Retrieval of aerosol optical and physical properties from ground-based spectral, multi-angular, and polarized sun-photometer measurements

An inversion algorithm to retrieve a complete set of aerosol optical and physical properties has been developed. The algorithm is based on sun-photometer measurements with emphasis on polarimetric observations. At present, these polarized sun-photometer observations are mainly provided by PHOTONS network included in AERONET worldwide network. With ground-based measurement of direct sun irradiance, as well as total and polarized sky radiance, most of key characteristics of atmospheric aerosols are retrieved, including spectral single-scattering albedo from UV to NIR wavelength, elements of scattering matrix F₁₁ and − F₁₂ (i.e. phase function and polarized phase function), complex refractive index, and size distribution. These parameters are fundamentally valuable in atmospheric physical and chemical studies. The theoretical accuracy is assessed based on three distinct bimodal log-normal aerosol models. The sensitivity studies to measurement uncertainties and to observing geometrical conditions are implemented to estimate the retrieval accuracy. Examples of typical retrievals when applying to real measurements are illustrated and compared with AERONET operational inversions. The particle shape-independent advantage of the retrieved single-scattering albedo, phase function, and polarized phase function is validated by considering a non-spherical aerosol model, which consisted of spheroid particles.     

Seasonal variation of columnar aerosol optical properties over Athens, Greece, based on MODIS data

A long-term (2000–2005) data set of aerosol optical properties obtained from the Moderate Resolution Imaging Spectroradiometer (MODIS) is analyzed focusing on the Greater Athens Area in the Eastern Mediterranean region. The MODIS aerosol optical depth standard product (AOD at 550 nm) and its respective ratio attributed to fine-mode particles (FM) are employed to evaluate the inter-annual and seasonal variability of the aerosol properties over Athens. Based on AOD₅₅₀ and FM values three specific aerosol types are discriminated corresponding to different aerosol load and optical properties. The aerosol types considered correspond to urban/industrial aerosols, coarse-mode particles and clean maritime conditions. This study focuses on the seasonal and year-to-year fluctuation of the number of occurrences as well as the AOD₅₅₀ and FM values of each aerosol type. The coarse-mode particles are observed mainly in the summer, while spring is the most favorable season for the occurrence of urban/industrial aerosols. On the other hand, clean maritime conditions occur mainly in the winter. The AOD₅₅₀ values for the coarse-mode particles are higher in spring, while the urban/industrial and clean maritime aerosols exhibit slightly higher values in the summer. The seasonal distribution of the aerosol properties is related to anthropogenic and dust emissions in the spring/summer period, but is modified by atmospheric dispersion and precipitation in late autumn/winter. The main conclusion of the study is that the coarse-mode particles exhibit much stronger inter-annual and seasonal variability compared to the urban/industrial aerosols. Finally, three cases corresponding to each aerosol type are analyzed with the aid of synoptic weather maps, air mass trajectories and MODIS data.     

Development and validation of a technique for merging satellite derived aerosol optical depth from SeaWiFS and MODIS

Merging time series of satellite derived aerosol products from independent missions can support aerosol science by combining in a consistent way temporally overlapping data sets and by increasing data coverage. A merging technique applied to satellite aerosol optical depth τ_a is presented and tested with SeaWiFS and MODIS-Aqua data. The technique relies on least squares fitting of the available τ_a spectra onto a linear or second-order polynomial relation between log-transformed τ_a and wavelengths. First, the sensor specific products are compared with field observations collected by a sun-photometer installed on the Acqua Alta Oceanographic Tower in the northern Adriatic Sea. Mean absolute percentage differences are approximately 21% at 412 and 443 nm, and increase with wavelength, with large overestimates in the red and near-infrared bands. The mean absolute differences are typically 0.04. When inter-compared, the 2 satellite products agree well, with mean absolute percentage differences lower than 20% at all wavelengths and little bias. The results of the comparison of the merger outputs with the field data are well in line with the validation results of the sensor specific products, and are comparable for the various merging procedures. The benefits of merging in terms of data coverage are briefly illustrated for the Mediterranean basin.     

