Antonella Boselli

Aerosol lidar intercomparison in the framework of the EARLINET project. 2.Aerosol backscatter algorithms

An intercomparison of aerosol backscatter lidar algorithms was performed in 2001 within the framework of the European Aerosol Research Lidar Network to Establish an Aerosol Climatology (EARLINET). The objective of this research was to test the correctness of the algorithms and the influence of the lidar ratio used by the various lidar teams involved in the EARLINET for calculation of backscatter-coefficient profiles from the lidar signals. The exercise consisted of processing synthetic lidar signals of various degrees of difficulty. One of these profiles contained height-dependent lidar ratios to test the vertical influence of those profiles on the various retrieval algorithms. Furthermore, a realistic incomplete overlap of laser beam and receiver field of view was introduced to remind the teams to take great care in the nearest range to the lidar. The intercomparison was performed in three stages with increasing knowledge on the input parameters. First, only the lidar signals were distributed; this is the most realistic stage. Afterward the lidar ratio profiles and the reference values at calibration height were provided. The unknown height-dependent lidar ratio had the largest influence on the retrieval, whereas the unknown reference value was of minor importance. These results show the necessity of making additional independent measurements, which can provide us with a suitable approximation of the lidar ratio. The final stage proves in general, that the data evaluation schemes of the different groups of lidar systems work well.

Aerosol observations by lidar in the nocturnal boundary layer

Aerosol observations by lidar in the nocturnal boundary layer (NBL) were performed in Potenza, Southern Italy, from 20 January to 20 February 1997. Measurements during nine winter nights were considered, covering a variety of boundary-layer conditions. The vertical profiles of the aerosol backscattering coefficient at 355 and 723.37 nm were determined through a Klett-modified iterative procedure, assuming the extinction-to-backscattering ratio within the NBL has a constant value. Aerosol average size characteristics were retrieved from almost simultaneous profiles of the aerosol backscattering coefficient at 355 and 723.37 nm, the measurements being consistent with an accumulation mode radius not exceeding 0.4 microm. Similar results in terms of aerosol sizes were obtained from measurements of the extinction-to-backscattering ratio profile at 355 nm performed on six nights during the measurement campaign. Backscattering profiles at 723.37 nm were also converted into profiles of aerosol liquid water content.

Volcanic ash concentration during the 12 August 2011 Etna eruption

Mount Etna, in Italy, is one of the most active volcanoes in the world and an ideal laboratory to improve volcano ash monitoring and forecasting. During the volcanic episode on 12 August 2011, an eruption column rose up to several kilometers above sea level (asl), and the volcanic plume dispersed to the southeast. From the video-surveillance system, we were able to estimate variations in the column height (peak value of 9.5 ± 0.5 km above sea level) with time. We derived the time-varying discharge rate (peak value of 60 m3 s−1) and determined the ash concentration using a volcanic ash dispersal model. The modeled ash concentration was compared with lidar measurements using different particle effective radius, and differences are within the error bars. Volcanic ash concentrations range from 0.5 to 35.5 × 10−3 g m−3. The comparison highlights that to improve volcanic ash forecasting during volcanic crises it is necessary to take into account the time-varying discharge rate of explosive eruptions.

Pulsed laser ablation of borocarbide targets probed by time-of-flight mass spectrometry

Abstract Excimer laser ablation of superconductive borocarbide material (YNi2B2C) in typical conditions for the deposition of superconductive thin films has been investigated using time-of-flight mass spectrometry. The mass spectra show the presence of all the target elemental ionized atoms as well as diatomics. The ablation yield of the metal ions is a strongly increasing function of the laser fluence, while the contrary is true for non-metal ions. The dependence of non-metal light mass diatomic ions on laser fluence indicates the presence of aggregation processes as the laser fluence is increased. Moreover, evidence of aggregation processes involving metallic ions at high laser fluence is also obtained by the mass spectra. An interesting aspect of our results is the observation of an ion spatial distribution characterized by the presence of the lighter species at the plume edges, while the heavier ones are concentrated at the plume center.

Characterization of atmospheric aerosol in the urban area of Napoli in the framework of EARLINET Project

Systematic Lidar measurements of tropospheric aerosols optical properties have been carried out in the urban area of Napoli (Southern Italy, 40°50’18”N, 14°10’59”E, 118 m above sea level), in the framework of EARLINET project. Lidar systematic measurements of aerosol backscattering and extinction profiles have been performed at laser wavelength of 351nm and were obtained from simultaneously measurements of elastic and N2 Raman signals. Following the EARLINET protocol, regular measurements have been performed three times a week in two days and they provide information on aerosol optical properties, with a final spatial resolution of 60m and a temporal resolution between 1 and 30 min. A statistical analysis in terms of integrated backscattering (BI), optical depth (OD), extinction to backscattering ratio (LR), and Dust Layer height (DL), obtained from measurements carried out in clear sky conditions over 30 months, has been realized. Further measurements have been performed during Saharan Dust transport events and some detailed observation of complete diurnal cycle has been carried out, in order to know the dynamic and the evolution of the Planetary Boundary Layer

Expectation maximization and the retrieval of the atmospheric extinction coefficients by inversion of Raman lidar data.

