I work in the field of remote sensing of the atmosphere, the radiative, optical, physical, and chemical properties of man-made (anthropogenic) and natural (e.g., dust, forest-fire smoke, sea salt) air pollution, and the impact of this pollution on present and future climate. My work is interdisciplinary. It covers various fields of traditional physics (lasers, optics, thermodynamics), atmospheric physics (atmospheric pollution, radiative transfer, and meteorology), mathematics (ill-posed inverse problems), and computational data analysis (data inversion, data mining, neural networks). My work is imbedded in a comparably young research field called LIDAR (Light Detection and Ranging), which can in simple terms be described as laser radar. The research field requires strong networking and concerted research effort of lidar groups worldwide in order to advance our research methods. Aside from our research, the training and teaching of the next generation of lidar specialists is one of our priorities. The establishment of central teaching and training facilities will allow us to develop standardized instrument and data analysis methods. Our lidar stations are in part organized in networks on different continents, like the European Aerosol Research Network (EARLINET) in Europe. We aim at establishing a global lidar network (GALION) which is supported by the World Meteorological Organization (WMO). A few airborne aerosol lidar systems are in operation, among which the first multiwavelength aerosol lidar, operated by NASA Langley Research Center since 2012, certainly is the greatest achievement of the past decade. Our community has started to improve the ground-based and airborne instrument technology such that it can be used on satellites orbiting Earth. These satellite activities are mainly coordinated by NASA, ESA, and JAXA. Satellite-lidars allow us to obtain a global view of air pollution that encircles the globe. This pollution not only has fundamental impact on climate, but also on the quality of the air we breathe, human health, and various ecoystems.
 Aerosol pollution studies with remote measurement and remote sensing methods (lidar, sunphotometer, starphotometer, and satellites) for climate impact studies;
 Development of aerosol models for radiative impact studies and ocean color retrievals;
 Development of lidar instrument techniques with focus on aerosol type identification and chemical characterization (Raman spectroscopy lidar, CARS lidar;
 Development and application of mathematical methods (inversion algorithms): determination of climate-relevant aerosol parameters;
 Synthesis of aerosol observations carried out with ground-based and space-borne active and passive remote sensors (for example CALIPSO, A-Train);
 Improvement of electro-magnetic scattering models for application in aerosol characterization;