Simultaneous retrieval of aerosol optical properties over the Pearl River Delta, China using multi-angular, multi-spectral, and polarized measurements

The optical properties of aerosol have been simultaneously retrieved over the Pearl River Delta (PRD), China during December 2009 from multi-angular, multi-spectral, and polarized airborne data. A new airborne Directional Polarimetric Camera (DPC) with high spatial resolution (4 m at 4000 m a.g.l.) is used to retrieve the aerosol optical properties, which is an experimental airborne instrument focused on monitoring aerosol particle pollution, dealing with the apportionment of sources and controlling air quality in cities. We present a case study of polarized observations performed during high air pollution episodes in the southeast of China. Exceptionally high values of the aerosol optical depth of up to 0.8 (at 0.865 μm) were observed in this case study. The spatial and temporal variability of aerosol optical properties over the Pearl River Delta region were analyzed using polarized measurements with high spatial resolution. To reduce the ambiguity in retrieving aerosol optical properties using DPC alone, ground-based measurements (Automatic CE318-DP polarized sun-sky radiometer, Raman Lidar) were used to constrain the inversion in terms of the key characteristics of a local aerosol model, including spectral complex refractive index, size distribution, and vertical distribution of aerosol optical parameters. The surface contribution to the polarized radiance was simulated using bidirectional polarized reflectance distribution function (BPDF), which was adjusted using DPC measurements at low altitude. The aerosol optical properties were retrieved using DPC over the Pearl River Delta, and are in good agreement with coincident sun photometer retrievals. The retrieval algorithm of aerosol optical properties using high spatial resolution polarized measurements proposed in this paper shows potential to retrieve the aerosol optical properties over cities.     

A new approach for mapping of Biological Soil Crusts in semidesert areas with hyperspectral imagery

Biological Soil Crusts (BSCs), consisting of cyanobacteria, algae, microfungi, lichens and bryophytes in varying proportions, live within or immediately on top of the uppermost millimeters of soil, where they form a more or less firm aggregation of soil particles and organisms. They mainly occur in soils of arid and semi-arid regions, which cover more than 35% of the earth's land surface and are assumed to play a major role as primary producers, C- and N-sinks and soil stabilizers.

In order to establish a methodology for mapping of BSCs, their spectral characteristics with respect to different crust types were analyzed. The resulting reflectance spectra of different crust types had a shallow absorption feature centered around 680 nm in common, in which they differed from the spectra of bare soil.

In October 2004, hyperspectral CASI data with a spatial resolution of 1 m were recorded in conjunction with field spectroscopic measurements in the Succulent Karoo, South Africa. Available spectral indices for Biological Soil Crusts were tested on the image but did not produce satisfying classifications. Therefore, an alternative approach was established based on spectral field data, field observations and the hyperspectral dataset. The newly developed Continuum Removal Crust Identification Algorithm (CRCIA) is based on small and narrow spectral characteristics, that were extracted by continuum removal and subsequently expressed as a set of logical conditions. Using this method, 16.2% of the area which covers 12 km² of gently sloping hills with some granite outcrops were classified as BSCs. Validation of the classification resulted in a Kappa index of 0.831.

In a next step, the methodology will be tested with regard to scale-dependent effects and applied to images covering areas with additional types of BSCs and soil to develop a robust and generally applicable method.     

PROSPECT-4 and 5: Advances in the leaf optical properties model separating photosynthetic pigments

The PROSPECT leaf optical model has, to date, combined the effects of photosynthetic pigments, but a finer discrimination among the key pigments is important for physiological and ecological applications of remote sensing. Here we present a new calibration and validation of PROSPECT that separates plant pigment contributions to the visible spectrum using several comprehensive datasets containing hundreds of leaves collected in a wide range of ecosystem types. These data include leaf biochemical (chlorophyll a, chlorophyll b, carotenoids, water, and dry matter) and optical properties (directional-hemispherical reflectance and transmittance measured from 400 nm to 2450 nm). We first provide distinct in vivo specific absorption coefficients for each biochemical constituent and determine an average refractive index of the leaf interior. Then we invert the model on independent datasets to check the prediction of the biochemical content of intact leaves. The main result of this study is that the new chlorophyll and carotenoid specific absorption coefficients agree well with available in vitro absorption spectra, and that the new refractive index displays interesting spectral features in the visible, in accordance with physical principles. Moreover, we improve the chlorophyll estimation (RMSE = 9 µg/cm²) and obtain very encouraging results with carotenoids (RMSE = 3 µg/cm²). Reconstruction of reflectance and transmittance in the 400-2450 nm wavelength domain using PROSPECT is also excellent, with small errors and low to negligible biases. Improvements are particularly noticeable for leaves with low pigment content.     

