Ten years of multiwavelength Raman lidar observations of free-tropospheric aerosol layers over central Europe : Geometrical properties and annual cycle
We present geometrical properties and seasonal variations of appearance of aerosol particle pollution in the free troposphere over the central European lidar site at Leipzig, Germany. The data set has been acquired with Raman lidar in the past 10 years in the framework of the German Lidar Network (1997-2000) and since 2000 in the framework of the European Aerosol Research Lidar Network (EARLINET). In summary we analyzed 1028 measurements. Geometrical depth of the pollution layers was 5 km were found in 10% of all cases. Traces of particle pollution were detected up to the height of the tropopause. Forest-fire burning in North America causes intrusion of particles into the stratosphere. Seven hundred seventeen of all observations were carried out on the basis of a regular measurement schedule which allows us to establish a statistic on the frequency of particle transport in the free troposphere. In 43% of the regular measurements we observed pollution above the continental boundary layer. The lofted particle layers largely result from intercontinental long-range transport. We use backward trajectory analysis to identify the main source regions of the lofted pollution layers. In 19% of all regular measurements, free-tropospheric pollution was advected from North America. Forest-fire smoke from Canada and anthropogenic pollution from urban areas of the United States of America and Canada were the sources of the particle layers. We find a strong seasonal dependence of occurrence of these layers with a peak in June-August of each year. In a few cases we observed forest-fire smoke advected from Siberia and east Asia with winds from westerly directions. Pollution advected from areas north of 70 degrees N presents another transport channel. That pollution consists of Arctic haze or mixtures of haze with anthropogenic pollution. The main occurrence of such particle layers is around springtime of each year. Import of mineral dust from the Sahara represents another transport path. Most of such cases are observed during late springtime and summertime. Free-tropospheric pollution advected from east and southeast Europe and Russia presents one transport channel from within the Euro-Asian continent.