Roberto Barbini

Self-absorption model in quantitative laser induced breakdown spectroscopy measurements on soils and sediments ☆

Abstract Application of laser induced breakdown spectroscopy (LIBS) in the quantitative analysis of elemental composition of soils with different origins and Antarctic marine sediments has been considered. The analytical method followed includes the usual plasma modeling at local thermal equilibrium (LTE) based on average temperature and electron density values, as well as spectra normalization, introduced in order to reduce the effects related both to the substrate optical and thermal properties and to the influence of laser parameters on quantitative data. The computational algorithm takes into account only atomic species and their first ionization states, which is sufficient at the plasma temperature measured in the experiments. Calibration curves are finally generated for each element of interest measured on certified samples with different provenience and matrix composition. In this paper a model is developed which takes into account the effects responsible for non-linearities in the relationship between line intensity and elemental concentration. The model properly includes line re-absorption and contributions from space regions with different plasma densities. Its application permits us to obtain the correlation coefficients between the LIBS measured and certified concentration of each element analyzed. These coefficients, specific for a given experimental layout and atomic lines data base, are successively applied in analytical LIBS measurements allowing for the direct determination of a single element concentration in any sample, regardless of its unknown matrix composition. The LIBS method presented here was tested on a priori unknown samples, and gave uncertainties in concentration varying from 15 to 40% over a large concentration range covering several orders of magnitude. The measuring error depends on element type, on the concentration value and also on the number of certified samples used for the initial calibration. The present results are already significant for some field application, such as on-board marine sediment analysis where a significant matrix variation with layer depth is common.

On board LIBS analysis of marine sediments collected during the XVI Italian campaign in Antarctica

Abstract The Laser-induced Breakdown Spectroscopy technique was applied on board the R/V Italica during the XVI Antarctic campaign (2000–2001) to carry out elemental chemical analysis of marine sediments collected using different sampling systems. To this end, a compact system has been built, which was suitable to operate also in the presence of mechanical vibrations, induced by the ship motion. Qualitative and quantitative analyses were performed on dried samples, without any further pre-treatment. Qualitative analyses have shown similar elemental composition among different collected sediments, except for significant differences in the case of rock fragments and manganese nodule. The latter also contains some heavy metals that in sediment layers were detected only in traces. The methodology to retrieve relative or absolute elemental concentration in heterogenous samples has been optimized and is scarcely sensitive to variations of sediment physical properties with depth, and to experimental parameters such as laser defocusing because of surface irregularities, and laser energy fluctuations. The relative distribution of the major elemental constituents, both from a bio-organic and mineral origin, was measured as a function of sediment depth. Measurements, once limited to specific spectral sections, and data analyses are fast and very reproducible. Most of the elements show a gradually varying distribution along the sampled core, except for silicon and barium, whose steep decrease with depth is strongly related to their biogenic origin. Quantitative LIBS analyses were performed on a limited number of samples and the results reported here, are comparable to the certified element contents in a reference sample of Antarctic sediments.

Differential lidar fluorosensor system used for phytoplankton bloom and seawater quality monitoring in Antarctica

A mobile fluorosensor system has been built to participate in oceanographic campaigns. During the first mission, carried out within the XIII Italian Antarctic Expedition (Research Vessel Italica, November 1997 to January 1998), seawater quality parameters (phytoplankton concentration, yellow matter, turbidity and biomass productivity) were remotely and locally monitored both in the Ross Sea and along the route back to New Zealand. Data were georeferenced by means of a GPS recorder to be subsequently plotted on thematic maps. The core of the system is a compact lidar fluorosensor capable of single or dual laser excitation of the target. In the single excitation mode, the detection of chromophores peculiar to various water impurities is possible, while dual laser excitation is necessary to monitor the phytoplankton photosynthetic activity on the chlorophyll emission channel. A lamp spectrofluorometer, a pulsed amplitude fluorometer (PAM) and a solar radiance detector completed the set of instruments onboard.

Lidar fluorosensor calibration of the SeaWiFS chlorophyll algorithm in the Ross Sea

Surface chlorophyll-a concentrations were measured in the Ross Sea and in the Antarctica-New Zealand transect by the lidar fluorosensor aboard the research vessel Italica during the 13th Italian Antarctic Oceanographic Campaign (December 1997-January 1998). The lidar measurements were compared to the data collected by the Sea-viewing Wide Field-of-view Sensor (SeaWiFS). The study area contained four distinct regions: coastal waters located east of Terra Nova Bay (chlorophyll-a 0.2-9 mg m m 3 ), offshore waters of the Ross Sea (chlorophyll-a 0.03-7 mg m m 3 ), offshore waters of the Ross Sea-New Zealand transect (chlorophyll-a 0.1-2 mg m m 3 ) and coastal waters located south of New Zealand (chlorophyll-a 0.2-9 mg m m 3 ). In coastal waters, lidar values were significantly higher than those of SeaWiFS. In offshore waters the discrepancy was lower and the experimental points were enough to calibrate the SeaWiFS chlorophyll-a algorithm against the lidar fluorosensor measurements in the Ross Sea.

