Is fluorescence of biogenic aerosols an issue for Raman lidar measurements?

During a measuring campaign in Lindenberg/Germany (14.5°E, 52.5°N) in August 2003, we observed an extended aerosol layer in the upper troposphere with our mobile Aerosol Raman Lidar (MARL). Backward trajectories indicated that this was a plume originating from forest fires. Water vapor Raman measurements performed with the same lidar showed a large discrepancy with co-located radiosonde measurements, which were not observed in undisturbed conditions. We interpret the unexpected properties of these aerosols as fluorescence induced by the laser beam at organic components of the aerosol particles. The detection of fluorescence from ambient aerosol with lidar systems has not yet been reported before. 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 induces inelastic backscatter signals which could be observed with the lidar. It provides for a new method to characterize atmospheric aerosols and should be taken into account when performing water vapor measurements with a Raman lidar.