Rodolfo Guzzi

Atmospheric aerosol optical properties: a database of radiative characteristics for different components and classes

A database management system has been realized that, by taking physical and chemical properties (the complex refractive index and the size distribution) of basic components as its starting point, allows the user to obtain optical properties of default as well as user-defined aerosol classes. Default classes are defined in accordance with the most widely known and used aerosol models. We obtain user-defined classes by varying the mixing ratio of components, creating new mixtures of default components, or by defining user components, thereby supplying the size distribution and the refractive index. The effect of relative humidity (RH) on the refractive index and the size distribution is properly accounted for up to RH = 99%. The two known mechanisms of obtaining classes from components are allowed (internal or external mixing).

Retrieval of aerosol properties over the ocean using Global Ozone Monitoring Experiment measurements: Method and applications to test cases

Satellite monitoring of aerosol properties using passive techniques is widely considered a crucial tool for the study of climatic effects of atmospheric particulate [Kaufman et al., 1997]. Only space-based observations can provide the required global coverage information on spatial distribution and temporal variation of the aerosol field. This paper describes a method for deriving aerosol optical thickness at 500 nm and aerosol type from Global Ozone Monitoring Experiment (GOME) data over the ocean under cloud-free conditions. GOME, flying on board the second European Remote Sensing satellite (ERS 2) since April 1995, is a spectrometer that measures radiation reflected from Earth in the spectral range 240–793 nm. The features of the instrument relevant to the aerosol retrieval task are its high relative radiometric accuracy (better than 1%), its spectral coverage, and its spectral resolution, which allows wavelengths in spectral regions free of molecular absorption (atmospheric windows) to be selected. The method presented is based on a pseudo-inversion approach in which measured reflectance spectra are fitted to simulated equivalents computed using a suitable radiative transfer model. The crucial aspects of this method are the high accuracy and the nonapproximate nature of the radiative transfer model, which simulates the spectra during the fitting procedure, and careful selection of candidate aerosol classes. A test application of the proposed method to a Saharan dust outbreak which occurred in June 1997 is presented, showing that in spite of the instruments low spatial resolution, information on both optical thickness and spectral characterization of the aerosol can be retrieved from GOME data. Preliminary comparisons of the results with independent estimates of the aerosol content show that a good correlation exists, encouraging planning of a systematic application of the method.

Atmospheric correction of data measured by a flying platform over the sea: elements of a model and its experimental validation

A model for the atmospheric correction of data remotely sensed over the sea at different altitudes is described in detail and validated through experimental data measured in the North Adriatic Sea during the Adria 83 campaign by a modular multispectral scanner.

Aerosol size spectra from spectral extinction data: the use of a linear inversion method

A modification of the Twomey-Phillips linear constrained inversion method is used to retrieve aerosol size spectra from simulated spectral optical depths in the wavelength range from 0.37 to 2.2 microm. It is found that the transition between nonphysical solutions to well-behaved ones, which is driven by the smoothing parameter gamma is gradual and that negative components, when present in the solution, always belong to the smallest particle range. When values of gamma greater than 0.05 are required to obtain well-behaved solutions, a strong bias on the retrieved solution is posed by the form of the first-guess solution. Moreover, even if the retrieved size distribution computed for low values of gamma is not well-behaved, much information on the shape of the true solution in the medium and large particle range is contained in the retrieved solution. The effect of realistic random errors added to the simulated optical depths is also discussed.

Spectroradiometer for ground-based atmospheric measurements related to remote sensing in the visible from a satellite

A spectroradiometer was developed for use on the ground and aboard ship. The instrument separates global and diffuse solar radiation with a rotating shading band. Calibration results are compared with those of other sun photometers. The instrument was also mounted on a stabilized platform during an experimental trial aboard ship on the Tyrrhenian Sea. Results are comparable with those obtained by handheld sun photometer.

Aerosol maps from GOME data

In this paper, we present a methodology to calibrate the surface reflectance seen by satellite and validate the aerosol optical properties retrieved by the GOME instrument. Data are also visualized in maps by a tool properly developed, named GOMEView. The validation procedure is based on ground measurements obtained by sunphotometers. Results show that calibration of the surface reflectance is crucial to obtain the best results, i.e. in agreement with the ground measurements. Aerosol data have also been classified on the basis of their optical properties evidencing for instance, the presence of desert aerosol over the sea along the west coast of Sahara. Cloud retrievals were also analyzed in terms of their occurrence and amount.

Effectiveness of the MS-method for computation of the intensity field reflected by a multi-layer plane-parallel atmosphere

Abstract A radiative transfer code based on the coupling of the currently labeled MS method (MS refers to the separation of the multiply and singly scattered radiation), with the reliable and widely used radiative transfer package DISORT is presented. We show that this code can be used to compute the intensity field reflected by a plane-parallel, non-emitting, aerosol loaded atmosphere with the same accuracy as a non-approximate model but maintaining a high computational speed. Results obtained for a two-layer atmosphere show that the single scattering features are clearly visible in the radiative field in the range from moderate to high aerosol optical thicknesses. Tests carried out in a reasonable range of viewing geometries, restricted to dark surfaces, and considering a significant set of aerosol optical properties, have shown that the present code is capable of attaining the same accuracy as DISORT but using a greatly decreased number of angular discretizations (streams), thereby reducing the computational time by a factor of between 2 and 10 with respect to DISORT , depending on the complexity of the scenario.

Water vapor absorption in the visible and near infrared: results of field measurements

Measurements of solar irradiance at the ground have been analyzed to obtain information on absorption from water vapor in the visible and near infrared. Great care has been taken in evaluating the aerosol optical thickness to obtain results compatible with theory. An automatic procedure is presented that eliminates the recordings in which modifications of the aerosol optical properties not monitored would seriously influence the determination of those of water vapor. Particular care is paid to assessing the error limits of the derived spectral attenuation parameters.

Theoretical and practical consideration of the construction of a zero-geometric-loss multiple-pass cell based on the use of monolithic multiple-face retroreflectors

The theory of the multiple-pass cell based on the use of retroreflectors is presented. As a result of this study, it is shown that it is possible to construct an enhanced White cell with zero geometric loss. Starting from theoretical considerations of the design of a new monolithic multiple-face retroreflector, a multiple-pass cell is proposed. Ray-tracing simulations indicate that this cell is easy to align and has zero geometric loss over a very long optical path.

Solar direct irradiance at the ground: A parametric approach

Abstract A parametric model, giving the solar direct irradiance at the ground, using as input parameters the precipitable water vapour and the optical thickness of particulate matter is presented. The validity of the parametrization is tested against a spectral model checked during field studies.