Search

The latest in a series of solar occultation satellite instruments, Stratospheric Aerosol and Gas Experiment (SAGE) III, was placed into orbit in December 2001, and data were obtained until March 2006. Measurements were made of the extinction attributable to aerosols and cloud at a number of wavelengths between 290 and 1550 nm. The analysis of data obtained by its predecessor, SAGE II, has shown that an intercomparison of such data at two or more wavelengths may be used to separate the effects of cloud and aerosol. This analysis has been done on a routine basis for many years using SAGE II data at 525 and 1020 nm and applied extensively to global studies of tropospheric cloud and aerosol. Here we describe the aerosol-cloud separation algorithm developed for use with the SAGE III data, which uses the extinction at 525, 1020, and 1550 nm. This algorithm is now being used to produce vertical profiles of cloud presence as a standard SAGE III data product. These profiles have a vertical resolution of 0.5 km and cover the altitude range from 6.0 to 30.0 km, and data are presently available from March 2002 onward. An outline is given of the development of this algorithm, the nature of the SAGE III data, and the algorithm performance. To maintain continuity with SAGE II cloud data, the relative performances of the SAGE II and SAGE III algorithms are also examined. An example of the application of the algorithm to SAGE III tropospheric data is shown and discussed. OCIS codes: 280.0280, 280.1100, 010.1290, 290.1090.

Methods for accurately characterizing aerosols are required for detecting biological warfare agents. Currently, fluorescence-based biological agent sensors provide adequate detection sensitivity but suffer from high false-alarm rates. Combining single-particle fluorescence analysis with laser-induced breakdown spectroscopy (LIBS) provides additional discrimination and potentially reduces false-alarm rates. A transportable UV laser-induced fluorescence-cued LIBS test bed has been developed and used to evaluate the utility of LIBS for biological-agent detection. Analysis of these data indicates that LIBS adds discrimination capability to fluorescence-based biological-agent detectors. However, the data also show that LIBS signatures of biological agent simulants are affected by washing. This may limit the specificity of LIBS and narrow the scope of its applicability in biological-agent detection. OCIS codes: 140.3440, 300.6210, 300.6360.

Simulation studies were carried out with regard to the feasibility of using combined observations from sunphotometer (SPM) and lidar for microphysical characterization of aerosol particles, i.e., the retrieval of effective radius, volume, and surface-area concentrations. It was shown that for single, homogeneous aerosol layers, the aerosol parameters can be retrieved with an average accuracy of 30% for a wide range of particle size distributions. Based on the simulations, an instrument combination consisting of a lidar that measures particle backscattering at 355 and 1574 nm, and a SPM that measures at three to four channels in the range from 340 to 1020 nm is a promising tool for aerosol characterization. The inversion algorithm has been tested for a set of experimental data. The comparison with the particle size distribution parameters, measured with in situ instrumentation at the lidar site, showed good agreement. OCIS codes: 010.0010, 010.1100, 010.1110, 010.3640, 120.0280.

A comprehensive analytical radiative transfer model for isothermal aerosols and vapors for passive infrared remote sensing applications (ground-based and airborne sensors) has been developed. The theoretical model illustrates the qualitative difference between an aerosol cloud and a chemical vapor cloud. The model is based on two and two/four stream approximations and includes thermal emission--absorption by the aerosols; scattering of diffused sky radiances incident from all sides on the aerosols (downwelling, upwelling, left, and right); and scattering of aerosol thermal emission. The model uses moderate resolution transmittance ambient atmospheric radiances as boundary conditions and provides analytical expressions for the information on the aerosol cloud that is contained in remote sensing measurements by using thermal contrasts between the aerosols and diffused sky radiances. Simulated measurements of a ground-based sensor viewing Bacillus subtilis var. niger bioaerosols and kaolin aerosols are given and discussed to illustrate the differences between a vapor-only model (i.e., only emission--absorption effects) and a complete model that adds aerosol scattering effects. OCIS codes: 280.1310, 280.1120, 290.1090, 290.4210, 010.1300, 300.6340.

Two aerosol extinction models have been developed using statistical analysis of long-term optical transmission measurements in Sweden performed at two locations from July 1977 to June 1982. The aerosol volume extinction coefficient for infrared (IR) radiation is calculated by the models with visibility, temperature, and air pressure as input parameters. As in the MODTRAN model, the IR extinction coefficient is proportional to the coefficient at 550 rim, which depends on the visibility. In the new models, the wavelength dependence of the extinction also depends on the visibility. The models predict significantly higher attenuation in the IR than does the Rural aerosol model from MODTRAN, which is commonly used. Comparison with the Maritime model shows that the new models predict lower extinction values in the 3-5 [micro]m region and higher values in the 8-12 [micro]m region. The uncertainties in terms of variance levels are calculated by the models. The properties of aerosols, and thereby the extinction coefficient, are partly correlated to local meteorological parameters, which enables the calculation of a mean predicted value. A substantial part of the variation is, however, caused by conditions in the source area and along the trajectory path of the aerosols. They are not correlated to the local meteorological parameters and therefore cause the variance in the models. OCIS codes: 010.0010, 010.1110, 010.1310, 010.1320, 290.2200.

