Scattering properties of sands 2 Results for sandsfrom different origins

Jean-Baptiste Renard1 Mirvatte Francis1 Edith Hadamcik2 Daniel Daugeron1

Benoicirct Couteacute1 Bertrand Gaubicher1 and Matthieu Jeannot1

1Laboratoire de Physique et Chimie de lrsquoEnvironnement et de lrsquoEspaceUniversiteacute drsquoOrleacuteans3A Avenue de la Recherche Scientifique F-45071 Orleacuteans cedex 2 France

2LATMOS 11 Boulevard drsquoAlembert F-78280 Guyancourt France

Corresponding author jbrenardcnrs‑orleansfr

Received 3 May 2010 revised 27 May 2010 accepted 1 June 2010posted 2 June 2010 (Doc ID 127844) published 16 June 2010

Mineral sand is a major component of aerosols in the atmosphere It is necessary to have a laboratorydatabase to interpret the remote sensing measurements of light scattered by such grains For thispurpose the PROGRA2 experiment is dedicated to the retrieval of polarization and brightness phasecurves in the visible wavelength domain of various grains that can be found in Earthrsquos atmosphereand in space The measurements of the scattered light by levitating clouds of grains are conductedat two wavelengths 6328 and 5435nm with PROGRA2-VIS Large grains (at least tens of micrometers)are studied in microgravity conditions during parabolic flights smaller (micrometer-sized) grains arelifted by an air draught in ground-based conditions The PROGRA2-SURF instrument allows measure-ments on the grains deposited on a plane surface at the same wavelengths New data for the scatteringproperties are presented for sands of various origins including fine clay The polarimetric phase curvesfor levitating grains are close to each other for all the samples (except for black sands) small discrepan-cies are mainly due to grainsrsquo light absorption differences The polarization curves for levitating grainsdiffer strongly from those of deposited grains (dry or wet) In particular these curves can be used tointerpret remote sensing measurements to distinguish between grains at ground and grains transportedby winds copy 2010 Optical Society of AmericaOCIS codes 0101100 1205410 1205820

1 Introduction

Mineral dust is a major component of Earth atmo-sphere aerosols Saharan sand seems to be the mainsource [12] and can be transported over large dis-tances [3] Remote sensing measurements of lightscattered by such mineral dust in the visible domaincan help to estimate their physical properties Thesemeasurements can be conducted from satellites [45]balloonborne instruments [6] and at ground [7]Although this can be done by remote sensing spectralreflectance [8] linear polarization could be a comple-mentary tool Polarization could help to distinguish

between deposited grains and transported materialsby winds in levitation in the air

In the previous paper [9] we started studies on thescattering properties of various sands This work al-lows us to enlarge the size of the database built withthe PROGRA2 experiment on numerous samplesmade of solid particulates that can be found inplanetary atmospheres comets and interplanetarymedium or deposited on surfaces such as lunar orasteroidal regoliths (for more information see the re-view byHadamcik et al [10] and the PROGRA2web-site at httpwwwicareuniv‑lille1frprogra2) It isdifficult from remote polarization observations to re-solve the ldquoinverse problemrdquo and such databases canhelp retrieve some physical properties of irregulargrains (size and size distribution structure albedo)

0003-693510183552-08$15000copy 2010 Optical Society of America

3552 APPLIED OPTICS Vol 49 No 18 20 June 2010

from the spatial variations and time evolution oftheir optical properties In fact scattering modelscan be used to retrieve the properties of regular-shaped particles [1112] but the theoretical calcula-tion for irregular-shaped particles are intricate andthe models are unable to perform when the shapesare unknown [13] In particular Muntildeoz et al com-pared scattering measurements of large Saharandust grains to various modeling calculations andnone of them can reproduce the measurements [14]This problem could be due to the rough surface of thegrains

Polarization phase curves depend on the size thestructure and the nature of the grains (refractive in-dex) Because of multiple scattering between the par-ticles polarimetric properties of particles depositedon a surface strongly differ from those of the samelifted particles [15ndash19] To well document the scatter-ing properties of various sands in different geophysi-cal conditions measurements must be conducted forboth deposited and levitating grains Lifting thegrains is not an easy task so different techniques canbe proposed The grains are carried along airflow andinjected in the field of view of the instrument [20ndash23]raised by an air draught [24] or observed under mi-crogravity conditions [25] Large grains with dia-meter greater than about 20 μm can be orientedwhen they are ejected from the pipe that carries theair flow at a speed greater than 2m=s so micrograv-ity presents the best conditions to obtain randomlyoriented large grains [9]

We use here the same concept of instruments per-forming polarimetric measurements The studiesconcern grains with diameters close to a few micro-meters as those that can be found in sand plume[26] to tens of micrometers such as those found atground but also on collection surfaces on NASA air-craft ER-2 and WB-57F in the lower stratosphere[27] The technique of measurements with PRO-GRA2 instruments is first described then the indi-vidual scattering curves for the sand samples arepresented and finally we propose a synthesis of themain results

2 Technique of Measurements

PROGRA2 is a French acronym (PRoprieacuteteacutes Op-tiques des GRains Astronomiques et Atmospheacuteri-ques) for ldquoOptical properties of astronomical andatmospheric grainsrdquo The two instruments used inthe visible domain are

ndash PROGRA2-VIS for measurements of levitatedgrains with an air-draught technique for grains witha diameter smaller than a few tens of micrometersand on microgravity conditions for larger grains and

ndash PROGRA2-SURF for measurements of grainsdeposited on surface The grains deposited on a cupare packed with a flat surface producing high pack-ing density and a flatter surface

