New global warming element

Amanda Francoeur, amanda.francoeur@laurin.com

TEMPE, Ariz. – Researchers at Arizona State University have found yet another culprit in the onslaught of global warming – and it’s called brown carbon.

Taking brown carbon into account in global circulation models, which previously were based only on estimated amounts of black carbon (soot) and other aerosol pollutants, may offer a more sensitive model for forecasting increases in the Earth’s temperature. The findings were published in the Aug. 8, 2008 issue of Science.

A cluster of carbonaceous aerosol particles was captured using carbon netting. The brown carbon particle is the large sphere, whereas the black carbon aggregate comprises much smaller particles. Including brown carbon in existing climate models will help researchers more accurately forecast temperature changes caused by global warming.

The brown carbon, which consists of carbonaceous aerosol particles, typically transpires from man-made sources such as fossil fuel combustion. Unlike greenhouse gases, which spread throughout the Earth’s troposphere (lower level of the atmosphere) and last for several years, aerosols have a short life span and remain only over regions where they are produced.

Identifying carbons

The investigators discovered the brown carbon by studying the optical absorption of ambient aerosols from East Asia using a transmission electron microscopy technique. The method couples information from the microscope’s monochromated electron source with electron energy-loss spectroscopy. “One advantage of this technique is that it permits us to characterize individual particles in a sample so there is no averaging, and you can perform a precise identification of aerosol species,” said Peter A. Crozier, an associate professor in the university’s School of Materials program.

However, the method comes with a few downsides: It isn’t automated, and sampling statistics are limited, Crozier noted.

Other methods determine only average light emissions produced by a group of particles; techniques such as laser scattering can measure optical properties using only a few wavelengths at a time. The electron microscopy technique can measure refractive indices from <400 to 1200 nm, fully encompassing the visible spectrum.

The researchers found that, unlike black carbons, which absorb light uniformly across the visible spectrum, brown carbons absorb light more in the blue, at a wavelength of 550 nm, and weakly in the red, constituting its brown color. These results indicate that brown carbon is a separate entity from black and from other weakly absorbing carbons.

According to Crozier, there are in fact many types of brown carbon, and those with deeper color absorb more light and produce more heat. The researchers say that climate prediction models must take into account these brown carbons because including only black carbon models might lead to an overestimation of temperature changes.