Grid-adaptive Fourier pseudospectral time domain model for the light scattering simulation of atmospheric nonspherical particles

PSTD (pseudospectral time domain) is recognized as one of the powerful models to accurately calculate the scattering properties of nonspherical particles. But it is only good at the computation in coarse spatial resolution, and large "staircase approximation error" will occur in the actual computation. To solve this problem, the variable dimension scheme is introduced to improve the PSTD computation, in which, the finer grid cells are set near the particle's surface. In order to ensure that the PSTD algorithm can be performed on non-uniform grids, we have improved the PSTD with the space mapping technique so that the FFT algorithm can be implemented. The performance of the improved PSTD (called "IPSTD" in this paper) is investigated from two aspects: for the calculation accuracy, the phase matrices calculated by IPSTD are compared with those well tested scattering models like Lorenz-Mie theory, T-matrix method and DDSCAT; for computational efficiency, the computational time of PSTD and IPSTD are compared for the spheres with different sizes. From the results, it can be found that, the IPSTD scheme can improve the simulation accuracy of phase matrix elements notably, especially in the large scattering angles; though the computational burden of IPSTD is larger than that of PSTD, its computational burden does not increase substantially.

Dimension-variable invariant imbedding (DVIIM) T-matrix computational method for the light scattering simulation of atmospheric nonspherical particles

The invariant imbedding (IIM) T-matrix method has shown great potential in light scattering field. However, the T-matrix need to be calculated through the matrix recurrence formula derived from the Helmholtz equation, thus its computational efficiency is much lower than Extended Boundary Condition Method (EBCM). To alleviate this problem, the Dimension-Variable Invariant Imbedding (DVIIM) T-matrix method is presented in this paper. Compared with the traditional IIM T-matrix model, the dimensions of the T-matrix and relevant matrices are gradually increasing as the iteration performed step by step, thus the unnecessary operations of large matrices can be avoided in early iterations. To optimally determine the dimension of these matrices in each iterative calculation, the spheroid-equivalent scheme (SES) is also proposed. The effectiveness of the DVIIM T-matrix method is validated from the modeling accuracy and calculation efficiency. The simulation results show that compared with traditional T-matrix method, its modeling efficiency can be improved notably, especially for the particles with large size and aspect ratio, where for the spheroid with a aspect ratio of 0.5, the computational time is cut down by 25%. Though the dimension of the T matrix is cut down in the early iterations, the computational precision of DVIIM T-matrix model is not decreased notably, and a good agreement is achieved between the calculation results of DVIIM T-matrix method, IIM T-matrix method and other well-validated models (like EBCM and DDACSAT), where the relative errors of the integral scattering parameters (e.g., extinction, absorption, scattering cross sections) are generally less than 1%.

SEVIRI Aerosol Optical Depth Validation Using AERONET and Intercomparison with MODIS in Central and Eastern Europe

This paper presents the validation results of Aerosol Optical Depth (AOD) retrieved from the Spinning Enhanced Visible Infrared Radiometer (SEVIRI) data using the near-real-time algorithm further developed in the frame of the Satellite-based Monitoring Initiative for Regional Air quality (SAMIRA) project. The SEVIRI AOD was compared against multiple data sources: six stations of the Aerosol Robotic Network (AERONET) in Romania and Poland, three stations of the Aerosol Research Network in Poland (Poland–AOD) and Moderate Resolution Imaging Spectroradiometer (MODIS) data overlapping Romania, Czech Republic and Poland. The correlation values between a four-month dataset (June–September 2014) from SEVIRI and the closest temporally available data for both ground-based and satellite products were identified. The comparison of the SEVIRI AOD with the AERONET AOD observations generally shows a good correlation (r = 0.48–0.83). The mean bias is 0.10–0.14 and the root mean square error RMSE is between 0.11 and 0.15 for all six stations cases. For the comparison with Poland–AOD correlation values are 0.55 to 0.71. The mean bias is 0.04–0.13 and RMSE is between 0.10 and 0.14. As for the intercomparison to MODIS AOD, correlations values were generally lower (r = 0.33–0.39). Biases of −0.06 to 0.24 and RMSE of 0.04 to 0.28 were in good agreement with the ground–stations retrievals. The validation of SEVIRI AOD with AERONET results in the best correlations followed by the Poland–AOD network and MODIS retrievals. The average uncertainty estimates are evaluated resulting in most of the AOD values falling above the expected error range. A revised uncertainty estimate is proposed by including the observed bias form the AERONET validation efforts.