A combination of laboratory experiments, theoretical modeling, and spacecraft observations is employed to characterize the aerosols in the atmosphere of Titan. The scattering properties of model aerosols were measured using the Microwave Analog Light Scattering Facility at the University of Florida and complemented with theoretical modeling of single scattering characteristics and radiative transfer in Titan's atmosphere. This study compares these modeling results with photopolarimetric observations made over a range of phase angles by the Pioneer 11 and Voyagers 1 and 2 spacecraft. Important results of this work include a survey of the scattering properties of different particle morphologies and compositions necessary to accurately interpret these observations without introducing non-physical assumptions about the particles or requiring additional free parameters to the radiative transfer models. Previous studies use calculation methods which, due to computing memory and processing time requirements, a priori exclude much of the parameter space that the microwave analog laboratory is ideal for exploring. The goal of the present work, to directly constrain aerosol physical characteristics, is addressed by studying in a consistent manner how a variety of particle morphologies and refractive indices affect the polarization and intensity reflected by Titan's atmosphere. Based on comparisons of model results to spacecraft observations, many model morphologies are excluded from further consideration. The most plausible physical particle models suggest that a combination of Rayleigh-like single particles and aggregates that are larger than those previously suggested and investigated [West, R.A., Smith, P.H., 1991. Evidence for aggregate particles in the atmospheres of Titan and Jupiter. Icarus 90, 330-333; Rannou, P., Cabane, M., Botet, R., Chassefiere, E., 1997. A new interpretation of scattered light measurements at Titan's limb. J. Geophys. Res. 102, 10997-11013] provide the best fit to the existing data. Additional laboratory experiments and more refined modeling awaits the results of the new rich observational dataset from the Cassini/Huygens encounter with Titan. Keywords: Experimental techniques; Radiative transfer; Titan

The inversion method for simultaneous gas ([O.sub.3], N[O.sub.2], HN[O.sub.2], [N.sub.2]O, C[H.sub.4], [H.sub.2]O, CFC-11, CFC-12, [N.sub.2][O.sub.5], and ClON[O.sub.2]) and aerosol retrievals from broadband continuous IR spectra of occultation measurements is described. Both gas and aerosol physical modeling with consideration of the multicomponent character of aerosol and polar stratospheric clouds (PSCs) are used to minimize the difference between measured and modeled transmittance spectra under smoothness constraints imposed on particle-size distributions for each PSC component and positive constraints on all gas and aerosol parameters. The method is tested by numerical simulations in which synthetic occultation measurements inherent to the improved limb atmospheric spectrometer are used. The study reveals that the method has significant advantages over other approaches based on offset or gas--window--channel aerosol correction for accurate gas retrievals and provides additional information on the particle-size composition, volume density, and chemical component character of PSCs. OCIS codes: 280.0280, 280.1100, 010.1280.

A new optical particle counter was developed to provide fast in situ sizing of cloud droplets in the Leipzig Aerosol and Cloud Interaction Simulator (LACIS). The new instrument features white light for the illumination of the sampling volume: two off-axis elliptical mirrors, providing a wide angle of collection for light scattered by particles; and an optically defined sampling volume. The smooth unambiguous response characteristic for water droplets allows direct conversion of the measured signal amplitudes into droplet diameters. Preliminary response measurements for dry polystyrol microspheres and water droplets, grown in the LACIS on NaCl particles, have shown good agreement with the corresponding calculated response curves. OCIS codes: 010.1100, 120.4570, 290.1090, 290.4020, 290.5850.

Linearly polarized radiation is sensitive to the microphysical properties of aerosols, namely, to the particle-size distribution and refractive index. The discriminating power of polarized radiation increases strongly with the increasing range of scattering angles and the addition of multiple wavelengths. The polarization and directionality of the Earth's reflectances (POLDER) missions demonstrate that some aerosol properties can be successfully derived from spaceborne polarimetric, multiangular measurements at two visible wavelengths. We extend the concept to analyze the retrieval capabilities of a spaceborne instrument with six polarimetric channels at 412,445, 555, 865, 1250, and 2250 nm, measuring approximately 100 scattering angles covering a range between 50 and 150 deg. Our focus is development of an analysis methodology that can help quantify the benefits of such multiangular and multispectral polarimetric measurements. To that goal we employ a sensitivity metric approach in a framework of the principal-component analysis. The radiances and noise used to construct the sensitivity metric are calculated with the realistic solar flux for representative orbital viewing geometries, accounting for surface reflection from the ground, and statistical and calibration errors of a notional instrument. Spherical aerosol particles covering a range of representative microphysical properties (effective radius, effective variance, real and imaginary parts of the refractive index, single-scattering albedo) are considered in the calculations. We find that there is a limiting threshold for the effective size (approximately 0.7 [micro]m), below which the weak scattering intensity results in a decreased signal-to-noise ratio and minimal polarization sensitivity, precluding reliable aerosol retrievals. For such small particles, close to the Rayleigh scattering limit, the total intensity provides a much stronger aerosol signature than the linear polarization, inspiring retrieval when the combined signals of intensities and the polarization fraction are used. We also find a strong correlation between aerosol parameters, in particular between the effective size and the variance, which forces one to simultaneously retrieve at least these two parameters. OCIS codes: 280.1100, 010.1110, 120.2130.