A detailed description of the instruments and ofthe data reduction can be found in Renard et al[28] and Hadamcik et al [10] Recently an improve-ment has been done on PROGRA2-SURFrsquos detectorsby changing the photodiodes to cameras In sum-mary the light sources are randomly polarized lasersat 6328 (red) and 5435nm (green) The laser beamsection is about 2mm in diameter Polarization mea-surements are performed using a beam splitter cubeand cover the 10degndash170deg scattering angle range Theperpendicular component of scattered light to thescattering plane (I1) and the parallel one (I2) are re-corded by two CCD cameras providing 25 images persecond with a resolution of about 10 μm per pixel

The scattered flux changes from one image to an-other with PROGRA2-VIS because of varying num-bers of grains in the field of view Thus a thirdcamera is used at a constant scattering angle of90deg (I3) to normalize the scattered flux On the otherhand the illuminated grains are all in the field ofview of the PROGRA2-SURF cameras and the totalincident brightness is constant Thus there is noneed to use the third camera

The degree of linear polarization P and the bright-ness B of the scattered light can be written as

P frac14 ethI1 minus I2THORN=ethI1thorn I2THORN eth1THORN

B frac14 I1thorn I2 eth2THORN

For lifted grains with PROGRA2-VIS the brightnessin arbitrary unit is normalized by I3

B frac14 ethI1thorn I2THORN=I3 eth3THORNFor geometric reasons the retrieval of the brightnesswith Eq (3) is difficult for levitating grains at scat-tering angles below 20deg and greater than 160deg (insuch angle regions the field of view of the I1 and I2cameras differs strongly from the field of view of theI3 camera so the normalization procedure becomesinaccurate)

PROGRA2-VIS and PROGRA2-SURF performphotometric measurements in relative units thusno absolute calibration is needed The following mustbe verified

1 The optical alignment must be checked to havethe same field of view and the same spatial resolu-tion on the two cameras

2 The beam splitter must work well The test isconducted at a scattering angle of 0deg using a polari-zation filter in front of the beam splitter Total extinc-tion must be obtained for one polarized componentcamera Then total extinction must be obtainedfor the second camera after rotating the filter by 90deg

3 The same flux is recorded by the cameras inthe case of a random-polarized light source This isdone by injecting the laser light through the beamsplitter at a scattering angle of 0deg The linearity ofthe camera is verified using neutral filters withdifferent attenuations

20 June 2010 Vol 49 No 18 APPLIED OPTICS 3553

4 The experimental scattering curves must con-verge toward 0 polarization values for scatteringangles decreasing to 0deg and scattering angles in-creasing to 180deg

For PROGRA2-SURF measurements on a black mir-ror are compared to the Fresnel curves The dis-crepancy corresponding to the error bars is about1 in absolute

The imaging system technique allows us to simpli-fy the optical alignment to easily correct the straylight contamination on each channel if necessaryand to sort out the polarization values per grain sizeclasses PROGRA2-VIS allows us to reject the imagescontaining typically more than 30 grains producingimportant multiple-scattering effects Results fromtens of images must be averaged in order to retrieverepresentative polarization values These errors areabout 1 (in absolute value) for polarization and 10(in relative values) for the brightness

With PROGRA2-SURF the cup that contains thesample is turned between each set of measurementsof a complete scattering curve in order to averagesurface irregularities Typically tens of series of mea-

surements are necessary to produce the final scatter-ing curve Nevertheless the individual polarizationvalues obtained for each set of measurements aremore scattered for grains having a diameter greaterthan 200 μm than for smaller grains This effect couldbe due to the surface roughness with respect to theincident light but this needs further investigation

3 Scattering Curves for Sands

A Studied Samples

In the previous paper [9] we limited our studies tothree kinds of sand from China Niger and Tunisiadeserts All the samples had more or less the sameocher color although their size distributions differsignificantly Here we extend the study to differentkinds of sand having different colors different sizedistributions and different origins (desert or coast)Table 1 gives the different samples analyzed hereImages recorded by PROGRA2-VIS allow the deter-mination of grainsrsquo size with diameter greater than50 μm The size of the fine clay is given by the

Table 1 Description of the Various Samples of Sand

Name of the Samples on the Figures Origin Size (μm) Color

Broken sand Morocco desert (Sahara) lt5 Pale ocherChina China desert (Gobi) 125 75 Dark ocherFiji Fiji coast 250 150 Pale beigeFine clay Tunisia (Douz) lt5 Dark ocherFrance Atlantic coast (Pyla dune) 300 100 Pale yellowHawaii Hawaii coast 200 150 BlackLebanon (heterogeneous sample) Mediterranean coast lt1mm GrayndashbeigeLibya Libya desert (Sahara) 150 100 OcherndashorangeMorocco Morocco desert (Sahara) 150 100 OcherndashorangeNiger Niger desert (Sahel) 250 100 OcherndashreddishTunisia Tunisia desert (Sahara) 150 50 Ocher

Fig 1 Electron-microscope image of Niger sand grains

Fig 2 (Color online) Maps from PROGRA2-VIS images for theChina sand at a scattering angle of 120deg The field of view is7mm in length Top brightness map (linear scale) bottom polar-ization map (dark gray corresponds to minus20 and white corre-sponds to 100)

3554 APPLIED OPTICS Vol 49 No 18 20 June 2010

provider [29] and the size of the broken Moroccosand is estimated from optical microscope observa-tions The fine clay is representative of the smallgrains that can be levitated by winds which cancross oceans and seas and are deposited later atground Also broken sand from Morocco was pro-duced by us to be compared to the natural fine clay

