Atmospheric correction of satellite ocean color imagery: the black pixel assumption

The assumption that values of water-leaving radiance in the near-infrared (NIR) are negligible enable aerosol radiative properties to be easily determined in the correction of satellite ocean color imagery. This is referred to as the black pixel assumption. We examine the implications of the black pixel assumption using a simple bio-optical model for the NIR water-leaving reflectance [rho(w)(lambda(NIR))](N). In productive waters [chlorophyll (Chl) concentration >2 mg m(-3)], estimates of [rho(w)(lambda(NIR))](N) are several orders of magnitude larger than those expected for pure seawater. These large values of [rho(w)(lambda(NIR))](N) result in an overcorrection of atmospheric effects for retrievals of water-leaving reflectance that are most pronounced in the violet and blue spectral region. The overcorrection increases dramatically with Chl, reducing the true water-leaving radiance by roughly 75% when Chl is equal to 5 mg m(-3). Relaxing the black pixel assumption in the correction of Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) satellite ocean color imagery provides significant improvements in Chl and water-leaving reflectance retrievals when Chl values are greater than 2 mg m(-3). Improvements in the present modeling of [rho(w)(lambda(NIR))](N) are considered, particularly for turbid coastal waters. However, this research shows that the effects of nonzero NIR reflectance must be included in the correction of satellite ocean color imagery.     

Atmospheric correction of ocean color sensors: computing atmospheric diffuse transmittance

Using the reciprocal equation derived by Yang and Gordon [Appl. Opt. 36, 7887-7897 (1997)] for atmospheric diffuse transmittance of the ocean-atmosphere system, I examined the accuracy of an analytical equation proposed by Gordon et al. [Appl. Opt. 22, 20-36 (1983)] in computing the atmospheric diffuse transmittance for wavelengths from 412 to 865 nm for both a pure Rayleigh and a two-layer Rayleigh-aerosol atmosphere overlying a flat Fresnel-reflecting ocean surface. It was found that for viewing angles up to approximately 40 degrees , the analytical formula produces errors usually between 2% and 3% for nonabsorbing and weakly absorbing aerosols and for aerosol optical thicknesses tau(a) 相似文献   

Remote sensing of ocean color and aerosol properties: resolving the issue of aerosol absorption

Current atmospheric correction and aerosol retrieval algorithms for ocean color sensors use measurements of the top-of-the-atmosphere reflectance in the near infrared, where the contribution from the ocean is known for case 1 waters, to assess the aerosol optical properties. Such measurements are incapable of distinguishing between weakly and strongly absorbing aerosols, and the atmospheric correction and aerosol retrieval algorithms fail if the incorrect absorption properties of the aerosol are assumed. We present an algorithm that appears promising for the retrieval of in-water biophysical properties and aerosol optical properties in atmospheres containing both weakly and strongly absorbing aerosols. By using the entire spectrum available to most ocean color instruments (412-865 nm), we simultaneously recover the ocean's bio-optical properties and a set of aerosol models that best describes the aerosol optical properties. The algorithm is applied to simulated situations that are likely to occur off the U.S. East Coast in summer when the aerosols could be of the locally generated weakly absorbing Maritime type or of the pollution-generated strongly absorbing urban-type transported over the ocean by the winds. The simulations show that the algorithm behaves well in an atmosphere with either weakly or strongly absorbing aerosol. The algorithm successfully identifies absorbing aerosols and provides close values for the aerosol optical thickness. It also provides excellent retrievals of the ocean bio-optical properties. The algorithm uses a bio-optical model of case 1 waters and a set of aerosol models for its operation. The relevant parameters of both the ocean and atmosphere are systematically varied to find the best (in a rms sense) fit to the measured top-of-the-atmosphere spectral reflectance. Examples are provided that show the algorithm's performance in the presence of errors, e.g., error in the contribution from whitecaps and error in radiometric calibration.     

Determination of the refractive index and size distribution of aerosol from dual-scattering-angle optical particle counter measurements

A method is presented for inferring both the refractive index and the size distribution of aerosol from observations of a dual-scattering-angle optical particle counter (OPC). An existing prototype of an OPC with 60 degree and 90 degree dual-scattering angles was used for the experiments. Based on the high sensitivity of the OPC response to the refractive index of particulates, two families of size distribution curves may be calculated. The solution of the refractive index corresponds to the superposition of the two size distributions. This method was applied to the simulation and to the field measurements conducted in Beijing and Hefei, and the results of both are presented.     

