Light absorption spectra and carbon mass of fine particle water-soluble components were measured during the summer of 2010 in the Los Angeles (LA) basin, California, and Atlanta, Georgia.Fresh LA secondary organic carbon had a consistent brown color and a bulk absorption per soluble carbon mass at 365 nm that was 4 to 6 times higher than freshly-formed Atlanta soluble organic carbon.Radiocarbon measurements of filter samples show that LA secondary organic aerosol (SOA) was mainly from fossil carbon and chemical analysis of aqueous filter extracts identified nitroaromatics as one component of LA brown SOA.Interpreting soluble brown carbon as a property of freshly-formed anthropogenic SOA, the difference in absorption per carbon mass between these two cities suggests most fresh secondary water-soluble organic carbon formed within Atlanta is not from an anthropogenic process similar to LA. Contrasting emissions of biogenic volatile organic compounds may account for these differences.

Light-absorbing soluble organic aerosol in Los Angeles and Atlanta: A contrast in secondary organic aerosol