Spatial characteristics of the difference between MISR and MODIS aerosol optical depth retrievals over mainland Southeast Asia

The difference between aerosol optical depths (AODs) retrieved from the Multi-angle Imaging SpectroRadiometer (MISR) and the Moderate Resolution Imaging Spectroradiometer (MODIS) is examined over mainland Southeast Asia from a spatial perspective. Though ideally the difference between these measurement methods should be small and randomly distributed over space, our analysis suggests that this difference has a strong negative relationship with MODIS AODs and tend to be spatially clustered. We quantify the spatial dependence in MISR/MODIS AOD differences and explore the extent to which the spatial patterns in these differences can be explained by variables that reflect the influence of physical environment and human activities. While these variables show a strong relationship with MISR/MODIS AOD differences, the results also suggest that further research is needed to fully understand the spatial dependence in these differences.     

Retrieval of aerosol type and optical thickness over the Mediterranean from SeaWiFS images using an automatic neural classification method

We present an automatic classification method based on topological neural network algorithms to retrieve aerosol optical properties from multi-spectral ocean-color satellite imagery. The first step of the method consisted in an unsupervised classification of a large set of clear-sky top of the atmosphere reflectance spectra measured by the sensor. We used the so-called Kohonen map which aggregates similar spectra into a reduced set of pertinent groups. The second step consisted in labeling these groups by clustering them with synthetic TOA reflectance spectra whose optical properties (i.e., aerosol type or optical thickness) are known. These synthetic spectra have been computed using a radiative transfer model. In the present study, we dealt with five aerosol types (maritime, coastal, tropospheric, oceanic and mineral) and several aerosol optical thickness values ranging from 0.05 to 0.8. These simulated spectra were then projected onto the Kohonen map to label each group of the map. The last step consisted in applying this method to the SeaWiFS imagery of the Mediterranean region for the years 1999 and 2000. The Kohonen map was “educated” from pixels randomly extracted during the year 1999 in this region. We accounted for the viewing geometry of the sensor by clustering the simulated spectra into ten groups of similar geometries, as defined by both scattering and sun zenith angles. The analysis of SeaWiFS images was performed pixel-by-pixel by selecting the suitable labeling (in terms of viewing geometry), then by identifying the closest spectrum in the Kohonen map, which finally gives the aerosol optical properties. This method led to accurate and coherent results, as shown by the comparison with in situ aerosol measurements provided by the AERONET station at Lampedusa and by the study of two aerosol events over the Mediterranean. One of the major advantages of this method is that it enables us to automatically identify the aerosol type and to retrieve the aerosol optical properties with a better accuracy than classical methods such as those used by SeaWifs. It gives accurate results for optical thickness values larger than 0.35 and is able to retrieve dust aerosols such as African dust aerosol (absorbing aerosol). These should ensure a more precise inversion of ocean-color imagery where the knowledge of atmospheric optical parameters is essential. Moreover the method is able to give probabilities for the estimate values of aerosol properties.     

Assessing the effect of hydrocarbon oil type and thickness on a remote sensing signal: A sensitivity study based on the optical properties of two different oil types and the HYMAP and Quickbird sensors

We measured the light absorption properties of two naturally occurring Australian hydrocarbon oils, a Gippsland light crude oil and a North West Shelf light condensate. Using the results from these measurements in conjunction with estimated sensor environmental noise thresholds, the theoretical minimum limit of detectability of each oil type (as a function of oil thickness) was calculated for both the hyperspectral HYMAP and multispectral Quickbird sensors. The Gippsland crude oil is discernable at layer thickness of 20 µm or more in the Quickbird green channel. The HYMAP sensor was found to be theoretically capable of detecting a layer of Gippsland crude oil with a thickness of 10 µm in approximately six sensor channels. By contrast, the North West Shelf light condensate was not able to be detected by either sensor for any thickness up to 200 µm. Optical remote sensing is therefore not applicable for detecting diagnostic absorption features associated with this light condensate oil type, which is typical of the chemistry of many hydrocarbon oils found in the Australian Northwest Shelf area and condensates world wide. We conclude that oil type is critical to the applicability of optical remote sensing for natural oil slick detection and identification. We recommend that a sensor- and oil-specific sensitivity study should be conducted prior to applying optical remote sensors for oil exploration.The oil optical properties were obtained using two different laboratory methods, a reflectance-based approach and transmittance-based approach. The reflectance-based approach was relatively complex to implement, but was chosen in order to replicate as closely as possible real world remote sensing measurement conditions of an oil film on water. The transmittance-based approach, based upon standard laboratory spectrophotometric measurements was found to generate results in good agreement with the reflectance-based approach. Therefore, for future oil- and sensor-specific sensitivity studies, we recommend the relatively accessible transmittance-based approach, which is detailed in this paper.     

