Raman lidar and sunphotometric measurements of aerosol optical properties over Thessaloniki, Greece during a biomass burning episode

Abstract

The influence of biomass burning smoke on the aerosol loading in the free troposphere over Thessaloniki, Greece (40.5°N, 22.9°E) is discussed in this paper. A selected case during summer 2001 is presented, when very high aerosol optical depth values were observed, benefiting from the synergy of various remote sensing instruments. The data that were collected allow the characterization of the optical properties of the aerosols in this region, where only little information has so far been available. Four-day back trajectories indicated that air masses were advected from Bulgaria and the northern coast of the Black Sea, where strong forest fires occurred in early August 2001. In order to investigate the optical properties of biomass burning aerosols, we used a two-wavelength lidar system that combines Raman and elastic-backscatter observations, in addition to a Brewer spectrophotometer, a nephelometer and a multi-filter rotating shadowband radiometer. The lidar measurements on 9 August 2001 recorded an integrated aerosol optical depth at 355nm of the order of 1.35 during cloud-free conditions. The estimated mean extinction-to-backscatter ratios from the Raman lidar were 60sr for 355nm and 50sr for 532nm. Estimated values of the single scattering albedo, using spectral UV measurements and modeling were of the order of 0.90, consistent with previous findings, indicating a weak contribution of absorption to the total extinction. The Angstrom exponent, calculated from the multi-filter rotating shadowband radiometer exhibited also high values around 1.78, indicating the presence of rather small particles. © 2003 Elsevier Ltd. All rights reserved.