The aerosol extinction measurements in the ultraviolet and visible wavelengths by the balloonborne spectrometer Spectroscopie d'Absorption Lunaire pour l'Observation des Minoritaires Ozone et [NO.sub.x] (SALOMON) show that aerosols are present in the middle stratosphere, above 25-km altitude. These observations are confirmed by the extinction measurements performed by a solar occultation radiometer. The balloonborne Laboratoire de Meteorologie Dynamique (LMD) counter instrument also confirms the presence of aerosol around 30-km altitude, with an unrealistic excess of micronic particles assuming that only liquid sulfate aerosols are present. An unexpected spectral structure around 640-nm observed by SALOMON is also detectable in extinction measurements by the satellite instrument Stratospheric Aerosols and Gas Experiment III. This set of measurements could indicate that solid aerosols were detected at these altitude ranges. The amount of soot detected up to now in the lower stratosphere is too low to explain these measurements. Thus, the presence of interplanetary dust grains and micrometeorites may need to be invoked. Moreover, it seems that these grains fill the stratosphere in stratified layers. OCIS codes: 010.1110, 280.1100.

in an effort to establish a more reliable set of optical cross sections for a variety of chemical and biological aerosol simulants, we have developed a flow-through photoacoustic system that is capable of measuring absolute, mass-normalized extinction and absorption cross sections. By employing a flow-through design we avoid issues associated with closed aerosol photoacoustic systems and improve sensitivity. Although the results shown here were obtained for the tunable C[O.sub.2] laser waveband region, i.e., 9.20-10.80 [micro]m, application to other wavelengths is easily achievable. The aerosols considered are categorized as biological, chemical, and inorganic in origin, i.e., Bacillus atrophaeus endospores, dimethicone silicone oil (SF-96 grade 50), and kaolin clay powder (alumina and silicate), respectively. Results compare well with spectral extinction measured previously by Fourier-transform infrared spectroscopy. Comparisons with Mie theory calculations based on previously published complex indices of refraction and measured size distributions are also presented. OCIS codes: 010.1100, 010.1110, 300.6430.

Marine sunphotometer measurements collected aboard the R/V Ronald H. Brown during the Aerosol Characterization Experiment--Asia (ACE-Asia) are used to evaluate the ability of complementary instrumentation to obtain the best possible estimates of aerosol optical thickness and Angstrom exponent from ships at sea. A wide range of aerosol conditions, including clean maritime conditions and highly polluted coastal environments, were encountered during the ACE-Asia cruise. The results of this study suggest that shipboard hand-held sunphotometers and fast-rotating shadow-band radiometers (FRSRs) yield similar measurements and uncertainties if proper measurement protocols are used and if the instruments are properly calibrated. The automated FRSR has significantly better temporal resolution (2 min) than the hand-held sunphotometers when standard measurement protocols are used, so it more faithfully represents the variability of the local aerosol structure in polluted regions. Conversely, results suggest that the hand-held sunphotometers may perform better in clean, maritime air masses for unknown reasons. Results also show that the statistical distribution of the Angstrom exponent measurements is different when the distributions from hand-held sunphotometers are compared with those from the FRSR and that the differences may arise from a combination of factors. OCIS codes: 010.0010, 120.0120, 280.0280, 290.0290.

The analytic eigenfunction inversion technique to retrieve aerosol columnar size distributions from Mie extinction measurements has been extended to include a priori information, specifically surface area. The earlier (standard) and new (subtracted/ techniques are compared by use of synthetic data that cover typical aerosol size distributions. Two different measurement wavelength ranges are considered. It is shown that the most appropriate inversion technique depends on the particle sizes, with the standard technique being more appropriate for small particles and the subtracted being more appropriate for large particles. Also presented is a simple method to determine whether a particular inversion technique is likely to produce meaningful results with a particular data set. OCIS codes: 010.1320, 280.1100. 010.1110, 280.1310, 290.4020.

This study concerns the development of a new inversion method for simultaneous gas and aerosol retrievals in the upper layers of the atmosphere from limb-viewing multiwavelength-transmission infrared measurements. In this method, concentrations of gas species such as [O.sub.3], N[O.sub.2], HN[O.sub.3], [N.sub.2]O, C[H.sub.4], and [H.sub.2]O, and spectral dependences of the aerosol extinction coefficient are retrieved simultaneously. When this is done, smoothness constraints on the desired spectral dependencies of the aerosol extinction coefficient are used as an a priori assumption. The method is used in the treating of synthetic transmission spectra of the Improved Limb Atmospheric Spectrometer, which is based on the solar occultation technique and was on board the Advanced Earth Observing Satellite. A set of numerical tests shows the efficiency of the method. OCIS codes: 010.1320, 280.1100, 280.1310, 290.1090.

