Aerosol particle direct and indirect radiative effects have been identified as key uncertainties for the prediction of the future global climate [IPCC, 2001]. To reduce these uncertainties several international field campaigns have been performed in the last years. These activities have focused on remote oceanic conditions (ACE 1) [Bates et al., 1998], intermittently polluted marine environments (ACE 2) [Raes et al., 2000], highly polluted conditions (INDOEX) [Ramanathan et al., 2002], and continental conditions (LACE 98) [Ansmann et al., 2002]. Because aerosol particles and clouds reflect, absorb, and emit solar and terrestrial radiation in the atmosphere, they also may affect the hydrological cycle. Micro-physical and optical properties of cloud particles are important for understanding of radiative interactions in the atmosphere, hence of the cloud feedback of Earth's climate. For example, a change in aerosol loading and albedo of marine stratocumulus clouds caused by anthropogenic emissions may significantly influence the radiative balance over the oceans and thus may influence the climate globally [IPCC, 2001].

Generally it is assumed that the direct and indirect aerosol effects in the Arctic do not influence the climate on a global scale, because of the low solar elevation at high latitudes and the fact that the Arctic represents a small part of the Earth's surface only. However, there may be significant regional radiative effects. The Arctic represents a sensitive ecosystem, which are susceptible to even small changes in the local climate. The already mentioned special conditions of usually high surface albedo and low solar elevations cause enhanced aerosol/cloud effects due to multiple scattering. It is suspected that this increased interaction between solar radiation and the aerosol particles/clouds magnifies their radiative impact. Thus, for a given aerosol distribution, the specific optical properties are enhanced in the Arctic. For the same reasons, results from field experiments at low latitudes are difficult to transfer to the Arctic and as a consequence there is an urgent need to conduct specific measurement programs in high latitude regions. In order to improve the knowledge about the origin, transport pathways, vertical structure of aerosol physical and chemical properties as well as the impact on climate in the Arctic, a combined effort of surface-based, airborne and spaceborne measurements is needed.

The Arctic is a sensitive environment where aerosol radiative properties and aerosol-cloud interactions with respect to natural and anthropogenic aerosol sources have been investigated previously. Past experiments conducted over the last 40 years have mainly focused on the Arctic Haze phenomenon that occurs during late winter and spring [Heintzenberg, 1989; Bodhaine and Dutton, 1993]. Optical properties of the aerosol during these events have been studied within projects such as the AGASP program [Schnell, 1984] or ASTAR 2000. The recent International Arctic Ocean Expeditions, using the Swedish icebreaker Odin, provided the most detailed information on aerosol in the central European Arctic. However, these observations were limited to the atmospheric boundary layer during the clean summer season. None of the past experiments so far had the capability to produce an over-determined set of observations of aerosol radiative properties. To assess the present and future impact on the Arctic climate by changes in the aerosol properties, it is requested to have an internal consistency between observed and simulated aerosol and cloud properties.

References

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