Background

Ice cores have proven to be an excellent tool for paleo-atmosphere studies and climate reconstructions. Sweden participates in several ongoing deep ice core drilling projects in Antarctica and on Greenland. We measure soluble ionic species and insoluble particles along these ice cores. Both ions and particles originate from the atmospheric aerosol, and reach the Antarctic or Greenland surface via dry or wet deposition. Since the atmospheric lifetime of aerosols is much shorter than of gases such as CO₂ or CH₄, they provide a qualitatively different kind of information about the past climate.

Insoluble particles are of particular interest in terms of interpreting changes in terrestrial ecosystems and circulation patterns. This material generally has two major sources: soot from incomplete combustion of a fossil fuel (biomass in the case of pre-industrial times) and soil dust. A major limitation on our ability to interpret the ice core measurements as past atmospheric concentrations is an incomplete understanding of the processes controlling the deposition of aerosols to the snow pack.

Very few direct measurements have been made for both dry and wet deposition of black carbon and soil dust. However, while for dry deposition a well-established theory describing the processes exists, wet deposition as yet lacks a satisfactory theoretical description and processes are far less understood. Hence our project aims at advancing the understanding of wet deposition of light absorbing aerosols.

Aims of the project

The long-term goal is to establish continuous measurements of absorbing aerosols in air and fresh snow at the Swedish station Wasa in Antarctica. Specific objectives are:

• to distinguish between crustal and combustion-derived aerosols
• to better understand the processes that control the deposition of absorbing aerosols in polar regions
• to estimate any potential changes in the absorption of solar radiation (and the surface energy balance) in polar regions due to the presence of absorbing aerosols

With this information we aim to refine interpretation of dust parameters from ice cores, revealing the interaction between climate and dust spanning several glacial cycles back in time. Our three week stay at Wasa was a reconnaissance study to evaluate the scientific suitability and technical feasibility of Wasa as a base for long-term monitoring of absorbing aerosols.

Field work

A six-hour flight from Cape Town in South Africa to the Russian base Novolazarevskaya in an Ilyushin 76 ML aircraft, and another seven-hour flight in a very much smaller Russian AN-2 aircraft brought us to the Swedish station Wasa in Dronning Maud Land, Antarctica. The view of Wasa and the neighbouring Finnish station Aboa on the nunatak Basen in the early morning light was spectacular. The following weeks were highly productive in beautiful scenery, with good facilities at the station and excellent logistic support. During our stay from 26 January to 13 February, 2003, our field programme included measurements of airborne absorbing aerosols, surface snow sampling, dust sampling on nunataks and surface albedo measurements.

Airborne absorbing aerosols

A single wavelength instrument (photo 2) was set up to measure the change in transmission of light through a filter substrate as particles are collected. The objective of this activity was to provide a baseline concentration value for absorbing aerosols that can be used in developing an automatic, multi-wavelength instrument for future year-round measurements. The local disturbance on the signal from activities at the stations Wasa and Aboa was also evaluated.

Surface snow sampling

Extensive snow chemistry studies have previously been carried out in the area around Wasa, comprising snow pit sampling (Stenberg and others, 1998 and 1999) and drilling of 100 m long ice cores at Camp Maudheimvidda (CM) 10 km away from Wasa (Holmlund and others, 2000). This time the study was focused on surface snow sampling along transects going out from the nunatak Basen to the drillsite CM and to the nunatak Plogen 20 km away. The objectives were to study the influence of the nunataks as local sources of crustal material and to establish a background value for absorbing aerosols in the snow pack valid for a larger area. Surface snow samples, down to 20 cm depth, were collected every 500 m along the transects. Three two-meter long firn cores were drilled at the drill site CM (photo 1) and divided into 20 cm long samples. The samples were melted in a microwave oven and filtered through a filter substrate. The filters were brought back home for analysis of light absorbing material on both single and multi-wavelength instruments as well as examination of mineralogy of any crustal material. The filtered melt water was collected and analysed for stable water isotopes (18O and D). These analyses confirmed what we could notice in the field; surface sublimation is a significant process in this area during summer. The effect of sublimation on the concentrations in the snow pack needs to be taken into account in all calculations of deposition fluxes and interpretation of air-snow transfer functions.

Surface albedo

A series of albedo measurements were carried out to investigate the effect of absorbing aerosols on the radiation balance at the snow surface and to determine the relationship between albedo and absorbing aerosols in the surface snow.

Point albedo measurements were performed at all snow-sampling locations using a portable albedometer mounted to a tripod in a surface parallel manner. In addition, an automatic station was operated from 29 January to 11 February, 2003, about 2 km from Wasa. Shortwave incoming and reflected radiation were stored every 5 minutes on a datalogger dug into the snow. The instruments were artificially ventilated to minimize measurements errors.

Svea, Heimefrontfjella

Although based at Wasa during our entire field season, on 26 January we had the chance to visit the field station Svea (photo 5) in the Heimefrontfjella region about 200 km inland from Wasa. Surface snow and crustal material from the nunataks were sampled and surface albedo measurements were performed at different sites in the vicinity of Svea. Local crustal material was found to contribute significantly to the total content of insoluble particles in the snow at these sites.

Outlook

Promoted by good weather conditions the planned field programme could be completed successfully. We could record high-resolution aerosol variations in the air and investigate the spatial variability of particles in near-surface snow and albedo. It is intended to establish a long-term monitoring system for aerosol measurements at Wasa, once the remaining technical problems will have been solved. These include issues like power supply during winter and operation of the soot photometer without regular maintenance.