We quantify the charge states of submicrometer aerosols and aromatic macromolecules in Titan's organic haze. The aerosol charge is balanced between the recombination of positive ions with the aerosol plus the ejection of electrons from the aerosol via the UV-driven photoelectric effect and the recombination of electrons with the aerosol. During the day, the dominant charge state for submicrometer aerosols is positive. Macromolecules composed of fewer than 32 carbon atoms with low electron affinities ( Key Words: Titan; stratospheric chemistry; aromatics; aerosol charging.

The sensitivity of aerosol optical depths [[tau].sub.1] and [[tau].sub.2] derived from the Advanced Very High Resolution Radiometer (AVHRR) channels 1 and 2, centered at [[lambda].sub.1] = 0.63 and [[lambda].sub.2] = 0.83 [micro]m, respectively, and of an effective Angstrom exponent [alpha], derived therefrom as [alpha] = -ln([[tau].sub.1]/[[tau].sub.2])/ln([[lambda].sub.1]/[[lambda].sub.2]), to calibration uncertainties, radiometric noise, and digitization is estimated. Analyses are made both empirically (by introduction of perturbations into the measured radiances and estimation of the respective partial derivatives) and theoretically (by use of a decoupled form of the single-scattering approximation of the radiative transfer equation). The two results are in close agreement. The errors, [DELTA] [[tau].sub.i] and [DELTA] [[alpha].sub.i], are parameterized empirically as functions of [[tau].sub.i], radiometric errors, and Sun and view geometry. In particular, the [alpha] errors change in approximately inverse proportion to [tau] and are comparable with, or even exceed, typical [alpha] signals over oceans when [tau] < 0.25. Their detrimental effect on the information content of the AVHRR-derived size parameter gradually weakens as [tau] increases. OCIS codes: 010.1110, 010.1310, 120.5630, 120.0280, 280.1100, 280.1310.

A fine calibration of the depolarization ratio is required for a detailed interpretation of lidar-observed polar stratospheric clouds. We propose a procedure for analyzing data by using atmospheric depolarization lidar. The method is based on a plot of [[delta].sub.T] versus (1 - [R.sub.T.sup.-1]), where [[delta].sub.T] is the total depolarization ratio and [R.sub.T] is the total backscattering ratio. Assuming that there are only spherical particles in some altitude ranges of the lidar data, the characteristics of the plot of [[delta].sub.T] versus (1 - [R.sub.T.sup.-1]) lead to a simple but effective calibration method for [[delta].sub.T]. Additionally, the depolarization of air molecules [[delta].sub.m] can be determined in the process of [[delta].sub.T] calibration. We compared determined values with theoretically calculated values for the depolarization of air to test the proposed method. The [[delta].sub.m] value was calculated from the lidar data acquired at Ny-[Angstrom]lesund (79 [degrees] N, 12 [degrees] E), Svalbard in winter 1994--1995. When only sulfate aerosols were present on 4 December 1994, [[delta].sub.m] was 0.46 [+ or -] 0.35%. When the particles consisted of sulfate aerosols and spherical particles of polar stratospheric clouds on 4 January 1995, [[delta].sub.m] was 0.45 [+ or -] 0.07%. Both [[delta].sub.m] values were in good agreement with the theoretically calculated value, 0.50 [+ or -] 0.03%. OCIS codes: 010.1100, 010.1310, 010.3640.

We have constructed a coolable spectroscopic cell for characterizing the physical and chemical properties of simulated atmospheric aerosol particles. The cell is designed for experiments in which the refractive indices, freezing temperatures, and the phase and chemical composition of a wide range of aerosol types are measured. The relatively large volume (0.075 [m.sup.3]) of the cell reduces wall--aerosol interactions and allows the aerosol residence time to exceed 2 h. The cell has been optically interfaced to Fourier-transform spectrometers to record broadband infrared, visible, and ultraviolet extinction spectra of aerosol particles and gas-phase components over a range of temperatures (180--300 K). The data generated with the cell have applications in remote sensing, radiative transfer models, heterogeneous atmospheric chemistry, and pollution studies. OCIS codes: 010.0010, 010.1110, 300.0300.

We used an instrumental synergy of both ground-based (sunphotometer) and spaceborne [POLDER (polarization and directionality of the Earth's reflectances) and Meteosat] passive remote-sensing devices to determine the aerosol optical thickness over the suburban area of Thessaloniki, Greece, from April 1996 to June 1997. The POLDER spaceborne instrument measures the degree of polarization of the solar radiance reflected by the Earth-atmosphere system. Aerosol optical thickness (AOT) retrieval needs an accurate estimate of the contribution of the ground surface to the top of atmosphere's polarized radiance. We tested existing surface reflectance models and fitted their parameters to find the best model for the Thessaloniki area. The model was constrained and validated by use of independent data sets of coincident sunphotometer and POLDER measurements. The comparison indicated that the urban AOT over Thessaloniki was retrieved by the POLDER instrument with an accuracy of +/- 0.05. From analysis of Meteosat data we found that approximately 40% of the days with high AOT (>0.18) are associated with African dust transport events, all observed in the period March-July. Excluding dust events, the 15-month AOT averages 0.12 +/- 0.04. During the 15-month period that the study was conducted, we observed aerosol pollution peaks with an AOT of >0.24 on 15 of the 164 days on which measurements were possible.