Investigation of the optical properties of P,As and Sb incorporated AlGaX alloys using full potential linearized augmented plane wave method

Dielectric function, absorption coefficient, reflectivity, optical conductivity, energy loss spectra, index of refraction and extinction coefficient of AlGaX (X=P, As, Sb) in rocksalt phase, are calculated using full potential linearized augmented plane wave (FP-LAPW) method. All optical parameters are calculated in the incident photon energy range 0–25 eV. The maximum value of optical conductivity observed is 15 000 S. Index of refraction has a maximum value of 4.6. A decrease in the absorption coefficient and optical conductivity is observed when X is replaced by P, As and Sb. The extinction coefficient increases when P is replaced by As and Sb. The high value (n=4.6$n=4.6$) of the index of refraction shows that AlGaX is a suitable alloy to be used in making advanced optical devices.     

Monitoring canopy biophysical and biochemical parameters in ecosystem scale using satellite hyperspectral imagery: An application on a Phlomis fruticosa Mediterranean ecosystem using multiangular CHRIS/PROBA observations

This study focuses on the potential of satellite hyperspectral imagery to monitor vegetation biophysical and biochemical characteristics through narrow-band indices and different viewing angles. Hyperspectral images of the CHRIS/PROBA sensor in imaging mode 1 (5 observation angles, 62 bands, 410-1005 nm) were acquired throughout a two-year period for a Mediterranean ecosystem fully covered by the semi-deciduous shrub Phlomis fruticosa. During each acquisition, coincident ecophysiological field measurements were conducted. Leaf area index (LAI), leaf biochemical content (chlorophyll a, chlorophyll b, carotenoids) and leaf water potential were measured. The hyperspectral images were corrected for coherent noises, cloud and atmosphere, in order to produce ground reflectance images. The reflectance spectrum of each image was used to calculate a variety of vegetation indices (VIs) that are already published in relevant literature. Additionally, all combinations of the 62 bands were used in order to calculate Normalized Difference Spectral Indices (NDSI_(x,y)) and Simple Subtraction Indices (SSI_(x,y)). The above indices along with raw reflectance and reflectance derivatives were examined for linear relationship with the ground-measured variables and the strongest relationships were determined. It is concluded that higher observation angles are better for the extraction of biochemical indices. The first derivative of the reflectance spectra proved to be very useful in the prediction of all measured variables. In many cases, complex and improved spectral indices that are proposed in the literature do not seem to be more accurate than simple NDSIs such as NDVI. Even traditional broadband NDVI is proved to be adequate in LAI prediction, while green bands seem also very useful. However, in biochemical estimation narrow bands are necessary. Indices that incorporate red, blue and IR bands, such as PSRI, SIPI and mNDVI presented good performance in chlorophyll estimation, while CRI did not show any relevance to carotenoids and WI was poorly correlated to water potential. Moreover, analyses indicated that it is very important to use a near red-edge band (701 nm) for effective chlorophyll index design. SSIs that incorporate 701 nm with 511 or 605 nm showed best performance in chlorophyll determination. For carotenoid estimation, a band on the edge of carotenoid absorption (511 nm) combined with a red band performed best, while a normalized index of two water absorption bands (945, 971 nm) proved to be an effective water index. Finally, the attempt to investigate stress conditions through pigment ratios resulted in the use of the band centred at 701 nm.     

