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Messung von optische Eigenschaften troposphärischer Aerosole in der Arktis = Measurements of optical properties for tropospheric aerosols in the Arctic
Schumacher, R. (2001). Messung von optische Eigenschaften troposphärischer Aerosole in der Arktis = Measurements of optical properties for tropospheric aerosols in the Arctic. Ber. Polarforsch. Meeresforsch. 386: 1-153
In: Berichte zur Polar- und Meeresforschung = Reports on Polar and Marine Research. Alfred-Wegener-Institut für Polar- und Meeresforschung: Bremerhaven. ISSN 1618-3193, more
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Document type: Project report

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  • Schumacher, R.

Abstract
    Tropospheric aerosols play an important role for the radiation budget of the atmosphere. Directly they influence the atmosphere by scattering and absorbing solar and terrestrial radiation. As a result of this effect both heating or cooling can be observed. Indirectly tropospheric aerosols contribute to cloud condensation by forming cloud condensation nuclei, and affect thereby the radiation and humidity budget of the atmosphere. Up to now both mechanisms are poorly understood and require additional measurements and modelling activities. Because of their high sensitivity the polar regions play a key role in climate change. The direct effect is of special importance due to the low humidity and the extreme radiative conditions (polar night and day, high surface albedo). In this thesis the occurrence of the so called " Arctic Haze" events is analyzed. During springtime anthropogenic aerosols from the main industrial areas (Eurasia, North America) are transported into the Arctic and the aerosol concentration can increase to values which are typical for the source regions. The vertical distribution of the extinction coefficient is one of the most important parameters for the radiation fluxes in the troposphere. This profile is measured with the Lidar technique. The Koldewey Aerosol Raman Lidar (KARL), located in Ny-Alesund (Spitsbergen, 78.95°N, 11.93°E) is a backscatter Raman lidar which acquires highly resolved profiles of the aerosol backscatter coefficient. The aerosol extinction coefficient is derived from the Raman scattered light by nitrogen molecules. The extinction coefficient determined by KARL can be compared with groundbased and airborne photometer measurements. During the German-Japanese ASTAR-2000 campaign in spring 2000 this method was applied sucessfully. 15 measuring flights with the aircraft "Polar 4 " and approximately 300 measuring hours with KARL provide a large and complex data set for the regional climate model. Based on this data two different methods for the derivation of the altitude resolved aerosol extinction profiles are applied. The Klett method, which needs only the elastical backscattered lidar signal, allows the determination of the extinction coefficient indirectly at day and nighttime. For the Klett method an assumption of the extinction-to-backscatter ratio, often called "lidar ratio", is necessary. This can be determined iteratively by combining with the airborne photometer measurements. With this method small scale inhomogenities and the high variability of fine structured aerosol layers can be recorded. In contrast to this the Raman method allows to calculate the extinction coefficient without an assumption of the lidar ratio. Here only the Raman backscattered light from nitrogen is used. This procedure is limited in temporal and spatial resolution because the Raman backscatter cross section is smaller than the elastic backscatter cross section by three orders of magnitude. A sensitivity study shows the feasibility of this method. During the ASTAR-2000 campaign different kinds of aerosol loadings were observed. Increased aerosol concentrations were measured with the groundbased photometer on 8 days with optical depths between 0.07 and 0.1. At 3 days very strong haze events with values greater than 0.15 could be analysed. Two different kinds of aerosol distributions were observed with KARL: thick aerosol layers near the ground up to 3 km and fine structured aerosol layers within the whole troposphere up to 8 km. In some cases stable aerosol layers over several days, on other days fine aerosol layers with high temporal and spatial variabilility have been measured. A good agreement between the photometer and lidar data was obtained. An aerosol data set based on these measurements will be implemented in the high resolution arctic regional model HIRHAM to make realistic estimations of the climate effect of tropospheric aerosols.

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