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Saisonalität und kurzperiodische Variabilität des Seesalz-Aerosols und des bodennahen Ozons in der Antarktis (Neumayer-Station) unter Berücksichtigung der Meereisbedeckung = Seasonal and short-periodic variability of seasalt aerosol and near-surface ozone in Antarctica (Neumayer-Station) in consideration of sea-ice coverage
Hofmann, J. (2000). Saisonalität und kurzperiodische Variabilität des Seesalz-Aerosols und des bodennahen Ozons in der Antarktis (Neumayer-Station) unter Berücksichtigung der Meereisbedeckung = Seasonal and short-periodic variability of seasalt aerosol and near-surface ozone in Antarctica (Neumayer-Station) in consideration of sea-ice coverage. Ber. Polarforsch. Meeresforsch. 376: 1-192
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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Keyword
    Marine

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  • Hofmann, J.

Abstract
    The present work deals with the seasonal variations and the short periodical variability of atmospheric trace compounds in Antarctica by taking sea ice coverage into consideration. The German Antarctic research station Neumayer (8° 15'W and 70° 39'S) has been selected for the measurements. Neumayer station accommodates a meteorological as well as an air chemistry observatory. The main objective of the investigations performed was the study of the seasonal behaviour of the sea ice coverage for the surroundings of Neumayer station under special consideration of air-mass transport variability. Based on that it was intended to study the correlation between the short periodical, temporal variability of the sea ice coverage, the short periodical, temporal changes of the sea-salt aerosol concentration and the near-surface ozone in the vicinity of Neumayer station. It was the aim to determine whether and how sea ice can influence an air mass on its way to Neumayer station. The originator of the investigations was an extensive analysis of the sea ice situation for the surroundings of the station on the basis of ice coverage data from the National Snow and Ice Data Center (NSIDC), Boulder. Two methods have been developed with the determination of area-related and trajectory-related ice coverages, which meet the requirements of an air mass transport related consideration of the sea ice coverage. Area-related ice coverage marks the ice coverage within fixed geographical regions, which have been derived from the main meteorological transport layers and represent areas over which a better part of the air mass transport towards Neumayer station takes place. Trajectory-related ice coverage designates the ice coverage within an area, which has been assigned to calculated back trajectories by means of a special method. Area-related ice coverages are above all suitable for statistical investigations of the pure ice coverage and - conditional on the method - a representative cross section for long distance transport. Trajetory-related ice coverages take into account the seasonal variablity of the ice coverage and the daily variations of the air transport conditions. Due to the limited length of the trajectories they represent mainly ice coverages in the vicinity of the station. An essential element of the sea ice coverage investigations was an analysis of the air mass transport towards Neumayer station. On the basis of seasonal freqeuncy distributions of the origins and averaged trajectories three main transport categories have been distinguished and the corresponding main catchment areas have been derived. These investigations were based on daily back trajectories which were made available by the Deutsche Wetterdienst (DWD) for the geographical area of Neumayer station and the years 1993-1996. About 70 % of all air mass transports are east transports. The starting point of the assigned trajectories is within the sector between 0° and 135° as seen from the station (azimuth angle). As main catchment area for east transports a region north of 60°S and 45°W has been determined. From this main catchment area air masses which can be attributed to east transports stream over the antarctic inland ice. West transports have a share of 25 % and the starting point of the assigned trajectories is between 225° and 360°. Their main catchment area is the Weddell Sea. Air masses which can be attributed to west transports move over sea in near-coast areas towards Neumayer station. South transports from a sector beween 135° und 225° play only a secondary role. Their origin is located on the inland ice south of Neumayer station. Consequently, south transports are not relevant for investigations of the sea ice coverage. Based on the predetermination of the main catchment areas area-related ice coverages for the easterly and westerly catchment areas have been determined for the period from 1979 to 1996. Starting from the available back trajectories, daily trajectory-related ice coverages for the period from 1993 till 1996 have been calculated. It could be shown, that there are substantial, primarily transport related differences in the ice coverage, which can influence an air mass on its way to Neumayer station. East transports are noticeable less exposed (about three months later in the annual cycle and over a shorter period) to the influence of the sea ice than west transports. Furthermore, it has been realised that west transports in the vicinity (1 to 3 days before reaching the station) move more frequently all-year-round over sea ice or sea areas whereas east transports in the vicinity move primarily during winter over shelf- or sea ice areas. From these observations the following conclusions can be drawn for the investigated atmospheric trace compounds: If the sea-salt aerosol which is transported to Neumayer station originates from polynias, then it should preferably reach the station via west transports since these traverse regions in near coastal areas where polynias frequently appear. Should the aerosol originate from ice-free sea areas, that means it would reach Neumayer station by long distance transport, then it should show up also with east transports. The near-surface transport of low-ozone air masses should preferentially be connected with west transports since - according to a hypothesis - ozone depletion should be connected to the existence of sea ice. In such a case polynias could play a crucial role, since at their rims sea-salt aerosol can be deposited and, by flooding with sea water, seasalt and thus bromine-containing species can be enriched. Therewith a further condition for the tropospheric ozone depletion in Antarctica would be fulfilled. Furthermore, the connection between the short periodical variability of the ice coverage and the short periodical variablity of concentrations of sea-salt aerosol and near-surface ozone have been investigated. For that purpose assumptions about the transport processes have been made, which have been checked by means of a cross-correlation analysis between the corresponding measurements and the various time series for ice coverage. It could be shown that the existing data do not show a causality between the short periodical variability of the sea ice coverage and the short periodical variability of the sea salt aerosol and near-surface ozone. An important result of this work is the extensive description of the sea ice coverage for the vicinity of Neumayer station. Methods for the investigation of the sea ice coverage have been developed, which analogously can also be applied for other measuring sites. The investigation of the correlation between sea ice coverage and atmospheric trace compounds primarily shows that the transport of trace compounds in Antarctica is the result of complex atmospheric processes, on which their occurrence and impact the sea ice coverage obviously plays only a secondary role.

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