Polarization measurement accuracy analysis and improvement methods for the directional polarimetric camera

The directional polarimetric camera (DPC) is a remote-sensing instrument for the characterization of atmospheric aerosols and clouds by simultaneously conducting spectral, angular, and polarimetric measurements. Polarization measurement accuracy is an important index to evaluate the performance of the DPC and mainly related to the calibration accuracy of instrumental parameters. In this paper, firstly, the relationship between the polarization measurement accuracy of DPC and the parameter calibration errors caused by the nonideality of the components of DPC are analyzed, and the maximum polarization measurement error of DPC in the central field of view and edge field of view after initial calibration is evaluated respectively. Secondly, on the basis of the radiometric calibration of the DPC onboard the GaoFen-5 satellite in an early companion paper [Opt. Express2813187 (2020)10.1364/OE.391078], a series of simple and practical methods are proposed to improve the calibration accuracy of the parameters-the diattenuation of the optics, absolute azimuth angle, and relative transmission corresponding to each pixel, thereby improving the polarization measurement accuracy of DPC. The calibration results show that, compared with the original methods, the accuracy of the diattenuation of the optics, relative azimuth angle, and relative transmission of three polarized channels obtained with the improved methods are improved from ±1%, 0.1 degree and ±2% to ±0.4%, 0.05 degree and ±0.2%, respectively. Finally, two verification experiments based on a non-polarized radiation source and a polarizing system were carried out in the laboratory respectively to verify the improvement of the parameters modified by the proposed methods on the polarization measurement accuracy of the DPC to be boarding the GaoFen-5 (02) satellite. The experimental results show that when the corrected parameters were employed, the average error in measuring the degree of linear polarization of non-polarized light source for all pixels in the three polarized bands and the maximum deviation of the degree of linear polarization between the values set by the polarizing system and the values measured by the DPC at several different field of view angles for each polarized spectral band are obviously reduced. Both the mean absolute errors and the root mean square errors of the degree of linear polarization obtained with the corrected parameters are much lower than those obtained with the original parameters. All of these prove the effectiveness of the proposed methods.

The Dark Target Algorithm for Observing the Global Aerosol System: Past, Present, and Future

The Dark Target aerosol algorithm was developed to exploit the information content available from the observations of Moderate-Resolution Imaging Spectroradiometers (MODIS), to better characterize the global aerosol system. The algorithm is based on measurements of the light scattered by aerosols toward a space-borne sensor against the backdrop of relatively dark Earth scenes, thus giving rise to the name “Dark Target”. Development required nearly a decade of research that included application of MODIS airborne simulators to provide test beds for proto-algorithms and analysis of existing data to form realistic assumptions to constrain surface reflectance and aerosol optical properties. This research in itself played a significant role in expanding our understanding of aerosol properties, even before Terra MODIS launch. Contributing to that understanding were the observations and retrievals of the growing Aerosol Robotic Network (AERONET) of sun-sky radiometers, which has walked hand-in-hand with MODIS and the development of other aerosol algorithms, providing validation of the satellite-retrieved products after launch. The MODIS Dark Target products prompted advances in Earth science and applications across subdisciplines such as climate, transport of aerosols, air quality, and data assimilation systems. Then, as the Terra and Aqua MODIS sensors aged, the challenge was to monitor the effects of calibration drifts on the aerosol products and to differentiate physical trends in the aerosol system from artefacts introduced by instrument characterization. Our intention is to continue to adapt and apply the well-vetted Dark Target algorithms to new instruments, including both polar-orbiting and geosynchronous sensors. The goal is to produce an uninterrupted time series of an aerosol climate data record that begins at the dawn of the 21st century and continues indefinitely into the future.

