Light-absorbing properties of particulate matter samples collected on quartz filters were studied using a UV/VIS spectrometer in the 330-1000 nm wavelength range. Variation in absorbing properties due to differences in particulate matter emission sources was investigated by analyzing samples collected in different environments: a roadway tunnel carrying diesel vehicles, diesel bus exhaust, and smoke plumes of burning savanna. The contribution of organic carbon to light absorption was assessed by comparing light transmission through samples that were treated with acetone and unaltered samples.

The spectral dependence of absorption by a roadway tunnel sample approximately conformed to the l^-1 dependence that is predicted from theoretical consideration of small black carbon spheres with constant refractive index. The spectral dependence of a biomass smoke sample at wavelengths larger than 600 nm was similar to that of the roadway tunnel sample. However, the biomass smoke sample exhibited enhanced absorption at shorter wavelengths (i.e., < 600 nm) compared to the tunnel sample. Much of this enhancement disappeared following extraction of organic carbon from the sample with acetone. These observations demonstrate that organic carbon in the biomass sample impacts the absorption of solar radiation. In contrast, light-absorption by the tunnel sample was not changed due to treatment with acetone, indicating that the organic carbon in the tunnel sample does not appreciably impact absorption of solar radiation.

We estimate that, for the biomass smoke sample, ~20% of the light absorption at 550 nm and ~50% of the light absorption at 400 nm is due to presence of organic carbon. The derivation of these numbers will be discussed at the conference. Additional samples will be analyzed to confirm these preliminary results.

The LBNL evolved gas analysis technique was used to measure the mass of organic and black carbon in each sample, as well as the mass of organic carbon extracted from the samples with acetone. Mass absorption cross-section (s, m² g^-1) was calculated by dividing light-attenuation (by the sample deposit on the quartz filter) by carbon mass concentration. Preliminary s values of black carbon (of the diesel bus sample) and acetone-extractable organic carbon (of the biomass smoke sample) are 36 and 6 m² g^-1 at 400 nm, respectively. We note that these s values correspond to particles deposited on quartz filters, and it is known that the reflective quartz filter fibers enhance particle light absorption compared to light absorption by the same particles suspended in the atmosphere.