Aerosol effect on the radiation budget is a critical component on global climate. In fact, depending on their typology, aerosols can absorb or scatter the incoming and outgoing radiation, and, depending on their size and composition, they can act as condensation nuclei, modifying cloud physical and radiative properties. The main difficulties in the determination of the direct and indirect effects of aerosols on the Earth's radiative balance are related to the high inhomogeneity and variability of atmospheric aerosol. In particular, the high variability of the tropospheric aerosols, in terms of concentration, shape, size distribution, refractive index and vertical distribution, makes the tropospheric aerosols one of the most uncertain elements in the estimation of Earth's radiation budget. Long-term measurements of vertical profiles of aerosol optical properties are needed to reduce these uncertainties. At CNR-IMAA (40°36'N, 15°44' E, 760 m above sea level), a lidar system for aerosol study is operative since May 2000, in the framework of EARLINET, the first lidar network for tropospheric aerosol study on continental scale.

CNR-IMAA lidar system

The CNR-IMAA lidar system is based on the combined Raman/elastic lidar technique and makes use of a Nd:YAG laser equipped with second and third harmonic generators. From May 2000 to August 2005, the system provided independent measurements of aerosol extinction and backscatter profiles at 355 nm and aerosol backscatter profiles at 532 nm. Subsequently, the CNR-IMAA lidar system for aerosol was upgraded to increase the number of retrievable parameters, in order to obtain more information about microphysical properties of the particles. In particular, since August 2005, this system can provide independent measurements of aerosol extinction and backscatter profiles at 355 and 532 nm, and of aerosol backscatter profiles at 1064 nm. In this way, lidar ratio profiles at 355 and 532 nm and Ångström exponent profiles at 355/532 nm are obtained. Multi-wavelength measurements (3 backscatter + 2 extinction) allow the determination of microphysical aerosol properties (refractive index, single-scattering albedo and effective particles radii). The high quality of the aerosol optical properties vertical profiles has been demonstrated within the quality assurance exercises performed within EARLINET [Böckmann et al., 2004; Pappalardo et al., 2004a]. Two further channels were added to the new CNR-IMAA aerosol lidar in order to detect the components of backscattered light polarized perpendicular and parallel to the direction of the linearly polarized transmitted laser beam at 532 nm. Starting from these two lidar signals and with the support of the total backscattered signal at 532nm, depolarization ratio profiles are obtained providing information about shape and orientation of aerosolic particles. This upgraded system is involved in the validation program of aerosol data products from the CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations) satellite mission, providing a reference point for depolarization ratio and aerosol backscatter at 532 and 1064 nm measurements.

Long-term measurements

Starting on May 2000, we perform three systematic lidar measurements per week according to the EARLINET schedule. On the base of systematic measurements, a climatological study has been carried out in terms of the seasonal behavior of the PBL height and of the aerosol optical properties calculated inside the PBL itself. All extensive properties show a significant seasonal dependence, except for lidar ratio at 355nm that, instead, does not depend on the season and is characterized by a single mode Gaussian distribution centered around 37 sr [Pappalardo et al., 2003]. Further measurements are performed in order to investigate particular events, like dust intrusions, volcanic eruptions and forest fires. Among cases with aerosol layer above the PBL, cases of Saharan dust intrusions at our site are identified by means of backtrajectory analysis and in accordance with satellite images. Because of the short distance between our site and the Sahara desert, about 1 day of Saharan dust intrusion every 10 days is observed [Mona et al., 2006]. The Saharan dust layers are observed between 1.8 and 9 km. A mean optical depth of 0.13 is observed inside the Saharan dust layer, reaching a maximum value of 0.68. The source origin is the central Sahara in about 65% of the cases, the western Sahara in about 31%, and only in 4 cases the eastern Sahara. A tri-modal Gaussian distribution has been found for lidar ratio values at 355 nm measured inside the dust layer, with a mean value for the central “pure” part of the layer of about 37sr [Mona et al., 2006]. During these 6 years of observations, very peculiar cases of volcanic aerosol emitted during Etna eruptions in summer 2001 and autumn 2002 have been observed at CNR-IMAA. In particular, in 2002, both direct and long path transport from volcano to our station shows significant differences in lidar ratio variability inside the volcanic aerosol layer [Pappalardo et al., 2004b; Villani et al., 2006]. In summer 2004, aerosol load in the free troposphere related to large forest fires burning occurred in Alaska and Canada has been observed at CNR-IMAA in the frame of ICARTT campaign. During this event a larger lidar ratio variability indicates cases characterized by mixing with local aerosol and desert dust.

Acknowledgment

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

References

Böckmann, C. et al., Aerosol lidar intercomparison in the framework of EARLINET. 2. Aerosol backscatter algorithms, Appl. Opt., 43, 4, 977-989, 2004.

Mona L., et al., Saharan dust intrusions in the Mediterranean area: three years of Raman lidar measurements, accepted for publication on J. Geophys. Res., 2006.

Pappalardo, G., et al., One year of tropospheric lidar measurements of aerosol extinction and backscatter, Ann. Geophys., 46, 401-413, 2003.

Pappalardo, G. et al., Aerosol lidar intercomparison in the framework of EARLINET. 3. Raman lidar algorithm for aerosol extinction, backscatter and lidar ratio, Appl. Opt., 43, 5370-5385, 2004a.

Pappalardo, G. et al., Raman lidar observations of aerosol emitted during the 2002 Etna eruption, Geophys. Res. Lett., 31, L05120, doi:10.1029/2003GL019073, 2004b.

Villani, M.G. et al., Transport of volcanic aerosol in the troposphere: the case study, in press J. Geophys. Res., 2006.