Parameterization of The Single-Scattering Properties of Dust Aerosols in Radiative Flux Calculations

In this study, we present parameterization schemes of dust single-scattering properties (SSPs) in order to establish a fast and accurate way to obtain the SSPs for dust shortwave radiative flux calculation. Based on the assumption that dust particles are spheroids, we represent a single nonspherical particle with a collection of monodisperse spheres that contain the same total surface area and volume as the original particle to convert the spheroid to a sphere. The SSPs of dust particles were parameterized in terms of the effective radius ( R e ) and imaginary part of the refractive index ( M i ). The averaged relative errors of the parameterized to the “exact” single-scattering properties, which refer to the results from the Mie theory program, are below 1.5%. To further quantify the impact of parametrization on the radiative flux simulation, we computed the radiative fluxes at both the top of the atmosphere (TOA) and the surface by using SSPs from the parameterization and the “exact”, respectively. The maximum relative errors were below 1% at both the TOA and the surface, proving that the SSPs of dust calculated by our parameterization schemes are well suited for radiative flux calculations. This parameterization differs from previous works by being formulated not only with R e but also with M i . We also investigated the sensitivity of dust-aerosol forcing to R e , M i , optical depth (τ), and solar zenith angle (SZA). The results show that the value of shortwave (SW) radiative forcing (RF) at the TOA changes from negative to positive as the M i is increasing, which means that, as the absorption of dust particles becomes stronger, more energy is kept in the atmosphere to heat the earth–atmosphere system. The SW RF gradually becomes less negative at the TOA and more negative at the surface with increasing R e , due to the decreases of reflection and transmission along with the single-scattering albedo decreasing. As the optical depth increases, the values of the SW RF decrease because of the strong attenuation for heavy loading. When SZA increases, the SW RF becomes more negative at both the TOA and the surface due to the long optical path at a large SZA. The errors induced from the parameterized SSPs of dust in the SW RF calculation are very small, which are less than 2.1%, demonstrating the accuracy of the parameterization and its reliability for climate model applications.

Sensitivity study on the contribution of scattering by randomly oriented nonspherical hydrosols to linear polarization in clear to semi-turbid shallow waters

The influence of hydrosol nonsphericity on the polarization characteristics of light under water is investigated by combining accurate single-scattering models for randomly oriented spheroidal scatterers with a radiative transfer model that employs Stokes formalism and considers refraction of direct unpolarized solar radiation and 100% linearly polarized radiation at the air-water interface followed by single scattering. Variations in what we call the "linear polarization phase function" (the degree of linear polarization as a function of scattering angle and the angle of linear polarization as a function of scattering angle) are examined for a wide range of spheroid aspect ratios and complex refractive indices of hydrosols. Implications for polarization-sensitive marine organisms and for remote sensing of the marine environment are discussed.

