European pollution outbreaks during ACE 2 : Lofted aerosol plumes observed with Raman lidar at the Portuguese coast

Abstract

During the Second Aerosol Characterization Experiment (ACE 2) in the summer of 1997, four pollution outbreaks from the European continent were monitored at Sagres (37 degreesN, 9 degreesW), Portugal, by applying the Raman lidar technique at a wavelength of 532 nm. The findings are presented in terms of vertical profiles of the volume extinction coefficient of the particles, the backscatter coefficient, the extinction-to-backscatter ratio (lidar ratio), the relative humidity, and the geometrical and optical depths of the marine boundary layer and the continental aerosol layers aloft. The lidar-derived optical depths are compared with results of simultaneously conducted star photometer observations. During all outbreaks, similar aerosol profiles were observed. Above the cold marine boundary layer the warm continental pollution plume extended from about 500 to a maximum height of 3500 m. Strong temperature inversions prohibited mixing of the two layers. The ACE 2 mean optical depths of the marine boundary laver and the aerosol layers aloft were about 0.05 and 0.1, respectively. Maximum values of the optical depth in the pollution plumes were close to 0.25 at 532 nm. On average, the free-tropospheric layers contributed about 60% to the total optical depth at the coast. Volume extinction coefficients were mainly between 30 and 70 Mm(-1) and the lidar ratios ranged from 30 to 80 sr in the polluted continental air.