As an example an electron-microscope image ofNiger sand is presented in Fig 1 For the same sam-ple brightness and polarization maps are presentedin Fig 2 It can be noticed that some ldquosmallrdquo grainsare present in the images Since small grains scattera small amount of light in comparison with light scat-tered by the largest grains we have verified thattheir contribution is negligible in the recorded fluxThus the size distribution given in Table 1 refersto the grains that contribute to the recorded flux

Irregular grains having a diameter larger than thewavelength of measurements are expected to pro-duce smooth polarization curves [1014] more or lesshaving a ldquobell shaperdquo The residual scatters of thedata on some curves are relevant of the measure-ment uncertainties On the following we will focuson some parameters of such curves the maximum

value of polarization its corresponding scatteringangle the presence or not of negative polarizationat large scattering angle and the value of the inver-sion angle (where the polarization curve crosses theldquozerordquo value)

B Deposited Grains

Figures 3 and 4 present the polarization curves atλ frac14 6328nm and at λ frac14 5435nm respectively forthe deposited samples Except at large scattering an-gles the polarization curve of fine clay is well belowthe curves of large grains

All the curves for the large grains exhibit the samebehavior as follows

ndash There is no obvious color effect at least takinginto account the PROGRA2-VIS measurementsuncertainties

ndash Negative values for scattering angle greaterthan 140deg are present The inversion angle is be-tween 130deg and 150deg Because of the error bars itis not possible to retrieve accurately the inversionangle value for the different samples

Fig 3 Polarization curves for the various deposited samplesmeasured at λ frac14 6328nm

Fig 4 Polarization curves for the various deposited samplesmeasured at λ frac14 5435nm

Fig 5 Brightness curves for the various deposited samplesmeasured at λ frac14 6328nm

Fig 6 Brightness curves for the deposited samples measured atλ frac14 5435nm

20 June 2010 Vol 49 No 18 APPLIED OPTICS 3555

ndash The maximum of polarization occurs for a scat-tering angle of around 30degndash40deg The differences forthe polarization values are partly due to differencesin light absorption by the materials

Figures 5 and 6 present the brightness curves atλ frac14 6328nm and at λ frac14 5435nm respectively forthe deposited samples The absolute values in rela-tive units can change from one sample to anotherbecause of the individual scattering properties ofthe sands as for polarization Nevertheless all thecurves are similar with a large brightness enhance-ment toward small scattering angles and smallerbrightness enhancement toward large scattering an-gles except for the Fiji sand for which the backwardscattering is more pronounced

It could be interesting to evaluate the effect of hu-midity on the grainsrsquo scattering properties Figures 7and 8 present the polarization and brightness curvesfor different kinds of wet sand The amount of addedwater from one sample to another can change so thecurves must be taken into account cautiously Never-

theless some tendencies are found when comparedto dry grains

ndash the maximum polarization increasesndash the value of the scattering angle at maximum

polarization increases slightly andndash the negative polarization branch disappears

The brightness curves are almost flat at large scat-tering angles

C Levitating Grains

As shown in the previous paper [9] the bestmethod toobtain random orientation of levitating grains largerthan a few tens of micrometers is to perform themeasurements under microgravity conditions Mea-surementswithPROGR2-VIS for such grains are con-ducted during parabolic flights onboard the dedicatedaircraft A300 ZeroGmanaged by theNovespace com-pany during campaigns funded by the French SpaceAgency (CNES) and the European Space Agency(ESA) The samples are put in a vial under vacuum

For the smaller grains here fine clay and brokensand the microgravity conditions are not mandatory

Fig 7 Polarization curves for the wet deposited samplesmeasured at λ frac14 6328nm

Fig 8 Brightness curves for the various wet deposited samplesmeasured at λ frac14 6328nm

Fig 9 Polarization curves for large grains levitating in micro-gravity conditions measured at λ frac14 6328nm

Fig 10 Polarization curves for large grains levitating in micro-gravity conditions measured at λ frac14 5435nm

3556 APPLIED OPTICS Vol 49 No 18 20 June 2010

and the measurements are conducted at groundusing an air-draught technique a small amount ofnitrogen is injected into a vial containing the sampleallowing the levitation of the grains for several sec-onds at least

On the polarization curves for the large grains(Fig 9 for λ frac14 6328nm and Fig 10 for λ frac145435nm) the higher polarization (Psim 75) is ob-tained with black grains This is in agreement withthe effect in which black large grains produce higherpolarization values than transparent large grains(see eg Ref [10]) The other sands produce more orless similar polarization values with maxima in the5ndash15 range depending on the grainsrsquo size andlight absorption by the samples No obvious wave-length effect is observed although the small discre-pancies between the two wavelengths could be due tothe difference in the grainsrsquo absorption Finally thereare no obvious negative values for polarization

The polarization curves for the fine grains (clayand broken sand) presented in Fig 11 for the reddomain are close to those of large grains On theother hand the fine clay presents a small color effect

with higher values in the green domain (presented inSection 4 in the synthesis of the results in Fig 14)

The brightness curves for the two kinds of finegrains are presented in Fig 12 and are similar Theyare typical of brightness curves of mineral aerosolswith a strong increase toward small scatteringangles

4 Discussion

The new polarization and brightness values obtainedare compared to those already done in previous stu-dies for Sahara sands and red clay For grains in lev-itation PROGRA2 polarization values are close tothose obtained with the experiments in which thegrains are carried in air jet stream [1422] Neverthe-less a careful analysis shows that our values areslightly higher than those already published andavailable in the Amsterdam database at httpwwwastrouvanlscatter This could be due to smalldifferences in the absorption properties of the grainsbut also because our method of measurements favorsthe contribution of large grains while the air jet