Atmospheric Correction of Satellite Ocean-Color Imagery During the PACE Era

The Plankton, Aerosol, Cloud, ocean Ecosystem (PACE) mission will carry into space the Ocean Color Instrument (OCI), a spectrometer measuring at 5nm spectral resolution in the ultraviolet (UV) to near infrared (NIR) with additional spectral bands in the shortwave infrared (SWIR), and two multi-angle polarimeters that will overlap the OCI spectral range and spatial coverage, i. e., the Spectrometer for Planetary Exploration (SPEXone) and the Hyper-Angular Rainbow Polarimeter (HARP2). These instruments, especially when used in synergy, have great potential for improving estimates of water reflectance in the post Earth Observing System (EOS) era. Extending the top-of-atmosphere (TOA) observations to the UV, where aerosol absorption is effective, adding spectral bands in the SWIR, where even the most turbid waters are black and sensitivity to the aerosol coarse mode is higher than at shorter wavelengths, and measuring in the oxygen A-band to estimate aerosol altitude will enable greater accuracy in atmospheric correction for ocean color science. The multi-angular and polarized measurements, sensitive to aerosol properties (e.g., size distribution, index of refraction), can further help to identify or constrain the aerosol model, or to retrieve directly water reflectance. Algorithms that exploit the new capabilities are presented, and their ability to improve accuracy is discussed. They embrace a modern, adapted heritage two-step algorithm and alternative schemes (deterministic, statistical) that aim at inverting the TOA signal in a single step. These schemes, by the nature of their construction, their robustness, their generalization properties, and their ability to associate uncertainties, are expected to become the new standard in the future. A strategy for atmospheric correction is presented that ensures continuity and consistency with past and present ocean-color missions while enabling full exploitation of the new dimensions and possibilities. Despite the major improvements anticipated with the PACE instruments, gaps/issues remain to be filled/tackled. They include dealing properly with whitecaps, taking into account Earth-curvature effects, correcting for adjacency effects, accounting for the coupling between scattering and absorption, modeling accurately water reflectance, and acquiring a sufficiently representative dataset of water reflectance in the UV to SWIR. Dedicated efforts, experimental and theoretical, are in order to gather the necessary information and rectify inadequacies. Ideas and solutions are put forward to address the unresolved issues. Thanks to its design and characteristics, the PACE mission will mark the beginning of a new era of unprecedented accuracy in ocean-color radiometry from space.

Retrieval of the Fine-Mode Aerosol Optical Depth over East China Using a Grouped Residual Error Sorting (GRES) Method from Multi-Angle and Polarized Satellite Data

The fine-mode aerosol optical depth (AODf) is an important parameter for the environment and climate change study, which mainly represents the anthropogenic aerosols component. The Polarization and Anisotropy of Reflectances for Atmospheric Science coupled with Observations from a Lidar (PARASOL) instrument can detect polarized signal from multi-angle observation and the polarized signal mainly comes from the radiation contribution of the fine-mode aerosols, which provides an opportunity to obtain AODf directly. However, the currently operational algorithm of Laboratoire d’Optique Atmosphérique (LOA) has a poor AODf retrieval accuracy over East China on high aerosol loading days. This study focused on solving this issue and proposed a grouped residual error sorting (GRES) method to determine the optimal aerosol model in AODf retrieval using the traditional look-up table (LUT) approach and then the AODf retrieval accuracy over East China was improved. The comparisons between the GRES retrieved and the Aerosol Robotic Network (AERONET) ground-based AODf at Beijing, Xianghe, Taihu and Hong_Kong_PolyU sites produced high correlation coefficients (r) of 0.900, 0.933, 0.957 and 0.968, respectively. The comparisons of the GRES retrieved AODf and PARASOL AODf product with those of the AERONET observations produced a mean absolute error (MAE) of 0.054 versus 0.104 on high aerosol loading days (AERONET mean AODf at 865 nm = 0.283). An application using the GRES method for total AOD (AODt) retrieval also showed a good expandability for multi-angle aerosol retrieval of this method.

The VIS-SWIR spectrum of skylight polarization

The skylight degree of linear polarization (DoLP) was previously shown to vary primarily with aerosol optical depth and underlying surface reflectance for visible-to-near-infrared (VNIR) wavelengths. This paper extends the study of skylight polarization to 2.5 μm in the short-wave infrared (SWIR). A successive-orders-of-scattering radiative transfer code was used to model skylight polarization with measured inputs that included aerosol properties retrieved from a ground-based solar radiometer (extrapolated into the SWIR) and spectral surface reflectance from a handheld spectrometer. The modeled DoLP depended heavily on the aerosol size distribution at SWIR wavelengths and on the aerosol optical depth at VNIR wavelengths. Once the aerosol optical depth became greater than the Rayleigh optical depth, the predicted polarization deviated significantly from Rayleigh scattering theory. The SWIR polarization spectrum generally decreased at wavelengths beyond 1 μm at a rate dependent on the aerosol size distribution. The surface reflectance affected the polarization in the same manner throughout the visible (VIS)-SWIR spectrum, with higher reflectance decreasing the skylight polarization. Validation measurements of SWIR skylight polarization in a 1.5-1.8 μm band are also shown. These measurements were made on clean and smoky days using a SWIR imaging polarimeter. In both simulations and measurements, the SWIR skylight polarization was greater in the smoky atmosphere than in the clean atmosphere.

