Raman Intensities of Mobility-Size Selected Sub-micron Black Carbon Particles

Raman Spectroscopy (RS) and IR spectroscopy employ the rotational and vibrational frequencies of molecules to identify compounds. Because Raman emission is a type of scattering, it can be measured for individual micron-sized particles using visible sources and detectors, and hence has several advantages for single-particle aerosol characterization. In measurements of Raman spectra of ambient aerosol using an aerosol Raman spectrometer, the number of spectra consistent with black carbon (BC) particles has been higher than those for other particle types, including the spectra of fluorescent materials or inorganic particles. This observation may appear surprising in part because BC has a low mass percentage in PM_2.5. Most BC in the atmosphere is formed as a result of incomplete combustion of biomass and hydrocarbon fuels and is found primarily in PM_2.5. The observations of high numbers of spectra of BC have been suggested to occur because BC has a higher Raman cross section than inorganic particles. Thus, very small, low mass, BC particles should be more readily observed than larger, more massive particles of typical inorganic species. However, relatively little research on the variations of Raman spectra of BC aerosols with particle size and concentration has been reported. This project aims to determine the correlation, if any, between the size of BC particles and their Raman intensities, for particles between 100 and 500 nanometers. In these experiments, the main type of BC studied is Printex 90. Raman spectra of different size particles and concentrations of Printex were measured with an aerosol Raman spectrometer (ARS). Particles of different mobility diameters were selected using a Sizing Mobility Particle Scanner (SMPS). The Raman intensities were then compared with particle mobility diameter. This work also attempts to establish the sampling conditions which result in single particle deposition of Printex 90 in the aerosol Raman spectrometer. Raman Spectra consistent with 100-300 nanometer particles were detected. In some cases concentrations of less than 100 particles per cubic centimeter exhibited weak or no Raman intensities in the characteristic D and G bands. At higher concentrations (>100 particles/cc) the Raman intensities were larger, and it appears that the Printex particles formed agglomerates.