Fig 11 Polarization curves for small grains lifted by air draughtmeasured at λ frac14 6328nm

Fig 12 Brightness curves for small grains levitating by airdraught measured at λ frac14 6328nm

Fig 13 Synthetic curves for deposited and levitating large sandgrains (excluding black sand) The error bars correspond to the dis-persion of the individual curves

Fig 14 Comparison between polarization curves for depositedand levitating fine clay

20 June 2010 Vol 49 No 18 APPLIED OPTICS 3557

4 The experimental scattering curves must con-verge toward 0 polarization values for scatteringangles decreasing to 0deg and scattering angles in-creasing to 180deg

For PROGRA2-SURF measurements on a black mir-ror are compared to the Fresnel curves The dis-crepancy corresponding to the error bars is about1 in absolute

The imaging system technique allows us to simpli-fy the optical alignment to easily correct the straylight contamination on each channel if necessaryand to sort out the polarization values per grain sizeclasses PROGRA2-VIS allows us to reject the imagescontaining typically more than 30 grains producingimportant multiple-scattering effects Results fromtens of images must be averaged in order to retrieverepresentative polarization values These errors areabout 1 (in absolute value) for polarization and 10(in relative values) for the brightness

With PROGRA2-SURF the cup that contains thesample is turned between each set of measurementsof a complete scattering curve in order to averagesurface irregularities Typically tens of series of mea-

surements are necessary to produce the final scatter-ing curve Nevertheless the individual polarizationvalues obtained for each set of measurements aremore scattered for grains having a diameter greaterthan 200 μm than for smaller grains This effect couldbe due to the surface roughness with respect to theincident light but this needs further investigation

3 Scattering Curves for Sands

A Studied Samples

In the previous paper [9] we limited our studies tothree kinds of sand from China Niger and Tunisiadeserts All the samples had more or less the sameocher color although their size distributions differsignificantly Here we extend the study to differentkinds of sand having different colors different sizedistributions and different origins (desert or coast)Table 1 gives the different samples analyzed hereImages recorded by PROGRA2-VIS allow the deter-mination of grainsrsquo size with diameter greater than50 μm The size of the fine clay is given by the

Table 1 Description of the Various Samples of Sand

Name of the Samples on the Figures Origin Size (μm) Color

Broken sand Morocco desert (Sahara) lt5 Pale ocherChina China desert (Gobi) 125 75 Dark ocherFiji Fiji coast 250 150 Pale beigeFine clay Tunisia (Douz) lt5 Dark ocherFrance Atlantic coast (Pyla dune) 300 100 Pale yellowHawaii Hawaii coast 200 150 BlackLebanon (heterogeneous sample) Mediterranean coast lt1mm GrayndashbeigeLibya Libya desert (Sahara) 150 100 OcherndashorangeMorocco Morocco desert (Sahara) 150 100 OcherndashorangeNiger Niger desert (Sahel) 250 100 OcherndashreddishTunisia Tunisia desert (Sahara) 150 50 Ocher

Fig 1 Electron-microscope image of Niger sand grains

Fig 2 (Color online) Maps from PROGRA2-VIS images for theChina sand at a scattering angle of 120deg The field of view is7mm in length Top brightness map (linear scale) bottom polar-ization map (dark gray corresponds to minus20 and white corre-sponds to 100)

3554 APPLIED OPTICS Vol 49 No 18 20 June 2010

provider [29] and the size of the broken Moroccosand is estimated from optical microscope observa-tions The fine clay is representative of the smallgrains that can be levitated by winds which cancross oceans and seas and are deposited later atground Also broken sand from Morocco was pro-duced by us to be compared to the natural fine clay

As an example an electron-microscope image ofNiger sand is presented in Fig 1 For the same sam-ple brightness and polarization maps are presentedin Fig 2 It can be noticed that some ldquosmallrdquo grainsare present in the images Since small grains scattera small amount of light in comparison with light scat-tered by the largest grains we have verified thattheir contribution is negligible in the recorded fluxThus the size distribution given in Table 1 refersto the grains that contribute to the recorded flux

Irregular grains having a diameter larger than thewavelength of measurements are expected to pro-duce smooth polarization curves [1014] more or lesshaving a ldquobell shaperdquo The residual scatters of thedata on some curves are relevant of the measure-ment uncertainties On the following we will focuson some parameters of such curves the maximum

value of polarization its corresponding scatteringangle the presence or not of negative polarizationat large scattering angle and the value of the inver-sion angle (where the polarization curve crosses theldquozerordquo value)

B Deposited Grains

Figures 3 and 4 present the polarization curves atλ frac14 6328nm and at λ frac14 5435nm respectively forthe deposited samples Except at large scattering an-gles the polarization curve of fine clay is well belowthe curves of large grains

All the curves for the large grains exhibit the samebehavior as follows

ndash There is no obvious color effect at least takinginto account the PROGRA2-VIS measurementsuncertainties

ndash Negative values for scattering angle greaterthan 140deg are present The inversion angle is be-tween 130deg and 150deg Because of the error bars itis not possible to retrieve accurately the inversionangle value for the different samples

Fig 3 Polarization curves for the various deposited samplesmeasured at λ frac14 6328nm

Fig 4 Polarization curves for the various deposited samplesmeasured at λ frac14 5435nm

Fig 5 Brightness curves for the various deposited samplesmeasured at λ frac14 6328nm

