Search

International audience; Polarimetry is one of the most promising types of remote sensing for improved characterization of atmospheric aerosol. Indeed, aerosol particles constitute a highly variable atmospheric component characterized by a large number of parameters describing particle sizes, morphologies (including shape and internal structure), absorption and scattering properties, amounts, horizontal and vertical distribution, etc. Reliable monitoring of all these parameters is very challenging, and therefore the aerosol effects on climate and environment are considered to be among the most uncertain factors in climate and environmental research. In this regard, observations that provide both the angular distribution of the scattered atmospheric radiation as well as its polarization state at multiple wavelengths covering the UV-SWIR spectral range carry substantial implicit information on the atmospheric composition. Therefore, high expectations in improving aerosol characterization are associated with detailed passive photopolarimetric observations. 475 The critical need to use space-borne polarimetry for global accurate monitoring of detailed aerosol properties was first articulated in the late 1980s and early 1990s. By now, several orbital instruments have already provided polarization observations from space, and a number of advanced missions are scheduled for launch in the coming years by international and national space agencies. The first and most extensive record of polarimetric imagery was provided by POLDER-I, POLDER-II, and POLDER/PARASOL multi-angle multi-spectral polarization sensors. Polarimetric observations with the POLDER-like design intended for collecting extensive multi-angular multi-spectral measurements will be provided by several instruments, such as the MAI/TG-2, CAPI/TanSat, and DPC/GF-5 sensors recently launched by the Chinese Space Agency. Instruments such as the 3MI/MetOp-SG, MAIA, SpexOne and HARP2 on PACE, POSP, SMAC, PCF, DPC-Lidar, ScanPol and MSIP/Aerosol-UA, MAP/Copernicus CO2 Monitoring, etc. are planned to be launched by different space agencies in the coming decade. The concepts of these future instruments, their technical designs, and the accompanying algorithm development have been tested intensively and analyzed using diverse airborne prototypes. Certain polarimetric capabilities have also been implemented in such satellite sensors as GOME-2/MetOp and SGLI/GCOM-C. A number of aerosol retrieval products have been developed based on the available measurements and successfully used for different scientific applications. However, the completeness and accuracy of aerosol data operationally derived from polarimetry do not yet appear to have reached the accuracy levels implied by theoretical sensitivity studies that analyzed the potential information content of satellite po-larimetry. As a result, the dataset provided by MODIS is still most frequently used by the scientific community , yet this sensor has neither polarimetric nor multi-angular capabilities. Admittedly polarimetric multi-angular observations are highly complex and have extra sensitivities to aerosol particle morphology, vertical variability of aerosol properties, polarization of surface reflectance, etc. As such, they necessitate state-of-the-art forward modeling based on first-principles physics which remains rare, and conventional retrieval approaches based on look-up tables turn out to be unsuitable to fully exploit the information implicit in the measurements. Several new-generation retrieval approaches have recently been proposed to address these challenges. These methods use improved forward modeling of atmospheric (polarized) radiances and implement a search in the continuous space of solutions using rigorous statistically optimized inversions. Such techniques provide more accurate retrievals of the main aerosol parameters such as aerosol optical thickness and yield additional parameters such as aerosol absorption. However, the operational implementation of advanced retrieval approaches generally requires a significant extra effort, and the forward-modeling part of such retrievals still needs to be substantially improved. Ground-based passive polarimetric measurements have also been evolving over the past decade. Although polarimetry helps improve aerosol characterization, especially of the fine aerosol mode, the operators of major observational networks such as AERONET remain reluctant to include polarimetric measurements as part of routine retrievals owing to their high complexity and notable increase in effort required to acquire and interpret polarization data. In addition to remote-sensing observations, polarimetric characteristics of aerosol scattering have been measured in situ as well as in the laboratory using polar nephelometers. Such measurements constitute direct observations of single scattering with no contributions from multiple scattering effects and therefore provide unique data for the validation of aerosol optical models and retrieval concepts. This article overviews the above-mentioned polarimetric observations, their history and expected developments , and the state of resulting aerosol products. It also discusses the main achievements and challenges in the exploitation of polarimetry for the improved characterization of atmospheric aerosols.

International audience

[Abstract] It is well known that the particulate matter (PM_) is a typical indicator of small particles in the atmosphere. In addition to providing impacts on climate and environment, these small particles can bring adverse effects on human health. We set up SPM sampler attached to our AERONET (Aerosol Robotics Network) station in Higashi-Osaka in Japan. The SPM sampler provides information about the concentrations of particles. The AEROENT program provides the spectral information about aerosol optical thickness (AOT) and Angstrom exponent (α). Simultaneous measurements show that a linear correlation between AOT and PM_. These results indicate that PM_ can be estimated from AOT. However AOT represents integrated values of column aerosol amount retrieved from optical property, while concentration of PM_ presents in-situ aerosol loading on the surface. Then simple way using linear correlation brings the discrepancy between observed and estimated PM_. In this work, we use cluster information about aerosol type to reduce the discrepancy. Our improved method will be useful for retrieving particulate matter from satellite measurements.