Simultaneously simulating the scattering properties of nonspherical aerosol particles with different sizes by the MRTD scattering model

In order to improve the computational efficiency of multi-resolution time domain (MRTD) scattering model, a multi-size synchronous-computational scheme (MSCS) is proposed. By using MSCS, the scattering properties of the particles with different sizes can be simultaneously calculated by MRTD model in one wave-particle interaction simulation. In this model, the pulse plane wave with a wide spectrum is taken as the incident light, and the light scattering simulation for particles with different sizes is transformed into the scattering calculation for a size-fixed particle at different wavelengths. To guarantee the stability and precision of the improved MRTD (IMRTD) model, the method to design model's input parameters, such as the spatial resolution, discrete time interval and pulse width, is proposed. To validate the accuracy of IMRTD model, its results are compared with those of Mie and T-Matrix theory, and the influence of spatial resolution on the precision of IMRTD is analyzed as well. At last, model's computational efficiency is also discussed. The simulation results show that, IMRTD method can calculate the scattering parameters of particles with different sizes simultaneously and accurately, where, in case that the pulse width is 5.56 × 10^-8ns, and the radius of the size-fixed particle is 0.5μm (its size parameter is 6.28), light scattering process by particles with size parameters up to 12.56 can be successfully simulated. With the increasing of spatial resolution, the simulation accuracy is improved for all particles, and the improvement for large particles is more notable than that for small ones. It can also be found that the computational efficiency of IMRTD is much higher than that of traditional version.

Polarization impacts on the water-leaving radiance retrieval from above-water radiometric measurements

Above-water measurements of water-leaving radiance are widely used for water-quality monitoring and ocean-color satellite data validation. Reflected skylight in above-water radiometry needs to be accurately estimated prior to derivation of water-leaving radiance. Up-to-date methods to estimate reflection of diffuse skylight on rough sea surfaces are based on radiative transfer simulations and sky radiance measurements. But these methods neglect the polarization state of the incident skylight, which is generally highly polarized. In this paper, the effects of polarization on the sea surface reflectance and the subsequent water-leaving radiance estimation are investigated. We show that knowledge of the polarization field of the diffuse skylight significantly improves above-water radiometry estimates, in particular in the blue part of the spectrum where the reflected skylight is dominant. A newly developed algorithm based on radiative transfer simulations including polarization is described. Its application to the standard Aerosol Robotic Network-Ocean Color and hyperspectral radiometric measurements of the 1.5-year dataset acquired at the Long Island Sound site demonstrates the noticeable importance of considering polarization for water-leaving radiance estimation. In particular it is shown, based on time series of collocated data acquired in coastal waters, that the azimuth range of measurements leading to good-quality data is significantly increased, and that these estimates are improved by more than 12% at 413 nm. Full consideration of polarization effects is expected to significantly improve the quality of the field data utilized for satellite data validation or potential vicarious calibration purposes.

Influence of polarimetric satellite data measured in the visible region on aerosol detection and on the performance of atmospheric correction procedure over open ocean waters

An original atmospheric correction algorithm, so-called multi-directionality and POLarization-based Atmospheric Correction (POLAC), is described. This algorithm is based on the characteristics of the multidirectional and polarimetric data of the satellite PARASOL (CNES). POLAC algorithm is used to assess the influence of the polarimetric information in the visible bands on the retrieval of the aerosol properties and the water-leaving radiance over open ocean waters. This study points out that the use of the polarized signal significantly improves the aerosol type determination. The use of the polarized information at one visible wavelength only, namely 490 nm, allows providing estimates of the Angstrom exponent of aerosol optical depth with an uncertainty lower than 4%. Based on PARASOL observations, it is shown that the detection of the fine aerosols is improved when exploiting polarization data. The atmospheric component of the satellite signal is then better modeled, thus improving de facto the water-leaving radiance estimation.

