Philippe Goloub

Remote sensing of aerosols over land surfaces from POLDER‐ADEOS‐1 polarized measurements

The polarization measurements achieved by the POLDER instrument on ADEOS-1 are used for the remote sensing of aerosols over land surfaces. The key advantage of using polarized observations is their ability to systematically correct for the ground contribution whereas the classical approach using natural light fails. The estimation of land surface polarizing properties from POLDER has been examined in a previous paper. Here we consider how the optical thickness, δ 0 , and Angstrom exponent, α , of aerosols are derived from the polarized light backscattered by the particles. The inversion scheme is detailed and illustrative results are presented. Maps of the retrieved optical thickness allow for detection of large aerosol features and, in the case of small aerosols, the δ 0 and α retrievals are consistent with correlative ground-based measurements. However, because polarized light stems mainly from small particles, the results are biased for aerosol distributions containing coarser modes of particles. To overcome this limitation, an aerosol index defined as the product AI = δα 0 is proposed. Theoretical analysis and comparison with ground-based measurements suggest that AI is approximately the same when using δ 0 and α related to the entire aerosol size distribution or derived from the polarized light originating from the small polarizing particles alone. This invariance is specially assessed by testing the continuity of AI across coastlines, given the unbiased properties of aerosol retrieval over ocean. Although reducing the information concerning the aerosols, this single parameter allows a link between the POLDER aerosol surveys over land and ocean. POLDER aerosol index global maps enable the monitoring of major aerosol sources over continental areas.

Climatological aspects of the optical properties of fine/coarse mode aerosol mixtures

[1] Aerosol mixtures composed of coarse mode desert dust combined with fine mode combustion generated aerosols (from fossil fuel and biomass burning sources) were investigated at three locations that are in and/or downwind of major global aerosol emission source regions. Multiyear monitoring data at Aerosol Robotic Network sites in Beijing (central eastern China), Kanpur (Indo-Gangetic Plain, northern India), and Ilorin (Nigeria, Sudanian zone of West Africa) were utilized to study the climatological characteristics of aerosol optical properties. Multiyear climatological averages of spectral single scattering albedo (SSA) versus fine mode fraction (FMF) of aerosol optical depth at 675 nm at all three sites exhibited relatively linear trends up to ~50% FMF. This suggests the possibility that external linear mixing of both fine and coarse mode components (weighted by FMF) dominates the SSA variation, where the SSA of each component remains relatively constant for this range of FMF only. However, it is likely that a combination of other factors is also involved in determining the dynamics of SSA as a function of FMF, such as fine mode particles adhering to coarse mode dust. The spectral variation of the climatological averaged aerosol absorption optical depth (AAOD) was nearly linear in logarithmic coordinates over the wavelength range of 440-870 nm for both the Kanpur and Ilorin sites. However, at two sites in China (Beijing and Xianghe), a distinct nonlinearity in spectral AAOD in logarithmic space was observed, suggesting the possibility of anomalously strong absorption in coarse mode aerosols increasing the 870 nm AAOD.

Measurement and modeling of the Saharan dust radiative impact: Overview of the Saharan Dust Experiment (SHADE)

[1] Aerosols are known to be important in determining Earth’s radiative balance. Dust aerosols are of particular interest since, in addition to their scattering and absorbing properties that affect the solar radiation, they also perturb the terrestrial radiation. Recent studies have shown that a significant proportion of mineral dust in the atmosphere may be of anthropogenic origin, and therefore they may have an important role in climate change by exerting a significant radiative forcing. However, the optical and radiative properties of dust are not yet very well-determined, and even the sign of the resulting forcing is still questionable. The Saharan Dust Experiment (SHADE) was designed to better determine the parameters that are relevant for computing the direct radiative effect. Two aircraft combining in situ and remote sensing instruments were coordinated with satellite overpasses and ground-based observations during the experiment, which was based in the Cape Verde area during the period 19–29 September 2000. These in situ and remotely sensed data provide new valuable information on the microphysical, optical properties, and radiative effects of a large mineral dust outbreak. In addition, a global chemical transport model was used for assessing the radiative impact of these events, which are shown to be important on regional and global scales. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 3359 Meteorology and Atmospheric Dynamics: Radiative processes; 3360 Meteorology and Atmospheric Dynamics: Remote sensing; KEYWORDS: Saharan dust, physical and optical properties, dust direct radiative forcing

Remote sensing of aerosols over land surfaces including polarization measurements and application to POLDER measurements

Ground-based measurements of the diffuse skylight and airborne measurements of the light reflected by land surfaces are examined, especially with regard to their polarization properties. The reported land surface reflections correspond to multidirectional polarized measurements performed by the Polarization and Directionality of Earth Reflectances (POLDER) airborne version on very clear days. These observations are analyzed for retrieving the polarization properties of scattering by terrestrial aerosols and reflection by ground targets, respectively. The results suggest that the polarized light is much more sensitive to atmospheric scattering than to reflection by natural surfaces, especially by vegetative cover. Theoretical modeling supports this hypothesis. Finally, application of these results to aerosol remote sensing over land surfaces from POLDER measurements is discussed.

