What controls chemical aerosol signals in Greenland ice cores

Gfeller, Gideon (2015). What controls chemical aerosol signals in Greenland ice cores (Unpublished). (Dissertation, Universität Bern, Philosophisch–naturwissenschaftliche Fakultät, Physikalisches Institut, Abteilung für Klima– und Umweltphysik)

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Aerosols and the related chemical processes can have a large influence on the radiative budget of the Earth and thus on climate. However, of all the factors affecting Earth’s radiative forcing, the influence of aerosols is the most difficult to quantify and is thus the one with the largest uncertainty.
Chemical impurities trapped in polar ice can give information about past atmospheric aerosol composition and thus help in better understanding the interaction between aerosols and climate. In the course of the North Greenland Eemian (NEEM) ice drilling project a core with an approximate length of 2500 m covering the last 130 000 years was obtained and analysed in the field to determine its chemical impurity content using the portable Bern Continuous Flow Analysis (CFA) system. The main species to be analysed were sodium (Na+), calcium (Ca2+), ammonium (NH+4 ) and nitrate (NO–3).
In addition, several firn cores were drilled and analysed in this thesis to gain information about the representativeness of these chemical impurities on different spatial scales. We conclude that to reconstruct the seasonality of the major ions it is sufficient to drill one core, whereas for the reconstructions of inter-annual variability, replicate coring is necessary. From this study, accurate concentration timeseries covering the period 1990 – 2010 AD and seasonalities representative for the atmospheric variability could be retrieved, which were contrasted to a comprehensive back-trajectory climatology in order to constrain the origin of the various aerosol concentration and to quantify the influence of aerosol transport on snow concentrations.
Over the past 30 years, 10 days air parcel back-trajectories have been calculated on 6-hour time intervals using the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model, which is based on the dataset assimilated by the European Centre for Medium-Range Weather Forecasts (ECMWF).
With the help of the air parcel back-trajectories, the source regions and the pathways of each of the chemical species reaching NEEM have been investigated. Three main air parcel trajectory clusters were identified; the first covers North America, the second includes the North Atlantic Ocean and Europe, and the third, representative of the zonal long-range transport, covers the northern half of the
northern hemisphere.
In addition a model has been established to account for the transport and the deposition of aerosols. This model has been applied on chemical impurity timeseries on different timescales such as seasonal and inter-annual timescales over the last 20 years for the firn cores, but also on longer timescales for the NEEM main core, reaching back into the last warm period, the Eemian. For the NEEM main core, it was found that transport and deposition explains about 26 % and 11 % of the Ca2+ ice concentration change between the Holocene and Greenland interstadials and stadials, respectively. For Na+, transport explains 90 % during Greenland interstadials and 74 % during Greenland stadials. For NH+4 and NO–3, the fast variations of the ice concentrations observed during DO-events over the last glacial period can almost entirely be explained by transport.
For the Eemian, an additional back-trajectory study was performed to gain insight into air parcel flow towards NEEM, investigating the impacts of five different Greenland ice sheet topographies on air transport. Whereas four of the topographies feature only western air transport towards NEEM, one topography stands out with anomalous eastern wind flow.
In addition to the NEEM core, the model was applied to the more highly resolved NH+4 concentrations obtained from the North Greenland Ice core Project (NGRIP) core after separating NH+4 fire peaks from the biogenic background from soil emissions. Results suggest that soil emissions increased on orbital timescaleswith warmer climate, through the reduction of the ice covered area and the consequent northward expansion of vegetation. Whereas sudden temperature changes seem to have had little effect on NH+4 soil emissions and transport, they had aneffect on the likelihood of fire occurence, which significantly increased during the same time interval.

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