Multi-Wavelength Measurement of Aerosol Optical Properties: Laboratory Intercomparison of In-Situ and Filter Based Techniques

Atmospheric aerosol absorption properties play a key role on the Earth radiation balance. Although both in-situ and filter-based instruments have been developed in the last years to measure aerosol optical properties, no reference instrument exists for their determination yet, due to particles complexity and technological issues. Filter-based methods are the most commonly used, even though multiple scattering and possibly loading effects have to be accounted for. In the Multi-Angle Absorption Photometer (MAAP) and the home-made multi-λ polar photometers PP_UniMI[1] and MWAA (Multi-Wavelength Absorbance Analyzer[2]) - this is accomplished by determining the total light transmitted and diffused in forward and back hemispheres and applying radiative transfer models. Opposite, some commercially available instruments as the Aethalometer, the Particle-Soot Absorption Photometer (PSAP), and the Tricolor Absorption Photometer (TAP) are based on light transmission only, and different correction schemes relying on measurements or assumptions on scattering properties and loading effects are needed to obtain absorption properties. In this work, optical properties of laboratory-generated particles and mixtures were measured at Jülich Forschungszentrum (Germany)[3]. The main goal was to test the performances of filter-based measurements of aerosol absorption coefficient by the above-mentioned polar photometers. Atomized particles of ammonium sulphate and/or Cabot soot or absorbing aerosol produced via an Inverted Flame Soot Generator were dried and sent in a mixing chamber; both in-situ and filterbased instruments were deployed downstream to retrieve the absorption coefficient of the test samples. Light extinction and scattering coefficients at 450 and 630 nm were obtained by two Cavity Attenuated Phase Shift CAPS PMSSA (Aerodyne Research). An integrating Nephelometer (TSI) measured total and back- scattering coefficients at 450, 550, and 700 nm. A MAAP (Thermo; λ=635 nm) and a TAP (Brechtel; λ: 467, 528, 652 nm) were employed to obtain filter-based aerosol absorption coefficients. At the end of the sampling line, a 47 mm diameter filter holder equipped with a Mass Flow Controller was used to produce aerosol samples to be measured off-line by multi- λ absorption coefficient with PP_UniMi (λ: 405, 532, 635, 780 nm) and MWAA (λ: 375, 407, 532, 635, 850 nm). In the presentation, a detailed validation of the multi-wavelength MAAP approach against different techniques applied in this study will be reported. Differences in multi-wavelength determination of the absorption coefficient obtained by the multi-wavelength MAAP approach and other methods will be evidenced, considering their role in the assessment of different aerosol components and sources during field campaigns.