During a measuring campaign in Lindenberg/Germany (14.5°E, 52.5°N), we observed in June 2003 an extended aerosol layer at 13 km altitude in the lowermost stratosphere with a mobile Aerosol Raman Lidar (MARL). The stratospheric layer created an inelastic backscatter signal that we detected with a water vapour Raman channel, but that could not have been produced by Raman scattering. Also, we find evidence for inelastic scattering by a smoke plume from a forest fire that we observed in the troposphere. We interpret the unexpected properties of these aerosols as fluorescence induced by the laser beam at organic components of the aerosol particles. Fluorescence from ambient aerosol had not yet been considered detectable by lidar systems. However, organic compounds such as polycyclic aromatic hydrocarbons sticking to the aerosol particles, or bioaerosol such as bacteria, spores or pollen fluoresce when excited with UV-radiation in a way that is detectable by our lidar system. Therefore, we conclude that fluorescence from organic material released by biomass burning creates, inelastic backscatter signals that we measured with our instrument. Fluorescence of aerosols shoul be taken into account for Raman lidar measurements of water vapor. On the other hand it also provides for a new technique to characterize atmospheric aerosols.
Helmholtz Research Programs > MARCOPOLI (2004-2008) > POL1-Processes and interactions in the polar climate system