EARLINET, the European Aerosol Research Lidar Network, is the first aerosol lidar network, established in 2000, with the main goal to provide a comprehensive, quantitative, and statistically significant data base for the aerosol distribution on a continental scale. At present, 25 stations distributed over Europe are part of the network. The network activity is based on scheduled measurements, a rigorous quality assurance program addressing both instruments and evaluation algorithms, and a standardised data exchange format. All the network stations perform measurements simultaneously at three fixed dates a week, so allowing to collect unbiased data. Lidar observations are routinely performed on a regular schedule of one daytime measurement per week around noon, when the boundary layer is usually well developed, and two night time measurements per week, in low background light conditions, in order to perform Raman extinction measurements. Further observations are devoted to monitor special events such as Saharan dust outbreaks, forest fires, photochemical smog and volcano eruptions. The EARLINET-ASOS (Advanced Sustainable Observation System) EC Project, started on 1 March 2006 will provide a contribution to the improvement of continuing observations and methodological developments that are urgently needed to provide the multi-year continental scale data set necessary to assess the impact of aerosols on the European and global environment and to support future satellite missions. The main objectives of the EARLINET-ASOS project are:
- to extend the development of the European Aerosol Research Lidar Network as a world
- leading instrument for the observation of the 4-dimensional spatio-temporal distribution of aerosols on a continental scale; this will result in accurate, well-defined, and easily accessible data products to be used in science and environmental services.
- to enhance the operation of the network with the aim to foster aerosol
- related process studies, validation of satellite sensors, model development and validation,  assimilation of aerosol data into operational models, and to build a comprehensive climatology of the aerosol distribution.

Acknowledgments The financial support of this work by the European Commission under grant  RICA-025991 EARLINETASOS is gratefully acknowledged.