On intra-annual EVI variability in the dry season of tropical forest: A case study with MODIS and hyperspectral data

Considerable controversy is associated with dry season increases in the Enhanced Vegetation Index (EVI), observed using the Moderate Resolution Imaging Spectroradiometer (MODIS), compared with field-based estimates of decreasing plant productivity. Here, we investigate potential causes of intra-annual variability by comparing EVI from mature forest with field-measured Leaf Area Index (LAI) to validate space-based observations. EVI was calculated from 19 nadir and off-nadir Hyperion images in the 2005 dry season, and inspected for consistency with MODIS observations from 2004 to 2009. The objective was to evaluate the possible influence of the view-illumination geometry and of canopy foliage and leaf flush on the EVI. Spectral mixture models were used to evaluate the relationship between EVI and the shade fraction, a measure that varies with pixel brightness. MODIS LAI values were compared with LAI estimated using hemispherical photographs taken in two field campaigns in the dry season. To keep LAI and leaf flush conditions as constant variables and vary solar illumination, we used airborne Hyperspectral Mapper (Hymap) data acquired over mature forest from another region on the same day but with two distinct solar zenith angles (SZA) (29° and 53°). Results showed that intra-annual variability in MODIS and nadir Hyperion EVI in the dry season of tropical forest were driven by solar illumination effects rather than changes in LAI. The reflectance of the MODIS and Hyperion blue, red and near infrared (NIR) bands was higher at the end of the dry season because of the predominance of sunlit canopy components for the sensors due to decreasing SZA from June (44°) to September (26°). Because EVI was highly correlated with the reflectance of the NIR band used to generate it (r of + 0.98 for MODIS and + 0.88 for Hyperion), this vegetation index followed the general NIR pattern, increasing with smaller SZA towards the end of the dry season. Hyperion EVI was inversely correlated with the shade fraction (r = − 0.93). Changes in canopy foliage detected from MODIS LAI data were not consistent with LAI estimates from hemispherical photographs. Although further research is necessary to measure the impact of leaf flush on intra-annual EVI variability in the Querência region, analysis of Hymap data with fixed LAI and leaf flush conditions confirmed the influence of the illumination effects on the EVI.     

Design and prototyping of a low-cost vehicle localization system with guaranteed convergence properties

This work presents a low-cost vehicle localization system, using measurements from one gyroscope, two wheel speed sensors and a GPS, to estimate the heading, velocity and position of a vehicle. Taking advantage of the nonholonomic constraints, the design of the observer (or “filter”) takes into account imperfections of the embedded sensor measurements, such as a slowly time-varying gyroscope bias or some uncertainty in the wheel diameter value. Thanks to a well-chosen nonlinear structure, the estimator is easy to tune, easy to implement, and well-behaved even at very low speed. Moreover, the proposed filter has guaranteed convergence properties when GPS is available and keeps providing good estimations during GPS losses. Simulation and experiment results in urban area illustrate the good performance of the algorithm.     

Comparison of bathymetry and seagrass mapping with hyperspectral imagery and airborne bathymetric lidar in a shallow estuarine environment

We compared hyperspectral imagery and single-wavelength airborne bathymetric light detection and ranging (lidar) for shallow water (<2 m) bathymetry and seagrass mapping. Both the bathymetric results from hyperspectral imagery and airborne bathymetric lidar reveal that the presence of a strongly reflecting benthic layer under seagrass affects the elevation estimates towards the bottom depth instead of the top of seagrass canopy. Full waveform lidar was also investigated for bathymetry and similar performance to discrete lidar was observed. A provisional classification was performed with limited ground reference samples and four supervised classifiers were applied in the study to investigate the capability of airborne bathymetric lidar and hyperspectral imagery to identify seagrass genera. The overall classification accuracy is highly variable and strongly dependent on the classification strategy used. Features from bathymetric lidar alone are not sufficient for substrate classification, while hyperspectral imagery alone showed significant capability for substrate classification with over 95% overall accuracy. The fusion of hyperspectral imagery and bathymetric lidar only marginally improved the overall accuracy of seagrass classification.     

iFind: 一个结合语义和视觉特征的图像相关反馈检索系统

给出了一个结合语义与视觉特征信息的图像相关反馈检索系统－－iFind。系统通过图像的标注信息构造语义网络，并在相关反馈中与图像的视觉特征相结合，有效地实现了在两个层次上的相关反馈，在基于内容的图像检索中取得了较为理想的效果，具有一定的应用价值。     

