Haze Optical Thickness Research Articles

Retrieval of haze properties and HCN concentrations from the three-micron spectrum of Titan

The 3 µm spectrum of Titan contains line emission and absorption as well as a significant haze continuum. The line emission has been previously analyzed in the literature, but that analysis has not properly included the influence of haze on the line emission. We report a new analysis of the 3 µm HCN emission spectrum using radiative transfer equations that include scattering and absorption by molecules and haze particles at altitudes lower than 500 km, where the influence of haze on the emergent spectrum becomes significant. Taking advantage of the dominance of resonant single scattering in the HCN ν3 fundamental and of the moderate haze optical thickness of the atmosphere around 3 µm, we adopt single dust and molecular scattering and present a formulation for the radiative transfer process. We evaluate the quantitative influence of haze scattering on the emission line intensities, and derive vertically-resolved single scattering albedos of the haze from model fits. We also present the resulting concentrations of HCN for altitudes below 500 km, where we find that the haze scattering significantly influences the retrieval of the concentrations of HCN. We conclude that the formulation we present is useful for the analysis of the HCN line emission from Titan and other similar hazy planetary or celestial objects.

A polarimetric investigation of Jupiter: Disk-resolved imaging polarimetry and spectropolarimetry

Context. Polarimetry is a powerful remote sensing tool to characterise solar system planets and, potentially, to detect and characterise exoplanets. The linear polarisation of a planet as a function of wavelength and phase angle is sensitive to the cloud and haze particle properties in planetary atmospheres, as well as to their altitudes and optical thicknesses.Aims. We present for the first time polarimetric signals of Jupiter mapped over the entire disk, showing features such as contrasts between the belts and zones, the polar regions, and the Great Red Spot. We investigate the use of these maps for atmospheric characterisation and discuss the potential application of polarimetry to the study of the atmospheres of exoplanets. Methods. We have obtained polarimetric images of Jupiter, in the B , V , and R filters, over a phase angle range of α = 4°–10.5°. In addition, we have obtained two spectropolarimetric datasets, over the wavelength range 500–850 nm. An atmospheric model was sought for all of the datasets, which was consistent with the observed behaviour over the wavelength and phase angle range. Results. The polarimetric maps show clear latitudinal structure, with increasing polarisation towards the polar regions, in all filters. The spectropolarimetric datasets show a decrease in polarisation as a function of wavelength along with changes in the polarisation in methane absorption bands. A model fit was achieved by varying the cloud height and haze optical thickness; this can roughly produce the variation across latitude for the V and R filters, but not for the B filter data. The same model particles are also able to produce a close fit to the spectropolarimetric data. The atmosphere of Jupiter is known to be complex in structure, and data taken at intermediate phase angles (unreachable for Earth-based telescopes) seems essential for a complete characterisation of the atmospheric constituents. Because exoplanets orbit other stars, they are observable at intermediate phase angles and thus promise to be better targets for Earth-based polarimetry.

Preliminary study of Venus cloud layers with polarimetric data from SPICAV/VEx

We present unique polarization data from the SPICAV-IR spectrometer onboard ESA׳s Venus Express (VEx) spacecraft and a first retrieval of cloud parameters. The polarization data have been collected from 2006 to 2010, and cover mostly the northern hemisphere, in the 0.65 to 1.7μm spectral range. They contain information about latitudinal and longitudinal variations in the properties of Venus clouds and hazes, and about temporal variations in these properties. The degree of polarization measured on a few test orbits is in agreement with previous observations from the ground and from Pioneer Venus. Using numerical modeling to interpret the nadir observations, we retrieve mean values of reff~1μm and νeff~0.07 for, respectively, the effective radius and variance of the cloud particle size distribution and a refractive index nr=1.42±0.02 at λ=1.101μm. We also derive an upper limit τh=0.17 at λ=1.101μm for the haze optical thickness at high latitudes. All these values are in good agreement with previous determinations.

Retrieval of the Haze Optical Thickness in North China Plain Using MODIS Data

China's industrialized regions have seen increasing occurrence of heavy haze caused by severe particle pollution. However, aerosol retrieval under these circumstances is often excluded from NASA's Moderate Resolution Imaging Spectrometer (MODIS) aerosol products due to cloud mask and suspected high surface reflectance. An algorithm to retrieve the haze aerosol optical thickness (HAOT) is developed using MODIS data to supplement the current MODIS retrieval algorithm. This method includes 1) haze identification, 2) the generation of a surface reflectance database using MODIS data in hazy conditions, and 3) the development of haze aerosol models with four aerosol components simulated by a global 3-D atmospheric chemical transport model (GEOS-Chem). This algorithm was used in combination with the MODIS dense dark vegetation algorithm to retrieve 1 km HAOT over North China Plain from March to September of 2008. The values of the retrieved HAOT values are mostly between 0.7–3, with a correlation coefficient of 0.82 with the Aerosol Robotic NETwork observations and a 19% mean relative difference. Retrieval uncertainties associated with the errors in haze detection, surface reflectance, and haze models were analyzed using ground measurements.

