Despite the importance of free-tropospheric particles on climate and air quality [McKendry et al., 2001; Jacob et al., 1999; Creilson et al., 2003; Collins et al., 2000; Prather et al., 2003] their optical and microphysical properties, their geometrical features, and particle transport mechanisms are still poorly understood. This lack of understanding becomes obvious in the first textbook review on present-day knowledge of long-range transport of atmospheric pollution [Stohl, 2004]. The review mainly focuses on trace gases from manmade pollution. The transport of gases from pollution sources can be described comparably well. Transport mechanisms of gases however cannot readily be applied to aerosol particles as, for instance, particle washout and other cloud processing mechanisms have additional influence on the presence and transport of particle pollution. For example, observations of particles carried out with lidar in the Indian Ocean [Ansmann et al., 2000; Müller et al., 2000; Franke et al., 2003; Léon et al., 2002] and in the framework of the European Aerosol Research Lidar Network (EARLINET) [Bösenberg et al., 2003] are in contradiction to the common assumption that measurements of trace gases can be used to explain the presence as well as the transport of particulate pollution [Lelieveld et al., 2001, 2002].
 Andreae et al.  carried out first detailed airborne observations of long-range transport of Euroasian emissions to the remote northeast Pacific troposphere in 1985. The authors observed air masses from east Asia that had traveled in heights around 3–6 km for 4–8 days before observation. Long-range transport of aerosols was observed over the North and South Pacific in the framework of airborne studies in 1990 [Clarke, 1993]. Jaffe et al.  reported on transport of Asian air pollution to North America. Bertschi et al.  and Bertschi and Jaffe  subsequently provided a detailed study of long-range transport to the northeast Pacific on the basis of aircraft flights in 2002. Lofted layers were observed from ground level to 6 km height. Geometrical depth of the layers of 0.2–3 km are reported. The measurements were carried out in the frame of PHOBEA (Photochemical Ozone Budget of the Eastern North Pacific Atmosphere) activities which lasted from 1999 until 2003. Studies on black carbon from biomass burning emission were carried out by Clarke et al.  over the Pacific Ocean and by Clarke et al.  in the outflow region of Southeast Asia.
 Aerosol pollution transport in the free troposphere over the North Atlantic was studied with airborne instrumentation in the vicinity of the Azore islands during ASTEX (Atlantic Stratocumulus Transition Experiment) in June 1992 [Clarke et al., 1997]. First detailed airborne studies of optical and microphysical properties of lofted aerosol pollution from North America were reported by Petzold et al.  and Fiebig et al. . The authors observed an forest-fire smoke plume that had been advected from west Canada to central Europe. These studies were carried out in the framework of LACE (Lindenberg Aerosol Characterization Experiment) [Ansmann et al., 2002]. LACE also provided the first data of such events over central Europe from ground-based Raman lidar observations [Wandinger et al., 2002].
 Knowledge on aerosol pollution long-range transport and particularly knowledge on black carbon in such plumes was significantly broadened by Petzold et al. , who carried out detailed airborne studies in summer 2004 in the framework of ICARTT (International Consortium for Atmospheric Research on Transport and Transformation). Clarke et al.  also reported on studies on forest-fire plumes over the North Atlantic in the framework of ICARTT.
 Optical and microphysical properties of boundary-layer aerosols which originate from local and regional emissions of particles and gases, usually are very different from free-tropospheric particles which are often advected over large distances from other continents. Information on the altitude in which particle layers appear is important for calculating aerosol radiative forcing. It depends on the atmosphere's albedo, and it makes a significant difference in the radiative impact whether the aerosol particles are above a cloudy boundary layer or within the boundary layer [Wagner et al., 2001; Keil and Haywood, 2003; Forster et al., 2001].
