- Email: [email protected]
01.O.01 Past aerosol composition from ice core studies
J Aero.t.I S¢'i., Vol. 25, Suppl. I, p. SI, 1994 Copyright(~)1994 El~vier Science Lid Printed in Great Britain. All rights re~rved 0021-8502/94 $7.00 + 0.00
Pergamon
01001 PAST AEROSOL COMPOSITION FROM ICE CORE STUDIES
Robert DELMAS Laboratoire de Glaciologie et G6ophysique de I'Environnement B.P. 96 38402 Saint-Martin d'H~.res cedex (France) Aerosols are important parameters to take into account in defining the optical properties and the budget radiation of the atmosphere. Moreover, they play an essential role in the regulation of the hydrological cycle, since water droplets and ice crystals in clouds form on aerosol particles called cloud condensation nuclei (CCN). They are indeed the second largest global climate forcing factor. Polar snow and ice is the only place where primary and secondary aerosol undisturbed records have been formed on the scale of the last hundreds millennia. High elevation glaciers located at mid or low latitudes cover shorter time periods. They are potentially important to reconstruct the history of regional pollution (Alpine glaciers), of El Nino events (Andes glaciers), of monsoon fluctuations (Himalayan glaciers) or of desert dust emissions (Alpine and central Asian glaciers). The use of the chemical records of ice cores for paleoenvironmental studies is intricate due to the nature of the polar aerosol. Before interpreting the past, the composition and the origin of the present chemical impurities deposited in polar snow have to be well understood. Gaseous and particulate impurities are advected to the polar areas by wind generally in the troposphere, and sometimes by a stratospheric pathway. The present contribution of the polar continental sources to this budget may be considered as negligible due to the aridity of the ice sheets. Three main aerosol categories have to be considered : 1. Dust emitted by continents under the action of weathering and wind, 2. Sea spray generated by the phenomenon of bubble bursting at the surface of the oceans, 3. Gas-derived aerosol produced by the chemical transformation of atmospheric trace gases. Generally speaking it can be emphasized that secondary aerosol (i.e. essentially H2SO4) is the main constituent of the background aerosol. The aerosol particles are incorporated in the snow flakes by nucleation scavenging at cloud level, by below-cloud scavenging (this process is, however, rather unimportant in clean air conditions) and by dry deposition onto the snow surface. The use of the glacial records for atmospheric chemistry purposes requires that mechanisms of transfer from the atmosphere to the snow be relatively well known. Glacio-chemists have worked empirically until now, assuming that these unknown processes can be neglected for interpreting ice data in terms of atmospheric data. The data obtained from recent Antarctic and Greenland deep ice cores provide precise information on the ice age environmental conditions. At this time, sea salt and overall crustal dust depositions were significantly higher. The biogeochemical cycles of S and N were also disturbed according to modifications in source intensity and transport of gaseous precursors.
AS IS S.I-C
S|
Pergamon
01001 PAST AEROSOL COMPOSITION FROM ICE CORE STUDIES
Robert DELMAS Laboratoire de Glaciologie et G6ophysique de I'Environnement B.P. 96 38402 Saint-Martin d'H~.res cedex (France) Aerosols are important parameters to take into account in defining the optical properties and the budget radiation of the atmosphere. Moreover, they play an essential role in the regulation of the hydrological cycle, since water droplets and ice crystals in clouds form on aerosol particles called cloud condensation nuclei (CCN). They are indeed the second largest global climate forcing factor. Polar snow and ice is the only place where primary and secondary aerosol undisturbed records have been formed on the scale of the last hundreds millennia. High elevation glaciers located at mid or low latitudes cover shorter time periods. They are potentially important to reconstruct the history of regional pollution (Alpine glaciers), of El Nino events (Andes glaciers), of monsoon fluctuations (Himalayan glaciers) or of desert dust emissions (Alpine and central Asian glaciers). The use of the chemical records of ice cores for paleoenvironmental studies is intricate due to the nature of the polar aerosol. Before interpreting the past, the composition and the origin of the present chemical impurities deposited in polar snow have to be well understood. Gaseous and particulate impurities are advected to the polar areas by wind generally in the troposphere, and sometimes by a stratospheric pathway. The present contribution of the polar continental sources to this budget may be considered as negligible due to the aridity of the ice sheets. Three main aerosol categories have to be considered : 1. Dust emitted by continents under the action of weathering and wind, 2. Sea spray generated by the phenomenon of bubble bursting at the surface of the oceans, 3. Gas-derived aerosol produced by the chemical transformation of atmospheric trace gases. Generally speaking it can be emphasized that secondary aerosol (i.e. essentially H2SO4) is the main constituent of the background aerosol. The aerosol particles are incorporated in the snow flakes by nucleation scavenging at cloud level, by below-cloud scavenging (this process is, however, rather unimportant in clean air conditions) and by dry deposition onto the snow surface. The use of the glacial records for atmospheric chemistry purposes requires that mechanisms of transfer from the atmosphere to the snow be relatively well known. Glacio-chemists have worked empirically until now, assuming that these unknown processes can be neglected for interpreting ice data in terms of atmospheric data. The data obtained from recent Antarctic and Greenland deep ice cores provide precise information on the ice age environmental conditions. At this time, sea salt and overall crustal dust depositions were significantly higher. The biogeochemical cycles of S and N were also disturbed according to modifications in source intensity and transport of gaseous precursors.
AS IS S.I-C
S|