Counting Carbon

To model climate change and plan new environmental policies, researchers must understand the myriad chemicals floating around in the atmosphere. And the atmosphere is swimming in carbon, such as greenhouse gases, volatile organic carbon (VOC) molecules, and secondary organic aerosols (SOA).

But to determine how we can regulate all these organic molecules, scientists must first determine where they come from. Now researchers describe a new isotopic fingerprinting technique that can help sort out atmospheric carbon's origins (Anal. Chem., DOI: 10.1021/ac1014436).

Water-soluble organic carbon (WSOC), one particular fraction of atmospheric carbon, forms when VOCs oxidize in the atmosphere into less-volatile derivatives. WSOC can both reflect sunlight back into space and seed clouds, says biogeochemist Õrjan Gustafsson of Stockholm University in Sweden, so they tend to have a cooling effect on climate.

WSOC can derive from man-made or natural sources, which researchers can distinguish isotopically. Plants synthesize VOCs, such as the chemicals responsible for that distinctive pine forest smell, from atmospheric CO₂, which contains some carbon-14. Meanwhile, fossil fuels' carbon-14 content has long since decayed and is essentially zero. So WSOC derived from living plants, including burned wood or biomass, has a different carbon-14/carbon-12 ratio than carbon released from fossil fuels.

In the past, researchers have detected these isotopic differences essentially by pumping air through filters and fingerprinting the trapped particulate material via accelerator mass spectrometry. Gustafsson and his colleagues modified these methods to specifically assess the WSOC fraction by adding a water-extraction step. The new method also measures the carbon-13/carbon-12 isotope ratio to gather information on the particular kind of plant matter, such as trees or grasses, that yielded the WSOC.

"This is in some sense 'environmental forensics,'" Gustafsson says.

The team tested the new method in a survey of Stockholm air in late summer 2009. They found that about 88% of the sampled carbon came from Sweden's boreal forests. Their data agrees with two other studies of WSOC that pinpoint biogenic emissions as important VOC sources in the U.S.

Gustafsson says that identifying what processes produce aerosols, such as burning fossil fuels or wood, can help regulators make more informed decisions regarding anthropogenic sources of atmospheric carbon.

But regulators should not forget about the biogenic fraction, says atmospheric aerosols expert Rodney Weber of Georgia Tech. He and other researchers have shown that biogenic WSOCs can be controlled partially by cutting other man-made pollutants, such as nitrogen oxides, because these anthropogenic chemicals help convert biogenic VOCs into organic aerosols.