Case study of a multi-layer aerosol structure in the eastern Mediterranean observed with the airborne polarized lidar ALEX during a STAAARTE campaign (7 June 1997)

We present a case study of tropospheric aerosol transport in the eastern Mediterranean, based on airborne measurements obtained south of Greece on 7 June 1997. Airborne observations (backscattering lidar at 0.532 mm with polarization measurements, in situ particle counters/sizers, and standard meteorological measurements) are complemented by monitoring with Meteosat visible and infrared images and a ground-based sun-photometer, air-mass back-trajectory computations, and meteorological analyses. The vertical structure of the lower troposphere appears complex with horizontal variability and a superposition of several turbid layers from the surface up to the clean free troposphere which is found above 2 to 4 km in altitude. We identify the presence of depolarising dust from northern Africa in the most elevated turbid layer, which is relatively humid and has clouds embedded. The lowermost troposphere likely contains pollution water-soluble aerosols from eastern continental Greece, and an intermediate layer is found with a probable mixture of the two types of particles. The column optical depth at 0.55 mm estimated from Meteosat is in the range 0.15&ndash;0.35. It is used to constrain the aerosol backscattering-to-extinction ratio needed for the backscattering lidar data inversion. The column value of 0.017 sr^&minus;1 is found applicable to the various aerosol layers and allows us to derive the aerosol extinction vertical profile. The aerosol extinction coefficient ranges from 0.03 km^&minus;1 in the lower clean free troposphere to more than 0.25 km^&minus;1 in the marine boundary layer. Values are <0.1 km^&minus;1 in the elevated dust layer but its thickness makes it dominate the aerosol optical depth at some places.