資料番号: PA0810045000

To monitor urban atmospheric particles, we have undertaken simultaneous measurements of aerosols and suspended particulate matter (SPM) at Kinki University Campus, Higashi–Osaka, Japan, since 2004. The largest dust event recorded during our long-term observations was detected during the spring of 2006. We also examined the relationship between PM_ concentrations and aerosol properties obtained from radiometry using a multi-spectral photometer located at a NASA/AERONET station. We found a linear correlation between aerosol optical thickness and PM_ concentrations for both ordinary days and days with dust events. This fact indicates that aerosol characteristics can be estimated from SPM data, and vice versa. Combining radiometric aerosol information with surface-level particulate mass measurements is useful in studying air quality and aerosol properties., 本文データは一部CiNiiから複製したものである。

[Abstract] Japanese observing satellite, called ADEOS-2 (Advanced Earth Observing Satellite-2), carried POLDER-2 (Polarization and Directionality of Earth Reflectances) and operated from April to the end of October in 2003. This work shows the aerosol information, over East Asia region from April to June derived from POLDER-2 data. Our retrieval algorithm for aerosols is based on the light scattering behavior of the atmospheric particles in the visible and near infrared wavelengths. It is shown that very thick aerosol layer has covered over East Asia during the period. The retrieved results are validated with ground based sun photometry., 本文データの一部は、CiNiiから複製したものである。

[Abstract] Simultaneous measurements of atmospheric aerosols and suspended particulate matter (SPM) have been undertaken at Kinki University campus in Higashi-Osaka in order to monitor the urban environment. The sun/sky photometry has been made as a NASA/AERONET station since 2002, and the SPM-613D (Kimoto Electric) has been taking measurements of the SPM concentrations such as TSP, PM_, PM_, and OBC at the same site since March 15, 2004. The relationship between aerosol properties obtained from radiometry with AERONET and the SPM measurements is examined. It is found that there is a linear correlation between SPM concentrations and aerosol properties, which indicates that aerosol characteristics can be estimated from SPM data, and vice versa. Furthermore chemical analysis of the SPM sampling strongly suggests the really complicated features of the atmospheric particles at Higashi-Osaka. In other words, it is shown that the air quality of the Higashi-Osaka site is poor due to not only anthropogenic particles by local emissions, such as diesel vehicles and chemical industries, but also due to dust particles coming from continental desert areas by large scale climatic conditions., 本文データの一部は、CiNiiから複製したものである。

[Abstract] The relationship is examined between aerosol properties obtained from radiometry with AERONET and the particulate mass simultaneously measured with a new instrument (SPM-613D). The AERONET site has been set up at Higashi-Osaka since 2002 to make measurements for understanding the characteristic features of the urban atmosphere. It is found that the SPM measurements classified as fine particles (PM2.5), coarse particles (TSP/PM10), and optical black carbon (OBC) are very useful for determining the dominating of particle size and air quality. There was a strong correlation between the SPM concentrations and aerosol properties (AOT/AI), which indicates that aerosol characteristics can be estimated from SPM data, and vice versa., 本文データの一部は、CiNiiから複製したものである。

[Abstract] POLarization and Directionality of the Earth's Reflectance (POLDER) provides the multiangle data of polarization and radiance of solar radiation reflected by the earth atmosphere-surface system in the visible and near-infrared wavelengths. This work describes a procedure to extract aerosol characteristics over the ocean on a global scale from the POLDER near-infrared data. Our algorithm is achieved by taking into account of each role of polarization and radiance in each optical property of aerosols. That is our aerosol retrieval is made based on selective combination of polarization and radiance not as simple combination. In practice, a look-up table which is compiled based on multiple scattering simulations in an atmosphere-sea surface model is available. The obtained monthly mean global maps present the Angstrom exponent, the optical thickness and the aerosol index. From our aerosol maps, it can be seen that: 1) the specific feature of maritime aerosols on a global scale, 2) the continental aerosols are blown over the ocean on a regional and/or temporal scale., 本文データの一部は、CiNiiから複製したものである。

[Abstract] This paper presents a procedure on how to derive the atmospheric water vapor content from ADEOS/POLDER data. A POLDER sensor, mounted on the Earth observation satellite ADEOS in 1996, is a unique sensor which can gather multi-(up to 14) directional polarization measurements of one target. Two channels of POLDER data in the near infrared wavelengths are examined in this study. The first channel is in the water vapor absorption band of 0.910 μm and the second is in the gas absorption-free band of 0.865 μm. In practice, a ratio of each reflectance for these two channels is used to estimate the total column water vapor content. This algorithm will be referred to as the two-channel ratio method. The measured water vapor content from the POLDER data is in close agreement with the ground measurements undertaken at several NASA/AERONET stations. Global distribution of water vapor content presents the typical characteristics of spatial and temporal changes over the whole period of the ADEOS satellite's deployment. The water vapor content has high values over the tropical zone and decreases with latitude. Furthermore, the Indian monsoon is clearly demonstrated in the water vapor map. It should be noted that the water vapor content retrieved from the POLDER data has been validated with AERONET measurements. The global map of water vapor is mutually compared with that of aerosol properties, which are derived from the POLDER polarization data. In addition to the comparison of water vapor content with aerosol loading, it has been found that the WA (water vapor-aerosol) index, i.e. the product of the water vapor content, aerosol optical thickness and Angstrom exponent, is a good indicator to classify the regional characteristics of the atmosphere-surface system. For example, high and low values in the WA index represent a bio-active area and a desert-dust area, respectively., 本文データの一部は、CiNiiから複製したものである。