Effects of stratospheric aerosols and thin cirrus clouds on the atmospheric correction of ocean color imagery: simulations

Using simulations, we determine the influence of stratospheric aerosol and thin cirrus clouds on the performance of the proposed atmospheric correction algorithm for the moderate resolution imaging spectroradiometer (MODIS) data over the oceans. Further, we investigate the possibility of using the radiance exiting the top of the atmosphere in the 1.38-microm water vapor absorption band to remove their effects prior to application of the algorithm. The computations suggest that for moderate optical thicknesses in the stratosphere, i.e., tau(s) < or approximately 0.15, the stratospheric aerosol-cirrus cloud contamination does not seriously degrade the MODIS except for the combination of large (approximately 60 degrees) solar zenith angles and large (approximately 45 degrees) viewing angles, for which multiple-scattering effects can be expected to be particularly severe. The performance of a hierarchy of stratospheric aerosol/cirrus cloud removal procedures for employing the 1.38-microm water vapor absorption band to correct for stratospheric aerosol/cirrus clouds, ranging from simply subtracting the reflectance at 1.38 microm from that in the visible bands, to assuming that their optical properties are known and carrying out multiple-scattering computations of their effect by the use of the 1.38-microm reflectance-derived concentration, are studied for stratospheric aerosol optical thicknesses at 865 nm as large as 0.15 and for cirrus cloud optical thicknesses at 865 nm as large as 1.0. Typically, those procedures requiring the most knowledge concerning the aerosol optical properties (and also the most complex) performed the best; however, for tau(s) < or approximately 0.15, their performance is usually not significantly better than that found by applying the simplest correction procedure. A semiempirical algorithm is presented that permits accurate correction for thin cirrus clouds with tau(s) as large as unity when an accurate estimate of the cirrus cloud scattering phase function is provided, and as large as 0.5 when a coarse approximation to the phase function is used. Given estimates of the stratospheric aerosol optical properties, the implementation of the algorithm by using a set of lookup tables appears to be straightforward.     

Simultaneous retrieval of aerosol refractive index and particle size distribution from ground-based measurements of direct and scattered solar radiation

Ground-based sunphotometer observation of direct and scattered solar radiation is a traditional tool for providing data on aerosol optical properties. Spectral transmission and solar aureole measurements provide an optical source of aerosol information, which can be inverted for retrieval of microphysical properties (particle size distribution and refractive index). However, to infer these aerosol properties from ground-based remote-sensing measurements, special numerical inversion methods should be developed and applied. We propose two improvements to the existing inversion techniques employed to derive aerosol microphysical properties from combined atmospheric transmission and solar aureole measurements. First, the aerosol refractive index is directly included in the inversion procedure and is retrieved simultaneously with the particle size spectra. Second, we allow for real or effective instrumental pointing errors by including a correction factor for scattering angle errors as a retrieved inversion parameter. The inversion technique is validated by numerical simulations and applied to field data. It is shown that ground-based sunphotometer measurements enable one to derive the real part of the aerosol refractive index with an absolute error of 0.03-0.05 and to distinguish roughly between weakly and strongly absorbing aerosols. The aureole angular observation scheme can be refined with an absolute accuracy of 0.15-0.19 deg. Offset corrections to the scattering angle error are generally found to be small and consistently of the order of -0.17. This error magnitude is deduced to be due primarily to nonlinear field-of-view averaging effects rather than to instrumental errors.     

