By studying the fs laser produced plume of different materials we show experimentally that the process of matter removal during ultrashort (fs) laser pulse irradiation followed by vacuum expansion is characterized by a number of general features, whichever the nature of the target material. In particular, fs laser ablation of solid targets at laser intensities of the order of ${10}^{12}-{10}^{13}\mathrm{W}∕{\mathrm{cm}}^2$, inevitably leads to the generation of nanoparticles of that material. This has been evidenced by atomic force microscopy analysis of less than one layer deposits showing that the produced nanoparticles have mean radii generally in the range 5–25 nm, with pretty narrow size distributions. These results are in very good agreement with the physical description and numerical predictions of recently published theoretical analyses of fs ablation processes.

We demonstrate that femtosecond laser ablation of silicon targets in vacuum is a viable route to the generation and deposition of nanoparticles with radii of ≈5–10 nm. The nanoparticles dynamics during expansion has been analyzed through their structureless continuum optical emission, while atoms and ions, also present in the plume, have been identified by their characteristic emission lines. Atomic force microscopy analysis of the material deposited at room temperature has allowed the characterization of the nanoparticles size distribution. Taking into account the emissivity of small particles we show that the continuum emission is a blackbody-like radiation from the nanoparticles. Our results suggest that nanoclusters are generated as a result of relaxation processes of the extreme material state reached by the irradiated target surface, in agreement with recently published theoretical studies.

Since 2000, regular lidar observations of the vertical aerosol distribution over Europe have been performed within the framework of EARLINET, the European Aerosol Research Lidar Network. A statistical analysis concerning the vertical distribution of the volume light extinction coefficients of particles derived from Raman lidar measurements at 10 EARLINET stations is presented here. The profiles were measured on a fixed schedule with up to two measurements per week; they typically covered the height range from 500 m to 6000 m above ground level (agl). The analysis is made for the planetary boundary layer (PBL) as well as for several fixed layers above ground. The results show typical values of the aerosol extinction coefficient and the aerosol optical depth (AOD) in different parts of Europe, with highest values in southeastern Europe and lowest values in the northwestern part. Annual cycles and cumulative frequency distributions are also presented. We found that higher aerosol optical depths in southern Europe compared to the northern part are mainly attributed to larger amounts of aerosol in higher altitudes. At 9 of the 10 sites the frequency distribution of the aerosol optical depth in the planetary boundary layer follows a lognormal distribution at the 95% significance level.

This paper presents a detailed description of LIRIC (LIdar-Radiometer Inversion Code) algorithm for simultaneous processing of coincident lidar and radiometric (sun photometric) observations for the retrieval of the aerosol concentration vertical profiles. As the lidar/radiometric input data we use measurements from European Aerosol Research Lidar Network (EARLINET) lidars and collocated sun-photometers of Aerosol Robotic Network (AERONET). The LIRIC data processing provides sequential inversion of the combined lidar and radiometric data. The algorithm starts with the estimations of column-integrated aerosol parameters from radiometric measurements followed by the retrieval of height dependent concentrations of fine and coarse aerosols from lidar signals using integrated column characteristics of aerosol layer as a priori constraints. The use of polarized lidar observations allows us to discriminate between spherical and non-spherical particles of the coarse aerosol mode.The LIRIC software package was implemented and tested at a number of EARLINET stations. Intercomparison of the LIRIC-based aerosol retrievals was performed for the observations by seven EARLINET lidars in Leipzig, Germany on 25 May 2009. We found close agreement between the aerosol parameters derived from different lidars that supports high robustness of the LIRIC algorithm. The sensitivity of the retrieval results to the possible reduction of the available observation data is also discussed.