[Abstract] OCTS and POLDER sensors were mounted on the satellite ADEOS in August 1996. In our present study, cloud detection and distinction of thermodynamic phase, i.e. water/ice-phase of cloud top, is derived from the combination of ADEOS/POLDER and OCTS. In general, cloud coverage map has been made from both of reflectance and brightness temperature. POLDER cloud detection has been done by reflectance and polarized reflectance at visible, near-infrared wavelengths. Especially, 2 spectral bands of oxygen absorption detection at wavelengths of 0.763 and 0.765μm (O_2-A band) can be used for instead of brightness temperature. However, removal of absorption by ozone and water vapor are strongly needed for using O_2-A band. In this work, cloud detection is done by reflectance and polarization data by POLDER and brightness temperature given by OCTS. Thermodynamic phase of the cloud top is distinguished by using polarized reflectance at 0.865μm given by POLDER and brightness temperature at 8.53, 10.9, 12.0μm by OCTS for each of cloud pixel. This approach shows improvement of the distinction of water/ice cloud phase.

[Abstract] This work proposes the combined use of ADEOS/POLDER- and OCTS-data for detection of the cloud coverage and its thermodynamic phase. On the basis of the satellite date obtained on November 10 in 1996, it is shown that infrared data at a wavelength of 10.9μm given by OCTS and near-infrared polarized reflectance at 0.865μm over wide range of scattering angle provided by POLDER are available for cloud screening and for classification of the thermodynamic phase of cloud top into ice and water..

[ABSTRACT] Radiative properties of aerosols and clouds on a global scale are strongly needed for better understanding of Earth's radiation budget. POLDER and OCTS sensors were mounted on the earth observing satellite ADEOS. POLDER is known as a unique sensor as polarization measurements and multi-(up to 14) directional viewing of one target. We intend to show that retrieval of aerosols/clouds are greatly improved with these multi-directional polarization information. OCTS sensor has 8 spectral bands from visible to near-infrared wavelength region similar to POLDER and four thermal bands in the wavelengths from 3 to 12μm. Here the 10.9μm-band is used to estimate the cloud top temperature because of the less influence of the gaseous absorption. In other word, this work proposes the combined use of POLDER- and OCTS-data for detection of the aerosol/cloud characteristics over the ocean and over the land.

[Abstract] A better atmospheric correction for space-based data given by the second generation of ocean color sensors, e.g. ADEOS/OCTS and POLDER, is the main aim of this work. The new sensors possess high radiometric sensitivity and multi-channels in the visible and near-infrared wavelengths. This paper focuses on retrieval algorithms for atmospheric aerosols using both radiance and polarization data. It is shown first that a heterogeneous aerosol model following the Maxwell-Garnett mixing rule with small water-soluble inclusions in an oceanic matrix is available to interpret both the OCTS and POLDER data in the near infrared wavelength. Then an atmospheric correction process, which is based on the radiative transfer process in an atmosphere-ocean model involving the retrieved aerosol models, is applied to the ocean color data in the visible spectrum. Finally our procedure provides the phytoplankton pigment concentration near the sea surface defined as an expected map of the normalized difference phytoplankton index (NDPI). We would like to emphasize that the aerosol retrieval is improved by a combination of radiance and polarization data. In addition, it is of interest to mention that both of the NDPI maps obtained from OCTS and POLDER show the seasonal flow pattern of the currents over the North Pacific Ocean near Japan on April 26 in 1997.

[Abstract] Atmospheric correction algorithms, which mean a procedure to remove scattered light in the Earth atmosphere from spaced-based data, are shown. In order to achieve better atmospheric correction, this paper proposes two subjects; one is how to determine aerosol characteristics by referring to both of radiance and polarization, and the other is introduction of atmospheric correction coefficients. Our atmospheric correction, which is based on radiative transfer process in an atmosphere-ocean model involving the retrieved aerosol model, is applied to ocean color data given by ADEOS/OCTS. It is of interest to mention that retrieval of atmospheric aerosols is improved by combination use of radiance and polarization, and also atmospheric correction process is progressed by using the correction coefficients.

[ABSTRACT] Atmospheric correction algorithms for ocean color data are presented. This paper proposes two subjects in order to achieve better atmospheric correction. One is a retrieval procedure for atmospheric aerosols by referring to both of radiance and polarization given by ADEOS/OCTS and POLDER. The other is introduction of atmospheric correction coefficients. Our atmospheric correction, which is based on radiative transfer process in an atmosphere-ocean model involving the retrieved aerosol distribution, is applied for the OCTS-ocean color data. It is shown that retrieval of atmospheric aerosols is improved by combination use of radiance and polarization, and atmospheric correction process is progressed by using the correction coefficients.