An automatic recorder for air/firn transfer studies of chemical aerosol species at remote glacier sites
Introduction
Permanent snow fields constitute vast natural receptacles for atmospheric aerosol particles and associated precursor gases. Provided that they are situated in the “dry snow zone”, the glacier masses successively formed from them may thus provide a genuine archive of past air chemical changes. Until now, several ice core studies performed at polar ice sheets (Oeschger and Langway, 1989; Delmas, 1995) and in a few cases also on non-temperated mountain glaciers (Wagenbach, 1989) have revealed important glacio-chemical changes over various time scales.
Presently, one of the most crucial shortcomings of these studies concern the interpretation of the chemical ice core signals in terms of the corresponding change in the local atmospheric loads (Bales and Wolff, 1995). For example, the interannual and even the interdecadal variability of mayor ions (Mayewski et al., 1986) and other particulate species observed in ice cores is generally much larger than what might be expected for the respective mean atmospheric levels. Furthermore, very strong seasonal cycles of chemical aerosol components prevail at both, polar and mid-latitudinal glacier sites (Dams and De Jonge, 1976; Mosher et al., 1993; Wagenbach, 1996) which, in conjunction with a possible seasonality of the net snow accumulation, may severely bias the net atmospheric information of glacio-chemical ice core records (Wagenbach, 1993a; Dibb, 1996).
Therefore, year round observations at the relevant ice core drill sites are urgently needed, to establish an empirical relationship between the variability of atmospheric loads and the resulting chemical firn stratigraphy. However, due to logistical constraints such information is presently only available through the DGASP campaign (Jaffrezo and Davidson, 1993) at Dye 3 (South Greenland) and, limited to the summer season, for the central Greenland position Summit (Jaffrezo et al., 1995). At the latter site, weekly year round records on the elemental aerosol composition are actually recovered by an automatic striker sampler (Heidam et al., 1993). PIXE-analyses applied here provide, however, only semiquantitative results on Cl and Na, no information on nitrogen species and no speciation of sulphur components.
We attempted to partly overcome this information lack by developing an automatic field station which is able to collect aerosol samples on filters for subsequent major ion analyses and to faithfully record the snow pack evolution unattended throughout the year. Here, we describe the technical design of this station along with the principal field experience gained so far from deploying the prototype device at the Alpine ice core drill site Colle Gnifetti (CG).
Section snippets
System requirements
The collection of aerosol samples should allow to establish a quasi-continuous record of major ion species commonly measured in ice cores. This means that also gas phase nitric acid, which may contribute significantly to the total nitrate deposited onto remote glaciers (Legrand and Kirchner, 1990) and may partly evaporate from inert aerosol filters (Zhang and McMurry, 1992) has to be collected as well. A maximum sampling interval of 1 month appears appropriate, since reworking of the snow
Test site
The first application of the automatic station was accomplished on a high elevation Alpine firn saddle (Colle Gnifetti, 4450 m asl), which we are using extensively for recovering several ice cores drilled to bed rock (Wagenbach et al., 1985; Wagenbach, 1993b, Wagenbach, 1996). This extremely exposed hanging glacier is characterized by the following glacio-meteorological conditions (mainly derived from our automatic weather station operated there):
• 10 m firn temperature −14.5°C (Haeberli and Funk,
Acknowledgements
This work is a contribution to the EUROTRAC subproject ALPTRAC and was funded by the German Ministry for Education and Research. We thank N. Beck and K. Geis for their commitment during many pilot investigations as well as H.G. Junghans, R. Fletterer and the work shop staff of the institute for invaluable help in constructing the device. We particularly acknowledge the logistical support provided by the Club Alpino Italiano, Air Zermatt and the Meteorological Observatory Locarno.
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