Fig 6 Brightness curves for the deposited samples measured atλ frac14 5435nm

20 June 2010 Vol 49 No 18 APPLIED OPTICS 3555

ndash The maximum of polarization occurs for a scat-tering angle of around 30degndash40deg The differences forthe polarization values are partly due to differencesin light absorption by the materials

Figures 5 and 6 present the brightness curves atλ frac14 6328nm and at λ frac14 5435nm respectively forthe deposited samples The absolute values in rela-tive units can change from one sample to anotherbecause of the individual scattering properties ofthe sands as for polarization Nevertheless all thecurves are similar with a large brightness enhance-ment toward small scattering angles and smallerbrightness enhancement toward large scattering an-gles except for the Fiji sand for which the backwardscattering is more pronounced

It could be interesting to evaluate the effect of hu-midity on the grainsrsquo scattering properties Figures 7and 8 present the polarization and brightness curvesfor different kinds of wet sand The amount of addedwater from one sample to another can change so thecurves must be taken into account cautiously Never-

theless some tendencies are found when comparedto dry grains

ndash the maximum polarization increasesndash the value of the scattering angle at maximum

polarization increases slightly andndash the negative polarization branch disappears

The brightness curves are almost flat at large scat-tering angles

C Levitating Grains

As shown in the previous paper [9] the bestmethod toobtain random orientation of levitating grains largerthan a few tens of micrometers is to perform themeasurements under microgravity conditions Mea-surementswithPROGR2-VIS for such grains are con-ducted during parabolic flights onboard the dedicatedaircraft A300 ZeroGmanaged by theNovespace com-pany during campaigns funded by the French SpaceAgency (CNES) and the European Space Agency(ESA) The samples are put in a vial under vacuum

For the smaller grains here fine clay and brokensand the microgravity conditions are not mandatory

Fig 7 Polarization curves for the wet deposited samplesmeasured at λ frac14 6328nm

Fig 8 Brightness curves for the various wet deposited samplesmeasured at λ frac14 6328nm

Fig 9 Polarization curves for large grains levitating in micro-gravity conditions measured at λ frac14 6328nm

Fig 10 Polarization curves for large grains levitating in micro-gravity conditions measured at λ frac14 5435nm

3556 APPLIED OPTICS Vol 49 No 18 20 June 2010

and the measurements are conducted at groundusing an air-draught technique a small amount ofnitrogen is injected into a vial containing the sampleallowing the levitation of the grains for several sec-onds at least

On the polarization curves for the large grains(Fig 9 for λ frac14 6328nm and Fig 10 for λ frac145435nm) the higher polarization (Psim 75) is ob-tained with black grains This is in agreement withthe effect in which black large grains produce higherpolarization values than transparent large grains(see eg Ref [10]) The other sands produce more orless similar polarization values with maxima in the5ndash15 range depending on the grainsrsquo size andlight absorption by the samples No obvious wave-length effect is observed although the small discre-pancies between the two wavelengths could be due tothe difference in the grainsrsquo absorption Finally thereare no obvious negative values for polarization

The polarization curves for the fine grains (clayand broken sand) presented in Fig 11 for the reddomain are close to those of large grains On theother hand the fine clay presents a small color effect

with higher values in the green domain (presented inSection 4 in the synthesis of the results in Fig 14)

The brightness curves for the two kinds of finegrains are presented in Fig 12 and are similar Theyare typical of brightness curves of mineral aerosolswith a strong increase toward small scatteringangles

4 Discussion

The new polarization and brightness values obtainedare compared to those already done in previous stu-dies for Sahara sands and red clay For grains in lev-itation PROGRA2 polarization values are close tothose obtained with the experiments in which thegrains are carried in air jet stream [1422] Neverthe-less a careful analysis shows that our values areslightly higher than those already published andavailable in the Amsterdam database at httpwwwastrouvanlscatter This could be due to smalldifferences in the absorption properties of the grainsbut also because our method of measurements favorsthe contribution of large grains while the air jet

Fig 11 Polarization curves for small grains lifted by air draughtmeasured at λ frac14 6328nm

Fig 12 Brightness curves for small grains levitating by airdraught measured at λ frac14 6328nm

Fig 13 Synthetic curves for deposited and levitating large sandgrains (excluding black sand) The error bars correspond to the dis-persion of the individual curves

Fig 14 Comparison between polarization curves for depositedand levitating fine clay

20 June 2010 Vol 49 No 18 APPLIED OPTICS 3557

provider [29] and the size of the broken Moroccosand is estimated from optical microscope observa-tions The fine clay is representative of the smallgrains that can be levitated by winds which cancross oceans and seas and are deposited later atground Also broken sand from Morocco was pro-duced by us to be compared to the natural fine clay

As an example an electron-microscope image ofNiger sand is presented in Fig 1 For the same sam-ple brightness and polarization maps are presentedin Fig 2 It can be noticed that some ldquosmallrdquo grainsare present in the images Since small grains scattera small amount of light in comparison with light scat-tered by the largest grains we have verified thattheir contribution is negligible in the recorded fluxThus the size distribution given in Table 1 refersto the grains that contribute to the recorded flux

Irregular grains having a diameter larger than thewavelength of measurements are expected to pro-duce smooth polarization curves [1014] more or lesshaving a ldquobell shaperdquo The residual scatters of thedata on some curves are relevant of the measure-ment uncertainties On the following we will focuson some parameters of such curves the maximum

value of polarization its corresponding scatteringangle the presence or not of negative polarizationat large scattering angle and the value of the inver-sion angle (where the polarization curve crosses theldquozerordquo value)

