Light absorbing properties of particles extracted from snow samples

Although BC is the dominating absorbing component in the atmosphere, there is evidence of numerous other aerosol components which are able to contribute to the aerosol light absorption. Such non-BC particles include mineral dust, volcanic ash, brown carbon as well as biological debris. Sooner or later different physical processes remove these airborne particles from the atmosphere and deposit them on the ground. If they are deposited on snow or ice surfaces these particles might induce melting processes by the absorption of solar radiation. Because snow is the most reflective natural surface on earth, already small amounts of absorptive impurities can significantly reduce the surface albedo. The contribution of the albedo reduction on the radiative balance of the atmosphere is usually assessed based on the mass concentration of refractory BC deposited in the snow. Ignoring the absorption contribution of the non-BC aerosol components might result in an underestimation of the albedo reduction. Therefore, the knowledge of the visible absorption coefficients of aerosol particles deposited in snow and ice, is essential for a realistic evaluation of the albedo effect on regional and global climate. To specify the absorbing properties of particles trapped in snow samples, we combined photo acoustic absorption spectroscopy with single particle black carbon mass analysis using a Single Particle Soot Photometer (SP2). In this way, the aerosol absorption coefficients in the visible spectral range and the refractory black carbon mass could be simultaneously derived. This type of analysis is quite challenging because of the low particle mass concentrations typically present in natural snow samples. In a first case study we used a set of snow samples from the Environmental Research Station (UFS) Zugspitze. Our results show significantly enhanced light absorption of the particles released from the snow samples compared to the particle absorption that can be linked to the refractory black carbon mass in the snow. Further analyses revealed a substantial portion of biological material within the snow that could be cause for the observed additional aerosol absorption.