The extinction spectra of aqueous sulfuric acid aerosols fully covering the mid-IR to visible regions from750 to 23,000 cm(super -1) (13.9-0.4 mu m) have been measured in the laboratory with a Fourier-transform spectrometer. Both large and small aerosol particles with compositions of approximately 60-70-wt. % H(sub 2)SO(sub 4) were generated and their spectra recorded at 230 and 294 K. The spectra were fitted to a model incorporating room-temperature refractive-index data and Mie theory calculations to characterize the composition and size distributions of the aerosol samples.

A novel technique applicable to remote sensing has been developed for determination of the chemical composition of microdroplets. Enhancement of stimulated anti-Stokes - Raman scattering (SARS) by external seeding of stimulated Raman scattering (SRS) at the Stokes shift lowers the detection limit of the minority species in multicomponent microdroplets. The technique is most useful in the investigation of microdroplets that contain fluorophores that can obscure the SRS signal. The SARS signal is to the blue of the pump laser and out of the fluorescence region of the fluorophore. Information about majority and minority species in multicomponent microdroplets can be determined from the enhanced SARS signals.

A previously reported method to determine profiles of the atmospheric aerosol extinction coefficients by use of a two-wavelength lidar was evaluated. It was shown that simultaneous lidar returns at two different wavelengths in combination with the assumptions of a fixed extinction-to-backscatter ratio for each wavelength and a constant ratio between the extinction profiles was not sufficient for solving the atmospheric extinction profile.

A continuous-wave focused Doppler lidar was used to obtain atmospheric aerosol backscatter measurements over western North America and the Pacific Ocean. Some quasi-vertical backscatter profiles were also obtained during various ascents and descents at altitudes that ranged from 0.1-12 km. Similarities and differences for aerosol loading over land and ocean were observed. The midtropospheric aerosol backscatter background mode was obtained, which was consistent with those of previous airborne and ground-based data sets.

An extension to the 355- and 1064-nm wavelengths of a numerical optical model originally developed at 532 nm is presented. The resulting parameterization allows estimates of stratospheric aerosol surface area, volume, and extinction-to-backscatter ratio from lidar measurements obtained at one of the two Nd:YAG laser wavelengths. Functional relationships that link single-wavelength backscatter to each of the physical variables are provided for sulfate aerosol types ranging from background to heavy volcanic under environmental conditions representative of the global lower stratosphere. The behavior of the functional relationships at the three Nd:YAG wavelengths is compared. Relative errors of model estimates range between 10% and 50%, depending on wavelength and backscatter cross sections. These values are comparable with the ones that characterize in situ particle counters. The inference of particle effective radius and the application of the method to the interpretation of supercooled polar stratospheric cloud observations are discussed.

Two iterative methods of inverting lidar backscatter signals to determine altitude profiles of aerosol extinction and altitude-resolved aerosol size distribution (ASD) are presented. The first method is for inverting two-wavelength lidar signals in which the shape of the ASD is assumed to be of power-law type, and the second method is for inverting multiwavelength lidar signals without assuming any a priori analytical form of ASD. An arbitrary value of the aerosol extinction-to-backscatter ratio ([S.sub.1]) is assumed initially to invert the lidar signals, and the ASD determined by use of the spectral dependence of the retrieved aerosol extinction coefficients is used to improve the value of [S.sub.1] iteratively. The methods are tested for different forms of altitude-dependent ASD's by use of simulated lidar-backscatter-signal profiles. The effect of random noise on the lidar backscatter signals is also studied.

This numerical study addresses the boundary value determination of the aerosol extinction coefficient from backscatter lidar measurements by use of the simultaneous evaluation of signals at 532 and 1064 nm. The basic equations are formulated for the most common case of a two-component atmosphere with variable aerosol extinction-to-backscatter ratios along the lidar line. The method proved to be quite stable for optically thick atmospheres even if the true profiles of the lidar ratios are not known exactly.

We present a retrieval scheme that can be used to derive the aerosol phase function and single-scattering albedo from the sky radiance over land. The retrieval algorithm iteratively corrects the aerosol volume scattering function, the product of the single-scattering albedo and the phase function, based on the difference between the measured sky radiance and the radiance calculated by solving the radiative transfer equation. It is tested first under ideal conditions, i.e., the approximations made in the retrieval algorithm totally agree with actual conditions assumed in creating the pseudodata for sky radiance. It is then tested under more realistic conditions to assess its susceptibility to measurement errors and effects of conditions not recognized in the retrieval algorithm, e.g., surface horizontal inhomogeneity, departures of the surface from Lambertian, and aerosol horizontal inhomogeneity. These simulations show that, in most cases, this scheme can retrieve the aerosol single-scattering albedo with high accuracy (within 1%) and can therefore be used to identify strongly absorbing aerosols. It can also produce meaningful retrievals of most aerosol phase functions: less than 5% error at 865 nm and less than 10% at 443 nm in most cases. Typically, the error in the volume scattering function is small for scattering angles [less than or equal to]90 [degrees], then increases for larger angles. Disappointing results in both the single-scattering albedo and the scattering phase function occur at 443 nm, either when there are large calibration errors in the radiometer used to measure the sky radiance or when the land reflection properties are significantly inhomogeneous.