B Deposited Grains

Figures 3 and 4 present the polarization curves atλ frac14 6328nm and at λ frac14 5435nm respectively forthe deposited samples Except at large scattering an-gles the polarization curve of fine clay is well belowthe curves of large grains

All the curves for the large grains exhibit the samebehavior as follows

ndash There is no obvious color effect at least takinginto account the PROGRA2-VIS measurementsuncertainties

ndash Negative values for scattering angle greaterthan 140deg are present The inversion angle is be-tween 130deg and 150deg Because of the error bars itis not possible to retrieve accurately the inversionangle value for the different samples

Fig 3 Polarization curves for the various deposited samplesmeasured at λ frac14 6328nm

Fig 4 Polarization curves for the various deposited samplesmeasured at λ frac14 5435nm

Fig 5 Brightness curves for the various deposited samplesmeasured at λ frac14 6328nm

Fig 6 Brightness curves for the deposited samples measured atλ frac14 5435nm

20 June 2010 Vol 49 No 18 APPLIED OPTICS 3555

ndash The maximum of polarization occurs for a scat-tering angle of around 30degndash40deg The differences forthe polarization values are partly due to differencesin light absorption by the materials

Figures 5 and 6 present the brightness curves atλ frac14 6328nm and at λ frac14 5435nm respectively forthe deposited samples The absolute values in rela-tive units can change from one sample to anotherbecause of the individual scattering properties ofthe sands as for polarization Nevertheless all thecurves are similar with a large brightness enhance-ment toward small scattering angles and smallerbrightness enhancement toward large scattering an-gles except for the Fiji sand for which the backwardscattering is more pronounced

It could be interesting to evaluate the effect of hu-midity on the grainsrsquo scattering properties Figures 7and 8 present the polarization and brightness curvesfor different kinds of wet sand The amount of addedwater from one sample to another can change so thecurves must be taken into account cautiously Never-

theless some tendencies are found when comparedto dry grains

ndash the maximum polarization increasesndash the value of the scattering angle at maximum

polarization increases slightly andndash the negative polarization branch disappears

The brightness curves are almost flat at large scat-tering angles

C Levitating Grains

As shown in the previous paper [9] the bestmethod toobtain random orientation of levitating grains largerthan a few tens of micrometers is to perform themeasurements under microgravity conditions Mea-surementswithPROGR2-VIS for such grains are con-ducted during parabolic flights onboard the dedicatedaircraft A300 ZeroGmanaged by theNovespace com-pany during campaigns funded by the French SpaceAgency (CNES) and the European Space Agency(ESA) The samples are put in a vial under vacuum

For the smaller grains here fine clay and brokensand the microgravity conditions are not mandatory

Fig 7 Polarization curves for the wet deposited samplesmeasured at λ frac14 6328nm

Fig 8 Brightness curves for the various wet deposited samplesmeasured at λ frac14 6328nm

Fig 9 Polarization curves for large grains levitating in micro-gravity conditions measured at λ frac14 6328nm

Fig 10 Polarization curves for large grains levitating in micro-gravity conditions measured at λ frac14 5435nm

3556 APPLIED OPTICS Vol 49 No 18 20 June 2010

and the measurements are conducted at groundusing an air-draught technique a small amount ofnitrogen is injected into a vial containing the sampleallowing the levitation of the grains for several sec-onds at least

On the polarization curves for the large grains(Fig 9 for λ frac14 6328nm and Fig 10 for λ frac145435nm) the higher polarization (Psim 75) is ob-tained with black grains This is in agreement withthe effect in which black large grains produce higherpolarization values than transparent large grains(see eg Ref [10]) The other sands produce more orless similar polarization values with maxima in the5ndash15 range depending on the grainsrsquo size andlight absorption by the samples No obvious wave-length effect is observed although the small discre-pancies between the two wavelengths could be due tothe difference in the grainsrsquo absorption Finally thereare no obvious negative values for polarization

The polarization curves for the fine grains (clayand broken sand) presented in Fig 11 for the reddomain are close to those of large grains On theother hand the fine clay presents a small color effect

with higher values in the green domain (presented inSection 4 in the synthesis of the results in Fig 14)

The brightness curves for the two kinds of finegrains are presented in Fig 12 and are similar Theyare typical of brightness curves of mineral aerosolswith a strong increase toward small scatteringangles

4 Discussion

The new polarization and brightness values obtainedare compared to those already done in previous stu-dies for Sahara sands and red clay For grains in lev-itation PROGRA2 polarization values are close tothose obtained with the experiments in which thegrains are carried in air jet stream [1422] Neverthe-less a careful analysis shows that our values areslightly higher than those already published andavailable in the Amsterdam database at httpwwwastrouvanlscatter This could be due to smalldifferences in the absorption properties of the grainsbut also because our method of measurements favorsthe contribution of large grains while the air jet

Fig 11 Polarization curves for small grains lifted by air draughtmeasured at λ frac14 6328nm

Fig 12 Brightness curves for small grains levitating by airdraught measured at λ frac14 6328nm

Fig 13 Synthetic curves for deposited and levitating large sandgrains (excluding black sand) The error bars correspond to the dis-persion of the individual curves

Fig 14 Comparison between polarization curves for depositedand levitating fine clay

20 June 2010 Vol 49 No 18 APPLIED OPTICS 3557

ndash The maximum of polarization occurs for a scat-tering angle of around 30degndash40deg The differences forthe polarization values are partly due to differencesin light absorption by the materials