Application of the finite-element method for the forward and inverse models in optical tomography

The development of an optical tomographic imaging system for biological tissue based on time-resolved near-infrared transillumination has received considerable interest recently. The reconstruction problem is ill posed because of scatter-dominated photon propagation, and hence it requires both an accurate and fast transport model and a robust solution convergence scheme. The iterative image recovery algorithm described in this paper uses a numerical finite-element solution to the diffusion equation as the photon propagation model. The model itself is used to compare the influence of absorbing and scattering inhomogeneities embedded in a homogeneous tissue sample on boundary measurements to estimate the possibility of separating absorption and scattering images. Images of absorbers and scatterers reconstructed from both mean-time-of-flight and logarithmic intensity data are presented. It is found that mean-time-of-flight data offer increased resolution for reconstructing the scattering coefficient, whereas intensity data are favorable for reconstructing absorption.     

SODA: A new method of in-scene atmospheric water vapor estimation and post-flight spectral recalibration for hyperspectral sensors: Application to the HyMap sensor at two locations

A new method of estimating per-pixel atmospheric column water vapor (ACWV) and potential differences in the reported band center wavelengths of the HyMap sensor has been developed. The new method uses variations of a second order derivative algorithm (SODA) to assess the impact of atmospheric residual features on calculated surface reflectance spectra after atmospheric compensation. The SODA method provides an alternative to the current band ratio techniques of ACWV estimation and also allows the same form of algorithm to be used for the estimation of possible band shifts. A comparison of in-situ measured surface reflectance at two field sites in Western Australia demonstrates improvement in the resulting spectra when post-flight updates are made to the reported HyMap band center wavelengths and applied during the atmospheric compensation process. The same SODA methodology was varied to estimate the ACWV on a per-pixel basis and found to significantly reduce the appearance of the underlying surface structure on the resulting ACWV images as well as improve the overall accuracy of the estimation. The ACWV estimated from the HyMap imagery at the two field sites was found to agree with in-situ atmospheric ACWV measurements to within 2% and represented a two fold increase in accuracy over a 3 band ratio Continuum Interpolated Band Ratio (CIBR) technique of ACWV estimation.     

Band depth analysis of CHRIS/PROBA data for the study of a Mediterranean natural ecosystem. Correlations with leaf optical properties and ecophysiological parameters

The absorption feature approach was used in CHRIS multiangular hyperspectral data in order to investigate its potential for ecosystem remote sensing. For that purpose, CHRIS images in mode 1 were acquired throughout a two-year period for a Mediterranean ecosystem dominated by the semi-deciduous shrub Phlomis fruticosa. During each acquisition, coincident in situ Leaf spectra and ecophysiological measurements (Leaf Area Index, leaf pigment content and leaf water potential) were conducted. After data preprocessing, absorption feature information was calculated for both CHRIS and Leaf spectra for the whole spectrum. Three common characteristic absorption features within the spectral areas 450-550 nm, 550-750 nm and 900-1000 nm were detected. Each spectral area was then examined separately and four characteristic parameters were calculated that described the pattern, magnitude and position of the maximum absorption. Correlations between CHRIS and Leaf spectra for each date and viewing angle (VA) were then conducted. All correlations, either on full continuum removed spectra or on spectral areas, showed high coefficients of determination, especially (i) in higher observation angles (VA + 55), (ii) during the wet season and (iii) in strong absorptions such as the “red absorption”. Subsequently, correlations between CHRIS and Leaf absorption parameters of selected spectral areas with field-measured ecophysiological parameters were examined. Ecophysiological parameters proved to be highly correlated to CHRIS and Leaf absorption parameters in magnitude and/or pattern of the absorption feature and less in wavelength of the maximum absorption. CHRIS VAs +/− 36 showed the highest correlations although the type of relation, linear or nonlinear, was not conclusive. Finally, a first comparison between narrowband spectral indices and absorption features in correlations with ecophysiological parameters showed that both methods provide significant and comparable results, with oblique angles showing best performance. However, ecophysiological parameters are generally better predicted linearly by narrowband spectral indices issued from CHRIS, with most significant differences appearing on pigments absorbing mainly within 450-550 nm.