Simultaneous determination of the aerosol complex index of refraction and size distribution from scattering measurements of polarized light

In this study we attempt to determine the aerosol complex index of refraction and size distribution from scattering measurements of polarized light. We illustrate that the scattering matrix elements M(2)(100 degrees ) and D(21) (150 degrees ) can be selected as an optimum set of matrix elements for determination of the complex index of refraction. We also illustrate that errors increase if we include insensitive scattering matrix elements in the determination of the complex index of refraction. A method is developed for the simultaneous determination of the complex index of refraction and the size distribution. In our method, we selected two sets of matrix elements, M (2) (100 degrees ) and D (21) (150 degrees ), for the determination of the complex index of refraction and others, which are much less sensitive to the complex index of refraction than M(2) (100 degrees ) and D(21) (150 degrees ), for the determination of the size distribution, based on their sensitivity analyses. A modified inversion library algorithm is adopted to solve the coupled system. Numerical experiments show that both the complex index of refraction and the size distribution can be determined with reasonable accuracy when we apply our method to scattering measurements of polarized light.     

Iterative method for the inversion of multiwavelength lidar signals to determine aerosol size distribution

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(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(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.     

Radiance reflected from the ocean-atmosphere system: synthesis from individual components of the aerosol size distribution

We describe a method by which the aerosol component of the radiance at the top of the atmosphere (TOA) can be synthesized from the radiances generated by individual components of the aerosol size-refractive-index distribution. The method is exact in the single-scattering approximation. For regimes in which the single-scattering approximation is not valid, the method usually reproduces the aerosol contribution with an error ?2-3% (and only rarely >3-4%) for Sun and viewing angles as large as 80° and 70°, respectively, and for aerosol optical thicknesses as large as 0.50. In the blue, where molecular scattering makes a dominant contribution to the TOA radiance, the percent error in the synthesized total radiance is significantly less than in the synthesized aerosol component and typically will be less than the radiometric calibration uncertainties of Earth-orbiting sensors. When the aerosol is strongly absorbing, the method can fail; however, the potential for failure is easy to anticipate a priori. An obvious application of our technique is to provide a basis for the estimation of aerosol properties with Earth-orbiting sensors, e.g., the Multiangle Imaging Spectroradiometer.     

Estimation of aerosol columnar size distribution and optical thickness from the angular distribution of radiance exiting the atmosphere: simulations

We report the results of simulations in which an algorithm developed for estimation of aerosol optical properties from the angular distribution of radiance exiting the top of the atmosphere over the oceans [Appl. Opt. 33, 4042 (1994)] is combined with a technique for carrying out radiative transfer computations by synthesis of the radiance produced by individual components of the aerosol-size distribution [Appl. Opt. 33, 7088 (1994)], to estimate the aerosol-size distribution by retrieval of the total aerosol optical thickness and the mixing ratios for a set of candidate component aerosol-size distributions. The simulations suggest that in situations in which the true size-refractive-index distribution can actually be synthesized from a combination of the candidate components, excellent retrievals of the aerosol optical thickness and the component mixing ratios are possible. An exception is the presence of strongly absorbing aerosols. The angular distribution of radiance in a single spectral band does not appear to contain sufficient information to separate weakly from strongly absorbing aerosols. However, when two spectral bands are used in the algorithm, retrievals in the case of strongly absorbing aerosols are improved. When pseudodata were simulated with an aerosol-size distribution that differed in functional form from the candidate components, excellent retrievals were still obtained as long as the refractive indices of the actual aerosol model and the candidate components were similar. This underscores the importance of component candidates having realistic indices of refraction in the various size ranges for application of the method. The examples presented all focus on the multiangle imaging spectroradiometer; however, the results should be as valid for data obtained by the use of high-altitude airborne sensors.     

Flow cytometric determination of size and complex refractive index for marine particles: comparison with independent and bulk estimates

We advance a method to determine the diameter D and the complex refractive index (n + n'i) of marine particles from flow cytometric measurements of forward scattering, side scattering, and chlorophyll fluorescence combined with Mie theory. To understand better the application of Mie theory with its assumptions to flow cytometry (FCM) measurements of phytoplankton cells, we evaluate our flow cytometric-Mie (FCM-Mie) method by comparing results from a variety of phytoplankton cultures with independent estimates of cell D and with estimates of n and n' from the inversion of bulk measurements. Cell D initially estimated from the FCM-Mie method is lower than independent estimates, and n and n' are generally higher than bulk estimates. These differences reflect lower forward scattering and higher side scattering for single-cell measurements than predicted by Mie theory. The application of empirical scattering corrections improves FCM-Mie estimates of cell size, n, and n'; notably size is determined accurately for cells grown in both high- and low-light conditions, and n' is correlated with intracellular chlorophyll concentration. A comparison of results for phytoplankton and mineral particles suggests that differences in n between these particle types can be determined from FCM measurements. In application to natural mixtures of particles, eukaryotic pico/nanophytoplankton and Synechococcus have minimum mean values of n' in surface waters, and nonphytoplankton particles have higher values of n than phytoplankton at all depths.     

