Progress on the analysis of aerosol optical properties from CARES

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Description

During June 2010, three multi-wavelength cavity ring-down extinction instruments were used at the T0 (Sacramento, California) and T1 (Cool, California) ground sites for the Carbonaceous Aerosols and Radiative Effects Study (CARES) field campaign to obtain aerosol optical properties and how they varied with size, relative humidity, and thermo-denuding. Light absorption coefficients were also measured at T0 using a photo-acoustic instrument operating at two wavelengths both before and after thermo-denuding. Some analysis of the rich data set has been completed and published, some is nearing completion, and some is planned for the near future. The analysis of the thermo-denuded aerosol extinction and absorption showed very little difference in the absorption when an optically significant amount of clear organic matter was removed from the particles (as judged by the change in extinction). This observation stands in contrast to the theoretical expectations that a clear coating on black carbon particles tends to increase the absorption coefficient via light focusing through lensing. Subsequent electron microscopy measurements suggest that a substantial fraction of the BC-containing particles sampled during CARES did not have a core-shell morphology (which leads to the largest enhancement factors), consistent with the optical measurements. In a separate line of investigation, the dependence of extinction or scattering on relative humidity, characterized by an optical hygroscopic aerosol growth factor, γ {where b_ext (%RH) = b_ext (dry) * (100% - %RH)^-γ} was found to depend strongly on particle size and composition, with some periods of higher than expected particle growth for mainly organic aerosols. The role of super-micron particles (D_p > 1.0 μm) in the observed optical growth is still being investigated and appears to have been important, but it still seems likely that some of the organic aerosols observed during CARES had greater than expected hygroscopicity. Planned future analyses also include the use of three-wavelength extinction coefficient data at both ground sites to produce a separation of fine and coarse mode scattering and an estimate of the effective radius for the observed particle distributions, potentially allowing a complete retrieval of size and hygroscopic growth, using only the optical data for future field campaigns and longer-term unattended monitoring applications.