Figures 5 and 6 present the brightness curves atλ frac14 6328nm and at λ frac14 5435nm respectively forthe deposited samples The absolute values in rela-tive units can change from one sample to anotherbecause of the individual scattering properties ofthe sands as for polarization Nevertheless all thecurves are similar with a large brightness enhance-ment toward small scattering angles and smallerbrightness enhancement toward large scattering an-gles except for the Fiji sand for which the backwardscattering is more pronounced

It could be interesting to evaluate the effect of hu-midity on the grainsrsquo scattering properties Figures 7and 8 present the polarization and brightness curvesfor different kinds of wet sand The amount of addedwater from one sample to another can change so thecurves must be taken into account cautiously Never-

theless some tendencies are found when comparedto dry grains

ndash the maximum polarization increasesndash the value of the scattering angle at maximum

polarization increases slightly andndash the negative polarization branch disappears

The brightness curves are almost flat at large scat-tering angles

C Levitating Grains

As shown in the previous paper [9] the bestmethod toobtain random orientation of levitating grains largerthan a few tens of micrometers is to perform themeasurements under microgravity conditions Mea-surementswithPROGR2-VIS for such grains are con-ducted during parabolic flights onboard the dedicatedaircraft A300 ZeroGmanaged by theNovespace com-pany during campaigns funded by the French SpaceAgency (CNES) and the European Space Agency(ESA) The samples are put in a vial under vacuum

For the smaller grains here fine clay and brokensand the microgravity conditions are not mandatory

Fig 7 Polarization curves for the wet deposited samplesmeasured at λ frac14 6328nm

Fig 8 Brightness curves for the various wet deposited samplesmeasured at λ frac14 6328nm

Fig 9 Polarization curves for large grains levitating in micro-gravity conditions measured at λ frac14 6328nm

Fig 10 Polarization curves for large grains levitating in micro-gravity conditions measured at λ frac14 5435nm

3556 APPLIED OPTICS Vol 49 No 18 20 June 2010

and the measurements are conducted at groundusing an air-draught technique a small amount ofnitrogen is injected into a vial containing the sampleallowing the levitation of the grains for several sec-onds at least

On the polarization curves for the large grains(Fig 9 for λ frac14 6328nm and Fig 10 for λ frac145435nm) the higher polarization (Psim 75) is ob-tained with black grains This is in agreement withthe effect in which black large grains produce higherpolarization values than transparent large grains(see eg Ref [10]) The other sands produce more orless similar polarization values with maxima in the5ndash15 range depending on the grainsrsquo size andlight absorption by the samples No obvious wave-length effect is observed although the small discre-pancies between the two wavelengths could be due tothe difference in the grainsrsquo absorption Finally thereare no obvious negative values for polarization

The polarization curves for the fine grains (clayand broken sand) presented in Fig 11 for the reddomain are close to those of large grains On theother hand the fine clay presents a small color effect

with higher values in the green domain (presented inSection 4 in the synthesis of the results in Fig 14)

The brightness curves for the two kinds of finegrains are presented in Fig 12 and are similar Theyare typical of brightness curves of mineral aerosolswith a strong increase toward small scatteringangles

4 Discussion

The new polarization and brightness values obtainedare compared to those already done in previous stu-dies for Sahara sands and red clay For grains in lev-itation PROGRA2 polarization values are close tothose obtained with the experiments in which thegrains are carried in air jet stream [1422] Neverthe-less a careful analysis shows that our values areslightly higher than those already published andavailable in the Amsterdam database at httpwwwastrouvanlscatter This could be due to smalldifferences in the absorption properties of the grainsbut also because our method of measurements favorsthe contribution of large grains while the air jet

Fig 11 Polarization curves for small grains lifted by air draughtmeasured at λ frac14 6328nm

Fig 12 Brightness curves for small grains levitating by airdraught measured at λ frac14 6328nm

Fig 13 Synthetic curves for deposited and levitating large sandgrains (excluding black sand) The error bars correspond to the dis-persion of the individual curves

Fig 14 Comparison between polarization curves for depositedand levitating fine clay

20 June 2010 Vol 49 No 18 APPLIED OPTICS 3557

and the measurements are conducted at groundusing an air-draught technique a small amount ofnitrogen is injected into a vial containing the sampleallowing the levitation of the grains for several sec-onds at least

On the polarization curves for the large grains(Fig 9 for λ frac14 6328nm and Fig 10 for λ frac145435nm) the higher polarization (Psim 75) is ob-tained with black grains This is in agreement withthe effect in which black large grains produce higherpolarization values than transparent large grains(see eg Ref [10]) The other sands produce more orless similar polarization values with maxima in the5ndash15 range depending on the grainsrsquo size andlight absorption by the samples No obvious wave-length effect is observed although the small discre-pancies between the two wavelengths could be due tothe difference in the grainsrsquo absorption Finally thereare no obvious negative values for polarization

The polarization curves for the fine grains (clayand broken sand) presented in Fig 11 for the reddomain are close to those of large grains On theother hand the fine clay presents a small color effect

with higher values in the green domain (presented inSection 4 in the synthesis of the results in Fig 14)

The brightness curves for the two kinds of finegrains are presented in Fig 12 and are similar Theyare typical of brightness curves of mineral aerosolswith a strong increase toward small scatteringangles

4 Discussion

The new polarization and brightness values obtainedare compared to those already done in previous stu-dies for Sahara sands and red clay For grains in lev-itation PROGRA2 polarization values are close tothose obtained with the experiments in which thegrains are carried in air jet stream [1422] Neverthe-less a careful analysis shows that our values areslightly higher than those already published andavailable in the Amsterdam database at httpwwwastrouvanlscatter This could be due to smalldifferences in the absorption properties of the grainsbut also because our method of measurements favorsthe contribution of large grains while the air jet