Derivation of the aerosol size distribution from a bistatic system of a muitiwavelength laser with the singular value decomposition method

Estimation of aerosol size distributions from measurements of scattered light intensity by the use of a bistatic system with a multiwavelength laser is presented. We investigated the effects of inversion of the scattered intensity on calculated aerosol size distribution in a numerical experiment. Two model aerosol size distributions were used, one a Junge type widely known as a typical example of the size distribution of suspended particles in the atmosphere and the other a log-normal type as an example of the monodisperse distribution. A singular value decomposition was applied to the inversion to infer the size distribution from the kernel function and the scattered light intensity. In the physical experiment, the size distributions were successfully inferred from analysis of the scattered light intensity from an artificial polystyrene latex aerosol.     

Measurement of the complex refractive index of liquids in the infrared using spectroscopic attenuated total reflection ellipsometry: correction for depolarization by scattering

With spectroscopic ellipsometry one can measure the real and imaginary parts of the refractive index of a medium simultaneously. To determine this index in the infrared for a number of technical liquids, use was made of attenuated total internal reflection at the glass-liquid interface of a specially designed prism. This attenuated total reflection approach warrants minimal signal loss and is, for strongly absorbing liquids, the only way to measure the complex refractive index. A surprising phenomenon, observed when BK-7 prism glass was used, is scattering in the vicinity of the absorption wavelengths of the glass. A simple model that can be used to describe the relations among absorption, scattering, and depolarization was successfully used to correct the measurements. Refractive indices for demineralized water, Freon 113, heptane, benzene, gas oil, and crude oil in the wave number range from 5000 to 10,000 cm(-1) (1-2 μm) are presented.     

Retrieval of atmospheric optical parameters from airborne flux measurements: application to the atmospheric correction of imagery

Methodologies that employ auxilliary flux data collected by upward- and downward-looking optical sensors to improve atmospheric corrections of airborne multispectral images are presented and evaluated. Such flux data often are collected in current airborne sensors to produce bidirectional reflectance factor (BRF) images and estimates of hemispherical-hemispherical reflectance. The fact that these images must then be corrected for atmospheric interference raises the question as to whether the auxilliary flux information can be employed to estimate some of the input parameters required by atmospheric correction models. Radiative transfer simulations are employed to demonstrate that the utilization of the downwelling and upwelling fluxes as a means of inferring intrinsic atmospheric optical information can be used to better characterize the local atmosphere and accordingly to improve the atmospheric corrections applied to the apparent BRF images.     

Separation of measurement of the refractive index and the geometrical thickness by use of a wavelength-scanning interferometer with a confocal microscope

An improved system for the separate measurement of the refractive index and the geometrical thickness that constitutes a hybrid configuration of a confocal microscope and a wavelength-scanning heterodyne interferometer with a laser diode is presented. The optical path difference can be measured in less than 1 s, which is 10 times quicker than with the low-coherence interferometry previously used, and with a resolution of 10 mum with a fixed reference mirror. Separate measurement of the refractive index and the geometrical thickness of glass plates was demonstrated by use of the arrangement in place of the low-coherence interferometer used previously.     

Particle size and refractive index retrieval from the backscattering spectrum of white light using the Twomey iterative method: simulation and experiment

The Twomey iterative method has been applied to the retrieval of hydrosol microphysical properties. In particular, we focused on the retrieval of single and multimode particle size distributions from both simulated and experimental backscattering spectra in the 400-800 nm wavelength range. Assuming a known refractive index, both single-mode and multimode distributions were successfully retrieved through the introduction of an initial distribution biased toward larger particles. The simulation results were experimentally verified with standard polystyrene particles suspended in water within the diameter range of 0.2-2 microm for both narrow and broad monomodal distributions as well as more complicated multimode distributions. Finally, the technique was extended to the retrieval of an unknown refractive index.