Retrieving radius, concentration, optical depth, and mass of different types of aerosols from high-resolution infrared nadir spectra

We present a sophisticated radiative transfer code for modeling outgoing IR radiation from planetary atmospheres and, conversely, for retrieving atmospheric properties from high-resolution nadir-observed spectra. The forward model is built around a doubling-adding routine and calculates, in a spherical refractive geometry, the outgoing radiation emitted by the Earth and the atmosphere containing one layer of aerosol. The inverse model uses an optimal estimation approach and can simultaneously retrieve atmospheric trace gases, aerosol effective radius, and concentration. It is different from existing codes, as most forward codes dealing with multiple scattering assume a plane-parallel atmosphere, and as for the retrieval, it does not rely on precalculated spectra, the use of microwindows, or two-step retrievals. The simultaneous retrieval on a broad spectral range exploits the full potential of current state-of-the-art hyperspectral IR sounders, such as AIRS and IASI, and should be particularly useful in studying major pollution events. We present five example retrievals of IASI spectra observed in the range from 800 to 1200 cm(-1) above dust, volcanic ash, sulfuric acid, ice particles, and biomass burning aerosols.

First results from a dual photoelastic-modulator-based polarimetric camera

We report on the construction and calibration of a dual photoelastic-modulator (PEM)-based polarimetric camera operating at 660?nm. This camera is our first prototype for a multispectral system being developed for airborne and spaceborne remote sensing of atmospheric aerosols. The camera includes a dual-PEM assembly integrated into a three-element, low-polarization reflective telescope and provides both intensity and polarization imaging. A miniaturized focal-plane assembly consisting of spectral filters and patterned wire-grid polarizers provides wavelength and polarimetric selection. A custom push-broom detector array with specialized signal acquisition, readout, and processing electronics captures the radiometric and polarimetric information. Focal-plane polarizers at orientations of 0 degrees and -45 degrees yield the normalized Stokes parameters q=Q/I and u=U/I respectively, which are then coregistered to obtain degree of linear polarization (DOLP) and angle of linear polarization. Laboratory test data, calibration results, and outdoor imagery acquired with the camera are presented. The results show that, over a wide range of DOLP, our challenging objective of uncertainty within +/-0.005 has been achieved.

A vector radiative transfer model for coupled atmosphere and ocean systems based on successive order of scattering method

A vector radiative transfer model has been developed for coupled atmosphere and ocean systems based on the Successive Order of Scattering (SOS) Method. The emphasis of this study is to make the model easy-to-use and computationally efficient. This model provides the full Stokes vector at arbitrary locations which can be conveniently specified by users. The model is capable of tracking and labeling different sources of the photons that are measured, e.g. water leaving radiances and reflected sky lights. This model also has the capability to separate florescence from multi-scattered sunlight. The delta - fit technique has been adopted to reduce computational time associated with the strongly forward-peaked scattering phase matrices. The exponential - linear approximation has been used to reduce the number of discretized vertical layers while maintaining the accuracy. This model is developed to serve the remote sensing community in harvesting physical parameters from multi-platform, multi-sensor measurements that target different components of the atmosphere-oceanic system.

Retrieval of aerosol properties over land surfaces: capabilities of multiple-viewing-angle intensity and polarization measurements

We investigate the capabilities of different instrument concepts for the retrieval of aerosol properties over land. It was found that, if the surface reflection properties are unknown, only multiple-viewing-angle measurements of both intensity and polarization are able to provide the relevant aerosol parameters with sufficient accuracy for climate research. Furthermore, retrieval errors are only little affected when the number of viewing angles is increased at the cost of the number of spectral sampling points and vice versa. This indicates that there is a certain amount of freedom for the instrument design of dedicated aerosol instruments. The final choice on the trade-off between the spectral sampling and the number of viewing angles should be made taking other factors into account, such as instrument complexity and the ability to obtain global coverage.