Vertical profiles of backscattering coefficients, optical thicknesses, and columnar size distributions of aerosols were obtained by simultaneous measurements with lidar, a sunphotometer, and an aureolemeter in Tsukuba, Japan, from November 1991 to December 1992. Several conspicuous characteristics were found in the relationship between aerosol size distribution and stratification. In summer an accumulation mode is dominant, and aerosols were heavily loaded in the planetary boundary layer. Turbid atmospheres with an abundance of large particles are observed in the middle troposphere in the spring. In autumn and winter the troposphere is clear so that columnar aerosol size distributions reflect stratospheric aerosols. During the observation period, volcanic aerosols that are due to the Mt. Pinatubo eruption were being loaded in the stratosphere. The mode radius in the volume size distribution of the stratospheric aerosol was observed to increase from 0.45 [[micro]meter] in November 1991 to 0.6 [[micro]meter] in October 1992, and decreased after October 1992. Total aerosol loading in the stratosphere was estimated to be maximum in the spring of 1992, minimum in the autumn of 1992, and increased again after the autumn of 1992.

The concentration of asbestos fiber aerosols can be monitored by measuring the polarization of laser light scattered by asbestos fibers. The principle of discriminating asbestos fibers is based on the theoretically expected difference in polarization at a scattering angle of 170 deg between cylindrical and spherical airborne particles; polarization at this scattering angle should be positive for cylindrical particles such as asbestos fibers but should be negative or close to zero for spherical mineral particles. We constructed an experimental asbestos real-time monitor that uses a strong electric field to align the airborne particles, that uses lasers having linear polarization with an equal amplitude in parallel and perpendicular components to the aligned long axis of particles, and that simultaneously detects the two components of the linear polarization of light scattered at 170 deg, i.e., close to the backscatter. Experiments that were performed to detect the light scattered from airborne standard asbestos fibers showed that the measured polarization fits theoretical prediction. The concentrations of airborne asbestos fibers obtained by the asbestos real-time monitor were consistent with those estimated by the standard phase contrast microscope method. Key words: Asbestos fibers, light scattering, polarization, cylindrical particles, aerosols.

In algorithms for the atmospheric correction of visible and near-IR satellite observations of the Earth's surface, it is generally assumed that the spectral variation of aerosol optical depth is characterized by an Angstrom power law or similar dependence. In an iterative fitting algorithm for atmospheric correction of ocean color imagery over case 2 waters, this assumption leads to an inability to retrieve the aerosol type and to the attribution to aerosol spectral variations of spectral effects actually caused by the water contents. An improvement to this algorithm is described in which the spectral variation of optical depth is calculated as a function of aerosol type and relative humidity, and an attempt is made to retrieve the relative humidity in addition to aerosol type. The aerosol is treated as a mixture of aerosol components (e.g., soot), rather than of aerosol types (e.g., urban). We demonstrate the improvement over the previous method by using simulated case 1 and case 2 sea-viewing wide field-of-view sensor data, although the retrieval of relative humidity was not successful. Key words: Ocean color, SeaWIFS, optical properties, multiple scattering, error minimization, humidity.

The use of powerful Raman backscatter lidars enables one to measure the stratospheric aerosol extinction profile independently of the backscatter, thereby obtaining additional information to aid in retrieving the physical characteristics of the sampled aerosol. We used principal component analysis to construct a self-consistent method for the retrieval of aerosol bulk physical and optical properties from multiwavelength elastic and/or inelastic Raman backscatter lidar signals. The procedure is applied to synthetic and actual lidar signals. We found that aerosol surface area and volume can be usefully estimated and that the use of Raman-derived extinction data leads to a notable improvement in the accuracy of the estimations. Key words: Aerosol, lidar, multiwavelength aerosol measurements.

The Stratospheric Aerosol and Gas Experiment (SAGE) III, scheduled for a first launch in mid-1998, will be making measurements of the extinction that is due to aerosols and gases at many wavelengths between 385 and 1550 nm. In the troposphere and wintertime polar stratosphere, extinction will also occur because of the presence of cloud along the optical path from the Sun to the satellite instrument. We describe a method for separating the effects of aerosol and cloud using the extinction at 525, 1020, and 1550 nm and present the results of simulation studies. These studies show that the new method will work well under background nonvolcanic aerosol conditions in the upper troposphere and lower stratosphere. Under conditions of severe volcanic contamination, the error rate for the separation of aerosol and cloud may rise as high as 30%. Key words: Satellite occultation measurements, Stratospheric Aerosol and Gas Experiment (SAGE) III, upper-tropospheric cloud, cloud discrimination.

