The proposed activity aims at establishing a bipolar network to obtain data needed to quantify properties of aerosols at high latitudes, including seasonal background concentrations by measurements of aerosol optical depth (AOD), spectral characterizations, and the evolutionary patterns of the natural and anthropogenic processes that perturb the aerosol cycles. An effort to quantify direct and indirect climate forcing by polar aerosols will be made through a set of closure experiments using observations in conjunction with model calculation and satellite data.

The co-operation in the frame of POLAR-AOD will allow the following:

1. - Definition of calibration procedures among the various sun-radiometers operating in Polar regions, in order to achieve homogeneous AOD (aerosol optical depth) evaluations and maintain a set of reference instruments in a high mountain station. Some of the instruments will be used to constitute a traveller system from one station to the other with the aim of attaining 'round robin' inter-comparison. Regular inter-comparison campaigns (biennial) will be organized in order to compare not only calibration constants but also instrumental characteristics, such as sensitivity and influence of diffuse light. The first inter-comparison campaign, scheduled for spring 2006 at Ny-Alesund, will be used also to define and adjust the methodology and calculation procedures, like that aimed at evaluating relative optical air mass and trace-gas corrections.

2 . - Improvement of "in-situ" optical and chemical measurements, with an effort to operate size and time-resolved aerosol composition and concentration sampling on a long-term base (EoI ID 557).

3. - Establishment of a data bank for spectral sun-photometric measurements (AOD archive), in-situ measurements and any other aerosol related parameters useful for assessing the quantification of aerosols and their radiative effects. This archive will fill the gap for the polar regions within the global aerosol climatology, where there is an urgent need for high-quality data on aerosol abundance and physico-chemical characteristics on a regional scale. The information is necessary to better constrain climate model simulations and improve the interpretation of remotely sensed data.