Scattering properties of sands. 2. Results for sands from different origins.

Mineral sand is a major component of aerosols in the atmosphere. It is necessary to have a laboratory database to interpret the remote sensing measurements of light scattered by such grains. For this purpose, the PROGRA2 experiment is dedicated to the retrieval of polarization and brightness phase curves, in the visible wavelength domain, of various grains that can be found in Earth's atmosphere and in space. The measurements of the scattered light by levitating clouds of grains are conducted at two wavelengths, 632.8 and 543.5nm, with PROGRA2-VIS. Large grains (at least tens of micrometers) are studied in microgravity conditions during parabolic flights; smaller (micrometer-sized) grains are lifted by an air draught in ground-based conditions. The PROGRA2-SURF instrument allows measurements on the grains deposited on a plane surface, at the same wavelengths. New data for the scattering properties are presented for sands of various origins, including fine clay. The polarimetric phase curves for levitating grains are close to each other for all the samples (except for black sands); small discrepancies are mainly due to grains' light absorption differences. The polarization curves for levitating grains differ strongly from those of deposited grains (dry or wet). In particular, these curves can be used to interpret remote sensing measurements to distinguish between grains at ground and grains transported by winds.

Scattering properties of sands. 1. Comparison between different techniques of measurements.

Measuring linear polarization of light scattered by a cloud of particles can help retrieve their physical properties. We present an extensive study of polarimetric measurements of sand grains that can be found on the surface and in the atmosphere of the Earth. Different techniques of measurements are compared using the Laboratoire de Météorologie Physique nephelometer on the ground and the Propriétés Optiques des Grains Astronomiques et Atmosphériques on the ground and in microgravity during parabolic flights. The techniques used on the ground bias the measurements. When the grains are lifted by an air draft, differentiation is produced in the size distribution and the nature of the floating particles. When the grains are carried along with the airflow, some grains become oriented along the flow direction at air speeds greater than a few meters per second, producing abnormal negative polarization. On the other hand, measurements conducted under microgravity permit the retrieval of the representative optical properties of the lifted sand grains with sizes greater than tens of micrometers.

Optical and physical properties of stratospheric aerosols from balloon measurements in the visible and near-infrared domains. III. Presence of aerosols in the middle stratosphere.

The aerosol extinction measurements in the ultraviolet and visible wavelengths by the balloonborne spectrometer Spectroscopie d'Absorption Lunaire pour l'Observation des Minoritaires Ozone et NOx (SALOMON) show that aerosols are present in the middle stratosphere, above 25-km altitude. These observations are confirmed by the extinction measurements performed by a solar occultation radiometer. The balloonborne Laboratoire de Météorologie Dynamique (LMD) counter instrument also confirms the presence of aerosol around 30-km altitude, with an unrealistic excess of micronic particles assuming that only liquid sulfate aerosols are present. An unexpected spectral structure around 640-nm observed by SALOMON is also detectable in extinction measurements by the satellite instrument Stratospheric Aerosols and Gas Experiment III. This set of measurements could indicate that solid aerosols were detected at these altitude ranges. The amount of soot detected up to now in the lower stratosphere is too low to explain these measurements. Thus, the presence of interplanetary dust grains and micrometeorites may need to be invoked. Moreover, it seems that these grains fill the stratosphere in stratified layers.

Optical properties of randomly distributed soot: improved polarimetric and intensity scattering functions.

Reference scattering curves for polarization and intensity produced by aggregates and agglomerates of ethylene and kerosene soot are obtained for scattering angles in the 10-170 degrees range. The polarization measurements were obtained with the Propriétés Optiques des Grains Astronomiques et Atmosphériques instrument for particles that levitate in microgravity during parabolic flights and on the ground by an air draught technique. The intensity measurements were obtained also on the ground with a Laboratoire de Metéorologie Physique nephelometer. The maximum polarization is of the order of 80% at a scattering angle of 80 degrees at lambda = 632.8 nm and approximately 75% at an angle of 90 degrees at lambda = 543.5 nm. The polarization increases by approximately 10% when the size of the agglomerate increases from 10 microm to a few hundred micrometers. The intensity curve exhibits a strong increase at small scattering angles. These reference curves will be used in the near future for the detection of stratospheric soot by remote-sensing measurement techniques.

Light scattering by dust particles in microgravity: polarization and brightness imaging with the new version of the PROGRA2 instrument.

A new version of the PROGRA2 instrument, dedicated to measuring the polarization phase function of various kinds of solid particles, allows obtaining maps of polarization and brightness with a spatial resolution of a few tens of micrometers. The measurements are conducted in microgravity during parabolic flights to ensure random distribution and orientation of the particles. The results of the first two sessions are presented. Comparison between measurements and Mie theory modeling for glass spheres shows that the instrument works well and that accurate results can be obtained even at small phase angles. Results for irregularly shaped particles are also presented.