Results of POLDER in-flight calibration

POLDER is a CNES instrument on board NASDAs ADEOS polar orbiting satellite, which was successfully launched in August 1996. On October 30, 1996, POLDER entered its nominal acquisition phase and worked perfectly until ADEOSs early end of service on June 30, 1997. POLDER is a multispectral imaging radiometer/polarimeter designed to collect global and repetitive observations of the solar radiation reflected by the Earth/atmosphere system, with a wide field of view (2400 km) and a moderate geometric resolution (6 km). The instrument concept is based on telecentric optics, on a rotating wheel carrying 15 spectral filters and polarizers, and on a bidimensional charge coupled device (CCD) detector array. In addition to the classical measurement and mapping characteristics of a narrow-band imaging radiometer, POLDER has a unique ability to measure polarized reflectances using three polarizers (for three of its eight spectral bands, 443 to 910 nm) and to observe target reflectances from 13 different viewing directions during a single satellite pass. One of POLDERs original features is that its in-flight radiometric calibration does not rely on any on-board device. Many calibration methods using well-characterized calibration targets have been developed to achieve a very high calibration accuracy. This paper presents the various methods implemented in the in-flight calibration plan and the results obtained during the instrument calibration phase: absolute calibration over molecular scattering, interband calibration over sunglint and clouds, multiangular calibration over deserts and clouds, intercalibration with Ocean Color and Temperature Scanner (OCTS), and water vapor channels calibration over sunglint using meteorological analysis. A brief description of the algorithm and of the performances of each method is given.

Validation of the first algorithm applied for deriving the aerosol properties over the ocean using the POLDER/ADEOS measurements

Global map of aerosol parameters (optical thickness, Angstrom exponent, and refractive index) are derived from the POLDER instrument on board the ADEOS-1 platform. This paper focuses on aerosol optical thickness and Angstrom exponent retrievals that are based on visible and near infrared back scattering measurements. Assessment of the retrieval quality is achieved by means of comparison with AERONET sunphotometer data. The results show that the POLDER measurements can be used to distinguish several aerosol types from their Angstrom coefficients, in addition to a precise estimate of the aerosol optical thickness.

Cloud droplet effective radius from spaceborne polarization measurements

The spaceborne POLDER instrument provided the first quantitative measurements of the Earth reflectance polarization characteristics. Many POLDER images of polarized light show cloudbow type features over cloud fields for scattering angles between 150 and 170°. This unexpected observation is attributed to the polarized radiance generated by single scattering by cloud droplets. It shows that, in many cases, the cloud droplet size distribution is very narrow. The multidirectional polarized radiance measurements can be inverted for an accurate estimate of the cloud droplet radius.

Aerosol Remote Sensing over Clouds Using A-Train Observations

Abstract The detection of aerosol above clouds is critical for the estimate of both the aerosol and cloud radiative impacts. In this study, the authors present a new method to retrieve the aerosol properties over clouds that uses the multiangle polarization measurements of the Polarization and Directionality of Earth Reflectances (POLDER)–Polarization and Anisotropy of Reflectances for Atmospheric Sciences Coupled with Observations from a Lidar (PARASOL) instrument. The method is illustrated and applied to a case study exploiting the coincident observations from other passive and active sensors of the NASA A-Train satellite constellation. The case study is relative to an elevated biomass burning aerosol layer that originates from southern Africa and is then transported over low-level clouds extending over the Atlantic Ocean. It is shown that the comparison between the cloud-top heights retrieved with the different passive techniques developed for the A-Train sensors can be used to detect the presence of a...

Characterization of aerosols over ocean from POLDER/ADEOS‐1

POLDER (POLarization and Directionality of the Earths Reflectance) is a French instrument, launched in August 1996 onboard the Japanese sun-synchronous ADvanced Earth Satellite System (ADEOS). Data were acquired until the loss of the platform June 30th 1997. This paper describes information on the global distribution of tropospheric aerosol over the ocean derived from the POLDER instrument. The aerosol remote sensing algorithm yields the aerosol optical thickness at 0.865 µm, τ and the Angstrom exponent, α, which allows the retrieval the aerosol sizes. Monthly synthesis of the global aerosol characteristics over the ocean are presented and described here for three months: November 1996, February, and May 1997. The major and most persistent feature is the Saharan dust outbreaks off West Africa. Biomass burning aerosols are observed during May off the western Mexican Coast and during November in Madagascar. Asian, Indonesian and eastern US pollution are also observed. For some of these areas, statistical distributions of τ and α and their spatial variability are discussed.

Intercomparison of Satellite Retrieved Aerosol Optical Depth over the Ocean

For an 8-month period aerosol optical depth (AOD) is compared, derived over global oceans with five different retrieval algorithms applied to four satellite instruments flown on board three satellite platforms. The Advanced Very High Resolution Radiometer (AVHRR) was flown on board NOAA-14, the Ocean Color and Temperature Scanner (OCTS) and the Polarization and Directionality of the Earth’s Reflectances (POLDER) on board the Advanced Earth Observing Satellite(ADEOS), and the Total Ozone Mapping Spectrometer (TOMS) on board the Earth Probe satellites. The aerosol data are presented on the same format and converted to the same wavelength in the comparison and can therefore be a useful tool in validation of global aerosol models, in particular models that can be driven with meteorological data for the November 1996 to June 1997 period studied here. Large uncertainties in the global mean AOD are found. There is at least a factor of 2 difference between the AOD from the retrievals. The largest uncertainties are found in the Southern Hemisphere, and the smallest differences mostly near the continents in the Northern Hemisphere. The largest relative differences are probably caused by differences in cloud screening.