced by biomass burning, biofuels and coal combustion, and black carbon in regions that employ a different mix of fossil-fuel black-carbon sources.

The researchers reported their results in Science.