An updated dynamical model for the circulation in the jovian stratosphere has been developed. For computation of the heating and cooling rates in the stratosphere, required to obtain the radiative flux convergence, the model makes use of exponential-sum derived coefficients from laboratory and correlated-k distributions from line listing data for the various atmospheric gases. The aerosol distribution at different latitudes was determined from comparisons of results from microphysical models and near-UV jovian images obtained by the Wide Field and Planetary Camera 2 on the Hubble Space Telescope. The largest aerosol content is found at latitudes south of 65 [degrees] S planetographic, the aerosols being produced at higher atmospheric levels and at higher rates than those at the corresponding northern latitudes. At midlatitudes, the aerosol production rate is less than about two orders of magnitude than that found at high latitudes. The distinct imaginary refractive index spectra of the aerosols found at the different latitudes reflect a compositional difference in the material being produced as a result of chemical reactions. This difference is evident in the shortest wavelength explored ([[Lambda].sub.eff], = 230 nm) between low to midlatitudes and the polar regions. The resulting heating rates are much higher in the stratosphere than those given by previous estimates. The corresponding radiative equilibrium temperatures are therefore hotter than previously reported, mainly above the 10-mbar level. A north-south asymmetry in the stratospheric aerosol distribution was found, which, in turn, produces a different pattern in the derived quantities from the dynamical model. In particular, the Eliassen-Palm flux divergence ([Epsilon]) shows a different pattern when compared with that inferred by Conrath et al. (1990, Icarus 83,255-281) and West et al. (1992, Icarus 100, 245-259). At the base of the stratosphere we inferred the same circulation pattern as found by Gierasch et al. (1986, Icarus 67, 456-483) and West et al. (1992) with upwelling over the zones and subsidence in belts, but with higher values in the magnitude of the vertical winds at low latitudes.

After the volcanic eruption of Mt. Pinatubo the degree of polarization of skylight during twilight over Beijing was monitored with a polarimeter aimed at the local zenith. We analyze the effect of changes in the scattering coefficient of atmospheric aerosols for the case of multiple scattering on skylight polarization at the zenith and then discuss the evolution of skylight polarization over Beijing during the posteruption period. As a reference and for comparison we also discuss the evolution of the aerosol optical depth retrieved from the combination of skylight polarization and backscattering ratio measured by the polarimeter and a lidar for the period beginning with the eruption of Mt. Pinatubo through the end of 1993. The contributions of atmospheric aerosols at different altitudes to the ground-observed twilight polarization depend on the solar zenith angle. For larger solar zenith angles, the skylight polarization is mostly sensitive to aerosol variations in the upper layer that range from 15 to 30 km. The twilight polarization at the zenith from June 1991 to mid-1994 shows different features for three periods: (1) From October 1991 to February 1992, volcanic dust traveled to mid-latitudes, and the degree of polarization decreased substantially. (2) From February 1992 to November 1993, volcanic dust was dispersed the minimum degree of polarization at the solar zenith angle of 93.5 [degrees] disappeared and the maximum increased. In addition, polarization for solar zenith angles less than 90 [degrees] also increased. (3) From November 1993 to May 1994, most of the volcanic dust had fallen off, the atmosphere was restored to the background state, and the skylight polarization approached the preeruption condition. Key words: Pinatubo aerosols, twilight polarization, retrieval.

We present an iterative algorithm for the simultaneous determination of water-leaving reflectances and aerosol optical thickness from the total outgoing radiances measured in the Coastal Zone Color Scanner (CZCS) visible and near-infrared channels. Numerical experiments were carried out to investigate the feasibility of the algorithm. The results show that the errors in determined water-leaving reflectance at 0.443 [[micro]meter] are approximately 10% in most cases. The errors in determined water-leaving reflectance at 0.55 [[micro]meter] are in the 4.05-7.2% range. The errors in the simultaneously determined aerosol optical thickness for all CZCS visible and near-infrared channels are less than approximately 10%. Key words: Water-leaving reflectance, aerosol optical thickness, remote sensing.

We present the results of the aerosol measurements carried out over the Aegean Sea during the Photochemical Activity and Solar Ultraviolet Radiation campaign held in Greece during June 1996. Simultaneous observations performed with a lidar and a double-monochromator spectrophotometer allowed us to retrieve the optical depth, the Angstrom coefficient, and the backscatter-to-extinction ratio. The Sun photometric data can be used to improve quantitative aerosol measurements by lidar in the Planetary Boundary Layer. Systematic errors could arise otherwise, because the value of the backscatter-to-extinction ratio has to be supplied. Instead this ratio can be retrieved experimentally by use of an iterative solution of the lidar equation. Key words: Aerosols, optical depth, backscatter-to-extinction ratio, Angstrom coefficient, lidar signal inversion.

A linear relationship between the variations in radiance measured in the visible and near infrared by satellites and variations of aerosol-extinction profile has been derived, hence reducing the problem to that of solving a Fredholm integral equation of the first kind. The retrieved profiles, by the linear constrained-inversion method, have proved to be accurate in the lower atmosphere, even if simultaneous changes are taking place in the stratosphere. Variations in the stratosphere, however, are retrieved with a lesser degree of accuracy and only as long as no variations occur in the tropospheric haze. Key words: Aerosol-profile retrieval, aerosol-profile sensitivity-weighting functions, aerosol kernel functions, ill-posed problems, linear constrained inversion.