We consider the problem of retrieving the aerosol extinction coefficient from Raman lidar measurements. This is an ill-posed inverse problem that needs regularization, and we propose to use the Expectation-Maximization (EM) algorithm to provide stable solutions. Indeed, EM is an iterative algorithm that imposes a positivity constraint on the solution, and provides regularization if iterations are stopped early enough. We describe the algorithm and propose a stopping criterion inspired by a statistical principle. We then discuss its properties concerning the spatial resolution. Finally, we validate the proposed approach by using both synthetic data and experimental measurements; we compare the reconstructions obtained by EM with those obtained by the Tikhonov method, by the Levenberg-Marquardt method, as well as those obtained by combining data smoothing and numerical derivation.

Lidar measurements of atmospheric transmissivity

SummaryThe possibility of performing atmospheric-transmissivity measurements by lidar is considered. In the present paper two distinct methods have been successfully applied. The first one is based on the detection of the Raman return from molecular nitrogen. An alternative method is based on the simultaneous detection of the elastic and N2 Raman returns. Measurements based on both techniques are discussed in detail, the second technique producing more accurate results. Through this technique an estimation of the Ångström coefficient can also be obtained.

Multiparametric tunable lidar system based on IR OPO laser sources

The present paper is devoted to the description of two distinct multiparametric lidar system presently under development. The first system is based on a Nd:YAG laser pumping a dye laser equipped with a dual wavelength device. The second lidar system is based on two optical parametric oscillator (OPO) lasers pumped by a Nd:YAG laser operating at a pulse repetition rate of 100 Hz. OPO lasers represent a new design for coherent radiation sources continuously tunable in the UV -- near IR range (up to 2 micrometer). This allows us to perform differential absorption lidar (DIAL) measurements in a spectral region where most of atmospheric pollutants display absorption lines. The selection of the wavelengths to be used for DIAL measurements is a non trivial task. In particular, a method to select the optimal (lambda) ON and (lambda) OFF has been developed and applied to several molecular gases. Both systems have been designed to carry out simultaneous measurements of atmospheric aerosols, water vapor profiles, temperature and density profiles, atmospheric transmissivity, and atmospheric pollutants concentration.

Contribution of EARLINET/ACTRIS to the summer 2013 Special Observing Period of the ChArMEx project: monitoring of a Saharan dust event over the western and central Mediterranean

ABSTRACT In the framework of the Chemistry-Aerosol Mediterranean Experiment (ChArMEx; http://charmex.lsce.ipsl.fr/) initiative, a field campaign took place in the western Mediterranean Basin between 10 June and 5 July 2013 within the ADRIMED (Aerosol Direct Radiative Impact on the regional climate in the MEDiterranean region) project. The scientific objectives of ADRIMED are the characterization of the most common ‘Mediterranean aerosols’ and their direct radiative forcing (column closure and regional scale). During 15–24 June a multi-intrusion dust event took place over the western and central Mediterranean Basin. Extra measurements were carried out by some EARLINET/ACTRIS (European Aerosol Research Lidar Network /Aerosols, Clouds, and Trace gases Research InfraStructure Network, http://www.actris.net/) lidar stations in Spain and Italy, in particular on 22 June in support to the flight over southern Italy of the Falcon 20 aircraft involved in the campaign. This article describes the physical and optical properties of dust observed at the different lidar stations in terms of dust plume centre of mass, optical depth, lidar ratio, and particle depolarization ratio. To link the differences found in the origin of dust plumes, the results are discussed on the basis of back-trajectories and air- and space-borne lidars. This work puts forward the collaboration between a European research infrastructure (ACTRIS) and an international project (ChArMEx) on topics of interest for both parties, and more generally for the atmospheric community.

Simultaneous application of the Raman and DIAL techniques for ground-based water vapor lidar measurements in the nocturnal boundary layer

High temporal and spatial resolution measurements by both space-borne and ground-based instruments are therefore necessary to monitor atmospheric water vapor for climate purposes. Lidar techniques can accomplish ground-based measurements of atmospheric water vapor with high space and time resolution. A lidar system capable to perform simultaneous measurements of atmospheric water vapor and aerosols has been developed in Potenza, Southern Italy, in the context of a cooperation between the Universita della Basilicata and Istituto di Metodologie Avanzate di Analisi Ambientale. An intensive measurement campaign was performed during 1997 in Potenza aimed to the simultaneous application of the Raman and DIAL techniques for water vapor ground- based lidar measurements. Raman and DIAL humidity measurements performed during 1997 in Potenza aimed to the simultaneous application of the Raman and DIAL techniques for water vapor ground-based lidar measurements. Raman and DIAL humidity measurements performed at night in the Planetary Boundary Layer have been compared with simultaneous radiosonde data obtained from both free and captive balloons. The agreement among Raman, DIAL and radiosonde data is found to be within 30 percent up to approximately 1.5 km. Results from this measurement campaign will be reported and discussed in this paper.