 Particle layers in the free troposphere often are optically very thin with an optical depth of 0.01–0.03 at 500 nm wavelength. Thus their direct effect on climate forcing may be low. However it is widely unknown how these aerosol particle layers influence formation of clouds and precipitation. They increase the free-tropospheric background aerosol load and may therefore have a comparably strong effect on indirect climate forcing, because of their large-scale structures, which may reach continental dimensions [Ansmann et al., 2005] and transport distances on the hemispheric scale [e.g., Damoah et al., 2004; Müller et al., 2003; Wandinger et al., 2002].
 Passive remote-sensing instrumentation is limited in documenting and characterizing long-range transport of particles in the free troposphere. For instance first results on long-term satellite observations of aerosol pollution on the global scale were reported by Husar et al. . Satellite passive sensors however cannot separate between particles in the boundary layer and pollution in the free troposphere. It still is a very challenging task to identify aerosols over land with such instruments. The retrieved parameters rather describe a mixture of particle types. The same also holds true for ground-based sensors like Sun photometers.
 Differences between free-tropospheric pollution and boundary layer particles are described by Wandinger et al. . The authors for the first time documented in detail an event of intercontinental long-range transport on the basis of vertically resolved measurements with multiwavelength Raman lidar. Forest-fire smoke generated over west Canada was transported to central Europe in the course of 6 days [Fiebig et al., 2002], and observed in the framework of the Lindenberg Aerosol Characterization Experiment 1998 (LACE 98) in the summer of 1998 [Ansmann et al., 2002].
 A detailed summary of our observations of lofted aerosol pollution during the Aerosol Characterization Experiment 2 (ACE 2) in southern Europe in 1997, LACE 98 in central Europe, and four campaigns during INDOEX in south Asia in 1999–2000 is given by Müller et al. [2007a].
 In the following years we intensified our observations of free-tropospheric pollution with our stationary Raman lidar in the framework of the German Lidar Network (AFS-Deutsches Lidarnetzwerk) [Bösenberg et al., 2001] and EARLINET [Mattis et al., 2004]. Mattis et al.  reported for the first time height-resolved observations of forest-fire smoke for a complete forest-fire season. On the basis of sophisticated model calculations of air-mass transport it was shown that some part of that smoke circled from its source regions in Siberia and North America around the globe [Damoah et al., 2004]. We determined optical and microphysical properties of smoke layers from Siberia and North America from measurements with our stationary multiwavelength Raman lidar [Müller et al., 2005]. We also observed transport of anthropogenic pollution from North America [Müller et al., 2005], and mixtures of Arctic haze and east European pollution advected from polar regions and east Europe [Müller et al., 2004]. Mattis et al. [2002b] report on optical properties of Sahara dust that was transported from North Africa to central Europe. A detailed case study on optical and microphysical properties of Sahara dust over Leipzig was discussed by Müller et al. . One large-scale event of transport of Sahara dust was observed at Leipzig and other lidar stations of the European Aerosol Research Lidar Network [Ansmann et al., 2003]. Modification of mineral-dust optical properties could be studied in detail with high vertical resolution.
 In this contribution we summarize 10 years of Raman lidar observations at Leipzig. This contribution to our knowledge is the first systematic, height-resolved study on long-range transport of aerosols. Particularly we document for the first time statistical results on aerosol pollution transport from North America to Europe, with information on the height distribution of that pollution. Our study thus fills a gap in our knowledge on hemispheric pollution transport, which primarily deals with trace gases, for example, ozone and carbon monoxide, and individual aerosol species, for example, sulfur and carbonaceous particles, leaving large gaps in a comprehensive assessment of surface particulate matter levels that arise from regional and long-range transport.
 We focus in this contribution on geometrical properties, and the annual, seasonal, and monthly variation of appearance of lofted pollution plumes advected on intercontinental distances from North America, polar areas, and North Africa. The results presented in this first of a series of papers can also be obtained by simple backscatter lidars. However, in future contributions we shall extend our results to quantitative descriptions of optical and microphysical particle properties of these pollution plumes, which requires multiwavelength Raman lidar.
 In section 2 we briefly discuss the methodology. We present the time series of our observations in section 3. In section 4 we discuss our results. We conclude our contribution with a summary and outlook in section 5.