We measured the location, size, and shape of Titan's shadow in five images taken with the Hubble Space Telescope in 1995. We inferred the altitude of Titan's optical limb at wavelengths of 337-954 nm to an accuracy of 15 km. At each wavelength, altitudes are constant north of -5 [degrees] and also constant but lower south of -50 [degrees] latitude, with a linear transition region inbetween. The amplitude of the variation of altitude with latitude increases from close to zero at wavelength 337 nm to 130 km at 954 nm. We conclude that Titan's aerosols are larger (0.3 [[micro]meter] mean radius) at northern latitudes where we probe the detached haze layer than at southern latitudes (0.1 [[micro]meter] south of -50 [degrees]) where we probe the main haze layer below. The geometric cross section of aerosols at probed altitudes ([approximately]300 km) does not show a significant latitudinal variation. The wavelength dependence of the size of Titan's shadow is not consistent with a spherical shape of aerosols, but gives evidence of their fractal nature.

Monitoring, probing, and sensing characteristics of aerosol clouds is difficult and complicated. Probing the characteristics of aerosols is most useful in the chemical and microelectronic industry for processing control of aerosols and emulsion, decreasing bit error rate in adaptive optical communication systems, and in acquiring data for atmospheric science and environment quality. We present a new mathematical and optical engineering model for monitoring characteristics of aerosol clouds. The model includes the temporal transfer function of aerosol clouds as a variable parameter in an electro-optic oscillator. The frequency of the oscillator changes according to changes in the characteristics of the clouds (density, size distribution, physical thickness, the medium and the particulate refractive indices, and spatial distribution). It is possible to measure only one free characteristic at a given time. An example of a practical system for monitoring the density of aerosol clouds is given. The frequency of the oscillator changes from 1.25 to 0.43 MHz for changes in aerosol density from 2000 to 3000 particulates [cm.sup.-3]. The advantages of this new method compared with the transmissometer methods are (a) no necessity for line-of-sight measurement geometry, (b) accurate measurement of high optical thickness media is possible, (c) under certain conditions measurements can include characteristics of aerosol clouds related to light scatter that cannot be or are difficult to measure with a transmissometer, and (d) the cloud bandwidth for free space optical communication is directly measurable. Key words: Aerosols, monitoring and probing, multiple scatter, electro-optic oscillator, clouds, light scatter, Monte Carlo simulation.

Estimation of particle-size distribution is analyzed for the complicated case of compound aerosols, in which particles are distinguished by sizes and optical constants. This task arises in a number of interesting practical situations when aerosol scatterers cannot be described with a common refractive index. This is an inverse problem with a large number of variables, and questions of formal inversion are of great importance here. They are discussed in detail, and an improved numerical-inversion method is proposed. The method provides a nonnegative and highly stable solution and makes it possible to include varied additional or a priori information. It is shown that the proposed technique is closely related to well-known linear and relaxation methods widely used in atmospheric optics. The algorithm for determination of bicomponent aerosol-size distribution is devised. It uses the intensity of light scattered at different angles and spectral-extinction measurements. In addition, the algorithm can incorporate a priori restrictions of size-spectra smoothness. A set of numerical examples illustrates the algorithm.

Calculations of direct climate forcing by anthropogenic aerosols commonly use radiative transfer parameters, including asymmetry parameter g. One method of obtaining the asymmetry parameter of a particle population is to convert measured values of the hemispheric-to-total-scatter ratio (backscatter ratio b) into their corresponding g values. We compare a conversion derived from Mie calculations with one derived from the Henyey-Greenstein (HG) phase function to show that the HG method systematically overestimates g for typical size distributions of accumulation-mode aerosols. A delta-Eddington radiative transfer calculation is used to show that a 10% overestimation of g can systematically reduce climate forcing as a result of aerosols by 12% or more. Mie computations are used to derive an empirical relationship between backscatter ratio and asymmetry parameter for log-normal accumulation-mode aerosols. This relationship can be used to convert the backscatter ratio to the asymmetry parameter, independent of geometric mean diameter [D.sub.gv] or complex refractive index m, but the conversion requires knowledge of the breadth [[Sigma].sub.g] of the size distribution. Key words: Climate forcing, aerosols, radiative transfer parameters, backscatter ratio, asymmetry parameter, light scattering.

The Raman wave-number shift for scattering at oxygen molecules is almost exactly 2/3 of the nitrogen Raman shift. This is used to build up a chain of three consecutive laser emission lines separated by 1/3 of the nitrogen Raman shift. It is shown that it is possible to determine aerosol extinction and backscatter coefficients without a priori assumptions from the received Rayleigh-Mie and Raman (oxygen and nitrogen) signals. Key words: Aerosol extinction coefficient, aerosol backscatter coefficient, Raman lidar, aerosol lidar, chain Raman lidar.

Laboratory studies of the radiation yields of complex organic materials manufactured from N2/CH4 gas mixtures with 10 or 0.1% CH4 reveals that these tholins are identical to organic aerosols in the atmosphere of Titan. Tholin yields in the experimental study are large compared to the natural production of tholin in satellites. Tholin in the satellites is produced by high-energy magnetospheric electrons that are different from plasma electrons.