Measurement of Wavelength-Resolved Light Absorption by Aerosols Utilizing a UV-VIS Extinction Cell

The principle, technical details, and performance of the long path extinction spectrometer (LOPES), a new folded–path extinction cell with a spectral range from the mid–UV (200 nm) to the near infrared (1015 nm), is presented. Using nonabsorbing glass beads the measured extinction spectrum of LOPES was validated by Mie calculations and was compared with scattering coefficients in the visible measured by a three–color integrating nephelometer (TSI, mod. 3563). For absorbing aerosols (here soot and biomass burning aerosol with single–scattering albedos of about 0.2 and 0.74 at 550 nm), LOPES was combined with a TSI 3563 nephelometer to determine the wavelength–dependent absorption coefficients from the difference of the extinction and scattering coefficients. These absorption coefficients were found to agree with the measurements by two state–of–the–art absorption techniques, the multiangle absorption photometry (MAAP) and photoacoustic spectrometry (PAS), which measure the absorption coefficient at the single wavelength of 532 nm and 670 nm, respectively. Finally, based on a comprehensive analysis of the TSI 3563 nephelometer errors and their propagation into the determination of the absorption coefficients from the LOPES and nephelometer data, we discuss implications for improving in situ measurements of the optical properties of atmospheric aerosols.

Acknowledgments

We thank Elisabeth Kranz for filter sampling and accomplishment of the thermographic carbon analysis. Georg Scheurig is thanked for providing SEM micrographs. This research was funded by the Max Planck Society and the Helmholtz Association of National Research Centres.