Study of the vertical structure of Saturn's atmosphere using HST/WFPC2 images

We have studied the vertical structure of hazes at six different latitudes (−60°, −50°, −30°, −10°, +30°, and +50°) on Saturn's atmosphere. For that purpose we have compared the results of our forward radiative transfer model to limb-to-limb reflectivity scans at four different wavelengths (230, 275, 673.2, and 893 nm). The images were obtained with the Hubble Space Telescope Wide Field Planetary Camera 2 in September 1997, during fall on Saturn's northern hemisphere. The spatial distribution of particles appears to be very variable with latitude both in the stratosphere and troposphere. For the latitude range +50° to −50°, an atmospheric structure consisting of a stratospheric haze and a tropospheric haze interspersed by clear gas regions has been found adequate to explain the center to limb reflectivities at the different wavelengths. This atmospheric structure has been previously used by Ortiz et al. (1996, Icarus 119, 53–66) and Stam et al. (2001, Icarus 152, 407–422). In this work the top of the tropospheric haze is found to be higher at the southern latitudes than at northern latitudes. This hemispherical asymmetry seems to be related to seasonal effects. Different latitudes experience different amount of solar insolation that can affect the atmospheric structure as the season varies with time. The haze optical thickness is largest (about 30 at 673.2 nm) at latitudes ±50 and −10 degrees, and smallest (about 18) at ±30 degrees. The stratospheric haze is found to be optically thin at all studied latitudes from −50 to +50 degrees being maximum at −10° ( τ=0.033). At −60° latitude, where the UV images show a strong darkening compared to other regions on the planet, the cloud structure is remarkably different when compared to the other latitudes. Here, aerosol and gas are found to be uniformly mixed down to the 400 mbar level.

Galileo Photopolarimetry of Jupiter at 678.5 nm

Brightness and linear polarization measurements at 678.5 nm for four south–north strips of Jupiter are studied. These measurements were obtained in 1997 by the Galileo photopolarimeter/radiometer. The observed brightness exhibits latitudinal variations consistent with the belt/zone structure of Jupiter. The observed degree of linear polarization is small at low latitudes and increases steeply toward higher latitudes. No clear correlations were observed between the degree of linear polarization and the brightness. The observed direction of polarization changes from approximately parallel to the local scattering plane at low latitudes to perpendicular at higher latitudes. For our studies, we used atmospheric models that include a haze layer above a cloud layer. Parameterized scattering matrices were employed for the haze and cloud particles. On a pixel-wise basis, the haze optical thickness and the single-scattering albedo of the cloud particles were derived from the observed brightness and degree of linear polarization; results were accepted only if they were compatible with the observed direction of polarization. Using atmospheric parameter values obtained from Pioneer 10 and 11 photopolarimetry for the South Tropical Zone and the north component of the South Equatorial Belt, this analysis yielded acceptable results for very few pixels, particularly at small phase angles. However, for almost all pixels, acceptable results were found when the parameterized scattering matrix of the cloud particles was adjusted to produce more negative polarization for single scattering of unpolarized light, especially at large scattering angles, similar to some laboratory measurements of ammonia ice crystals. Using this adjusted model, it was found that the derived latitudinal variation of the single-scattering albedo of the cloud particles is consistent with the belt/zone structure, and that the haze optical thickness steeply increases toward higher latitudes.

A Universal Phase Function for Jovian Clouds from the Visible to Near-IR Wavelength Region. I. Zone Phase Function

Abstract A set of wavelength-dependent scattering phase functions for Jupiter's clouds has been obtained; the Mie theory was employed to simulate well-defined scattering phase functions deduced from an analysis of the Pioneer 10 photometry data (Tomasko et al. 1978, AAA 021.099.011). A reasonable fit to the cloud-phase functions for the bright zone (STrZ) was obtained with a particle effective radius, reff, of 0.96 μ m and the real part of the refractive index, nr, being 1.591 for the blue and 1.535 for the red, slightly higher than the nominal values for ammonia ice. The derived Mie phase functions were tested against several STrZ data. The appropriateness of these phase functions has been demonstrated through the fact that they reproduce not only the observed limb-to-limb intensity profiles, but also the correct wavelength dependence of the haze optical thickness for a wide wavelength range from visible to near-infrared.

