| Summary: | In order to improve knowledge on the properties, evolution and climate impact of the tropospheric aerosol in both polar regions, two aircraft experiments have been performed in the Arctic from Longyearbyen/Svalbard, namely ASTAR (Arctic Study on Tropospheric Aerosol and Radiation) 2004 (in May) and ASTAR 2007 (in March/April), as well as one experiment in coastal Antarctica (in December/January 2006/7), using either the AWI Dornier-228 or the DLR Falcon research aircraft. In this contribution the main focus will be on an overview of first results of the ASTAR 2007 experiment, which took place in a season where the Arctic troposphere in general is expected to be influenced by Arctic haze pollution. Observations during the measurement period revealed rather low aerosol number concentrations and no apparent Arctic haze event with exceptionally high aerosol concentrations and reduced visibility in the boundary layer. On the other hand, measurements of nitrogen oxides (NOy) mixing ratios showed rather high levels which are typical for the Arctic winter/spring time. Furthermore, during many flights thin layers with aerosol concentrations clearly higher than background were observed mainly in the lower and middle troposphere, clearly marked also by enhanced sulfur dioxide mixing ratios. Different composition of these layers in terms of aerosol properties and trace gases (e.g. particle absorption coefficient, non-volatile particle fraction, carbon monoxide, NOy) point to different sources of these layers. FLEXPART analyses of air mass transport show that we in general have seen rather aged air masses. We will use the comparison to the ASTAR 2004 and the Antarctic measurements to assess the "cleanliness" of the Arctic troposphere in terms of particle properties.
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