Abstract
Abstract. Atmospheric aerosol particles like mineral dust, volcanic ash and combustion particles can reduce Earth's snow and ice albedo considerably even by very small amounts of deposited particle mass. In this study, a new laboratory method is applied to measure the spectral light absorption coefficient of airborne particles that are released from fresh snow samples by an efficient nebulizing system. Three-wavelength photoacoustic absorption spectroscopy is combined with refractory black carbon (BC) mass analysis to determine the snow mass-specific and BC mass-specific absorption cross sections. Fullerene soot in water suspensions are used for the characterization of the method and for the determination of the mass-specific absorption cross section of this BC reference material. The analysis of 31 snow samples collected after fresh snowfall events at a high-altitude Alpine research station reveals a significant discrepancy between the measured snow mass-specific absorption cross section and the cross section that is expected from the BC mass data, indicating that non-BC light-absorbing particles are present in the snow. Mineral dust and brown carbon (BrC) are identified as possible candidates for the non-BC particle mass based on the wavelength dependence of the measured absorption. For one sample this result is confirmed by environmental scanning electron microscopy and by single-particle fluorescence measurements, which both indicate a high fraction of biogenic and organic particle mass in the sample.
Highlights
Light-absorbing atmospheric particles like black carbon (BC), brown carbon (BrC), mineral dust or volcanic ash are eventually removed from the atmosphere by dry and wet deposition
Since these microphysical details of the soot particles are very sensitive to the actual formation and subsequent treatment conditions (Gorelik et al, 2002), it is conclusive that the mass-specific absorption cross section of fullerene soot (MACFS) has an even higher variability between different fullerene soot batches compared to what is expected from the SP2 mass sensitivity only
There is no clear correlation between the fresh snow samples and ambient air eBC mass concentration, the enhanced air eBC mass concentration observed at the end of March and beginning of April might have resulted in additional deposition of BC particles in the snow surface that is reflected – with a time lag of several days – in the measured snow refractory BC mass mixing ratio
Summary
Light-absorbing atmospheric particles like black carbon (BC), brown carbon (BrC), mineral dust or volcanic ash are eventually removed from the atmosphere by dry and wet deposition. For a reliable assessment of the radiative forcing by lightabsorbing impurities in snow and ice, the response of the snow albedo to the presence of light-absorbing particles has to be understood from a physical basis This is not a trivial task as the spectral albedo of snow depends on the mass mixing ratio of the absorbing particles in the snowpack, and on the chemical composition, the microphysical properties and the spectral absorption properties of the particles in addition to the snow grain size distribution and the spatial variations of these parameters (Flanner et al, 2007). There is a need for more studies that address the question on the microphysical nature and the optical properties of particles in snow and ice. In a large-area study on lightabsorbing impurities in Arctic snow, Doherty et al (2010) applied the integrating sandwich with integrating sphere technique (ISSW; Grenfell et al, 2011) to measure the snow mass-specific spectral absorption cross section, σabs, on filter samples.
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