Analysis of temporal variations of the polarization of Venus observed by Pioneer Venus Orbiter

Time variations of the disk‐averaged polarization of Venus at wavelengths of 270, 365, 550, and 935 nm have been analyzed in terms of average haze optical thickness, cloud‐top pressure, and cloud absorption optical thickness at 270 nm (presumably due to gaseous SO2 absorption). The polarization data were deduced from observations made by the Pioneer Venus Orbiter between 1978 and 1990. Newton‐Raphson iteration was used to derive the atmospheric parameter values from these observations. The required multiple scattering calculations were performed using the adding/doubling method. The atmospheric model consists of a cloud layer and a haze layer. The cloud and haze particles are composed of a concentrated sulfuric acid solution and have an effective radius of 1.05 μm and 0.25 μm, respectively. It was found that the disk‐averaged haze optical thickness, cloud‐top pressure, and cloud absorption optical thickness at 270 nm varied irregularly between 1978 and 1990. No trends were identified for the cloud‐top pressure and cloud absorption optical thickness at 270 nm, but we did deduce a decrease in haze optical thickness during this 12 year period. Specific numbers are: (1) a haze optical thickness at 365 nm of about 0.25 in 1978 and less than 0.1 in 1990, (2) a cloud‐top pressure that varies between 5 and 20 mbar, and (3) a cloud absorption optical thickness that varies between 0.0 and 4.0. Finally, our analysis revealed that the disk‐averaged cloud‐top pressure and haze optical thickness behaved anomalously in 1983.

A biwavelength analysis of Pioneer Venus polarization observations

A new method for analyzing Pioneer Venus polarimetry data on a pixel‐wise basis is presented. Quasi‐simultaneous observations at two wavelengths (550 and 935 nm) are combined and compared with results of multiple scattering calculations. In this manner, hypotheses about particle size distributions in the upper part of the Venus atmosphere are tested. Particles composed of a sulfuric acid solution are considered, and a distinction is made between large and small particles, called cloud and haze particles, respectively. Three model atmospheres have been investigated: (1) a single layer containing cloud particles, (2) a single layer containing a mixture of cloud and haze particles, and (3) a two layer model with an upper layer composed of haze particles and a lower layer containing cloud particles. It is found that all three models agree with the observations at phase angles near 20°, but that the first model cannot be made to agree with the observations for phase angles near 90°. This confirms the presence of haze particles in the Venus atmosphere found earlier by Kawabata et al. [1980]. We find that the haze particles may be situated either above or mixed within the main cloud deck of Venus. We derived effective radii between 0.85 and 1.15 μm for the cloud particles, and effective radii of 0.2 or 0.3 μm for the haze particles. When the haze particles are situated above the cloud layer, the haze optical thickness can take values of up to 0.6 at 550 nm. When the haze particles are mixed with the cloud particles, their contribution to the total atmospheric scattering coefficient at 550 nm can become as large as 70%.

Properties of haze in the atmosphere of Triton

Modeling the formation of haze by methane photolysis products requires knowledge of the methane altitude profile and optical data at two wavelengths to determine the two free parameters: the cross‐section‐weighted mean radius rc and the particle material density ρ. Using Voyager ultraviolet spectrometer measurements of CH4 and haze combined with the haze brightness profile determined by the narrow angle camera, we infer a haze optical thickness of 0.024 at 1500 Å and 7.8 × 10−3 in the spectral range of the narrow angle camera centered at 4700 Å, ρ/γ = 0.36±0.1 g cm−3 (γ is the quantum yield of condensate), and values of rc varying from 0.1±0.02 μm at 30 km to 0.15 ± 0.03 μm near the surface. Other auxiliary properties of the haze are also determined. The value found for ρ/γ corresponds to a packing coefficient of 0.6γ if C2H4 is the main condensible species.

CCD spectroscopic observations of Saturn, Uranus, Neptune, and Titan during the 1990 apparitions

High signal-to-noise CCD long-slit spectra of Saturn, Uranus, Neptune, and Titan have been recorded at the coudé focus of the 1.52-m telescope at Calar Alto Observatory (Spain) on June 1990. The spectra were obtained in different spectral regions in the wavelength range 4900–9300 Å. The nominal resolution was about 1.2 Å pixel −1. The spatial resolution was 1.2″, which allowed us to obtain spatially resolved spectra on different regions across Saturn's disk. Our model atmospheres for Saturn are consistent with an enrichment of carbon of a factor of two with respect to the solar abundance. The derived haze optical thickness is 13±2, in accordance with previous measurements carried out during the 1986 apparition. The spectra of Uranus, Neptune, and Titan compare well with previous absolute spectrophotometry of these objects by Neff et al. (1984, Icarus 60, 221–235).