Analysis of simultaneous chlorophyll measurements by lidar fluorosensor, MODIS and SeaWiFS

The ENEA (Italian Agency for New Technologies, Energy and the Environment) lidar fluorosensor (ELF), aboard the research vessel Italica, measured continuously surface chlorophyll-a concentrations during the Italy–New Zealand and New Zealand–Italy transects (13 November–18 December 2001 and 28 February–1 April 2002, respectively). The ELF measurements were compared with the data collected by the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) and the Moderate Resolution Imaging Spectroradiometer (MODIS). This study pointed out advantages, disadvantages and possible synergies of lidar fluorosensor and spaceborne radiometers. In particular, the SeaWiFS and MODIS bio-optical algorithms have been calibrated with the ELF measurements. The differences between the performances of the two spaceborne radiometers are also briefly discussed.

Shipborne laser remote sensing of the Venice lagoon

Abstract A complex active remote sensing system has been used on board a small ship to extensively monitor the water quality of the Venice lagoon and nearby open sea. The system was composed of an UV lidar fluorosensor directly pointing to the sea surface and a laser fluorometer monitoring the water inside a cell continuously filled by a pump from a water depth of 1m. During the cruise, both apparata were collecting data in parallel. Laser-Induced Fluorescence (LIF) signals at selected wavelengths were acquired to monitor distributions of the different species. DOM and chlorophyll features were detected by the lidar fluorosensor upon excitation at lambda 355nm, while organic pollutants and oils were scanned in the fluorometer cell by laser emitting at lambda 266nm. In both cases, the accompanying water Raman signal was collected and used for normalizing spectral intensities. Absolute concentrations of different species were obtained off-line, when possible, by calibrating LIF intensities against analytica...

Remotely sensed primary production in the western Ross Sea: results of in situ tuned models

The Southern Ocean plays an important role in the global carbon cycle and, as a consequence, in the planetary climate equilibrium. The Ross Sea is one of the more productive regions in the Southern Ocean, due to strong phytoplankton blooms occurring during summer. Satellite remote sensing is a powerful tool for investigating such phenomena, especially if the bio-optical algorithms are tuned with in situ data. In this paper, after having compared the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) and the ENEA Lidar Fluorosensor (ELF), the SeaWiFS chlorophyll a (Chl a) algorithm is tuned in the Ross Sea by means of the ELF measurements. The Chl a concentrations obtained in this way have been the basis for estimating productivity values and their evolution during summer 1997–98. Three primary production models have been used, providing information on their accuracy and performance in the Antarctic environment. Our investigations suggest that the primary production was lower than usual during the period 3 December 1997–16 January 1998.

Simultaneous Measurements of Remote Lidar Chlorophyll and Surface CO2 Distributions in the Ross Sea.

A comparison between continuous lidar fluorosensor chlorophyll-a measurements and partial pressure of CO2 (pCO2) in surface water is presented. The data were gathered during the XVI Italian Antarctic expedition in the Western Ross Sea and crossing the Southern Ocean. Anticorrelation between CO2 levels and chlorophyll-a determinations was observed throughout the cruise in the late austral summer 2000/01. The survey revealed the presence of high productivity in polynya regions and close to the ice edges. The lidar data, reported here, are consistent with the results of previous campaigns conducted in the same investigated area. The relationship between pCO2 and chlorophyll-a concentration are discussed in terms of biologically driven phenomena responsible for the pCO2 undersaturation with respect to the atmospheric value during the austral summer.

Comparison of SeaWiFS, MODIS‐Terra and MODIS‐Aqua in the Southern Ocean

The surface chlorophyll‐a concentrations measured by SeaWiFS, MODIS‐Terra and MODIS‐Aqua are compared in the Southern Ocean in summer 2003. The radiometers generally agree within their estimated accuracy. Residual discrepancies could be reduced by regional calibrations of the bio‐optical algorithms.

Application of a lidar fluorosensor system to the continuous and remote monitoring of the Southern Ocean and Antarctic Ross Sea: results collected during the XIII and XV Italian oceanographic campaigns

A mobile fluorosensor laboratory, including as main constituent a lidar fluorosensor apparatus for remote measurements at water surface and ancillary instruments dedicated to local analyses, was developed for extensive biooptical monitoring of the sea waters. Continuous remote and local measurements of Chl-a (Chlorophyll a) and CDOM (Chromophoric Dissolved Organic Matter) were performed during the XIII (1997/1998) and XV (1999/2000) marine campaigns in the South-western Ross Sea and along the Southern Ocean transects to New Zealand as part of the Italian Research Programme for Antarctica. Chla and CDOM fluorescence maps of the investigated areas revealed the bio-optical peculiarity of coastal zones and seasonal changes encountered.