Fig 11 Polarization curves for small grains lifted by air draughtmeasured at λ frac14 6328nm

Fig 12 Brightness curves for small grains levitating by airdraught measured at λ frac14 6328nm

Fig 13 Synthetic curves for deposited and levitating large sandgrains (excluding black sand) The error bars correspond to the dis-persion of the individual curves

Fig 14 Comparison between polarization curves for depositedand levitating fine clay

20 June 2010 Vol 49 No 18 APPLIED OPTICS 3557

stream method could favor the study of the smallestgrains (following the results presented in [9]) ThePROGRA2-VIS brightness curves for fine grainsalthough less accurate are in good agreement withthe curves obtained with the jet stream method

For deposited grains our polarization and bright-ness values are in excellent agreement with those al-ready published for red clay at scattering anglesgreater than 120deg [19] The negative polarization va-lues in such angle domain are then confirmed

From all the PROGRA2measurements an averageldquosyntheticrdquo curve for large grains is obtainedexclud-ing only the values obtained for black sand In Fig 13the polarization curves for deposited and levitatinggrains strongly differ at all scattering angles theyalso differ from the curves of the wet grains

The same conclusion can be obtained consideringthe smaller grains representative of those in the low-er atmosphere as presented in Fig 14 for the fineclay polarization curves Also the brightness curvesdiffer strongly as presented in Fig 15 (the curves arenormalized in order to have the same value at a 90degscattering angle)

Polarization and brightness observations could bean additional tool from interpretation of remote sen-sing measurements of aerosols in the lower tropo-sphere and their source at ground Although theycannot be used alone to identify the origin of the sandgrains (except if they are black) they will allow easydetermination if the grains are deposited at groundor are floating in the air For such a purpose the mea-surements must be compared at scattering angles ofaround 30deg and greater than 120deg Also it could bepossible to distinguish between dry and wet depos-ited sands by analyzing the polarization values ataround 50deg where wet grains produce higher polar-ization values

5 Conclusion

Within the framework of the PROGRA2 project newdata are available for the scattering properties ofsands of various origins including broken grains

and fine clay Excluding black sands that can producehigh polarizations values all the polarization curvesfor large grains having a diameter greater than50 μm are similar (with small differences due to thedifferent natures of the grains) with no obvious coloreffect On the other hand a small color effect is foundfor fine clay The polarization curves for levitatinggrains differ strongly from those of deposited grains(dry and wet) These new data are available on thePROGRA2 web site httpwwwica reuniv‑lille1frprogra2

The PROGRA2 database includes now the scatter-ing curves of various natures of solid particulates(sand soot meteoritic material etc) that are ex-pected to be present in the middle atmosphere [30ndash32] As an immediate application this databasewill be a useful tool for the interpretation of theballoonborne radiometer MicroRADIBAL [632] per-forming measurements of scattered light includingpolarization to better determine the various naturesof the nonliquid aerosols in the stratosphere

The PROGRA2 project is mainly funded by theFrench Space Agency (CNES) The maintenance ofthe experiment is funded by a contractwith theEnvir-onnement-SA companyWe thank the Novespace andthe Direction Geacuteneacuterale de lrsquoarmement (DGA) Essaisen Vol teams for the parabolic flights We also thankF Dulac (Laboratoire des Sciences du Climat et delrsquoEnvironnementCommissariat agrave lrsquoEnergie Atomi-que) forproviding the fine clay sampleand the ICAREthematic center

References1 X Li H Maring D Savoie K Voss and J M Prospero ldquoDom-

inance of mineral dust in aerosol light scattering in the NorthAtlantic trade windsrdquo Nature 380 416ndash419 (1996)

2 J M Prospero and P J Lamb ldquoAfrican droughts and dusttransport to the Caribbean climate change implicationsrdquoScience 302 1024ndash1027 (2003)

3 I ChiapelloCMoulin andJMProspero ldquoUnderstanding thelong-term variability of African dust transport as recorded inboth Barbados surface concentrations and large-scale TotalOzone Mapping Spectrometer (TOMS) optical thicknessrdquo JGeophys Res 110 D18S10 (2005)

4 F M Breacuteon J L Deuzeacute D Tanreacute and M Herman ldquoValida-tion of spaceborne estimates of aerosol loading from Sunphotometer measurements with emphasis on polarizationrdquoJ Geophys Res 102 17187ndash17195 (1997)

5 T Kusaka T Ema and N Taniguchi ldquoExtraction of opticalproperties of yellow sand dust from satellite-level data overeast Asiardquo in Proceedings of Geoscience and Remote SensingSymposium IGARSSrsquo98 (IEEE 1998) Vol 2 pp 876ndash878

6 C Brogniez N Huret S Eckermann E D Riviegravere M PirreM Herman J-Y Balois C Verwaerde N Larsen andB Knudsen ldquoPolar stratospheric cloud microphysical proper-ties measured by the microRADIBAL instrument on 25January 2000 above Esrange and modelling interpretationrdquoJ Geophys Res 108 8332 (2003)

7 T Kusaka F Satou and Y Hayato ldquoOptical properties of Kosaaerosols estimated from multispectral polarizationrdquo ProcSPIE 4891 413ndash418 (2003)

8 G S Okin and T H Painter ldquoEffect of grains size on remotelysensed spectral reflectance of sandy desert surfacesrdquo RemoteSens Environ 89 272ndash280 (2004)

Fig 15 Comparison between brightness curves for deposited andlevitating fine clay

3558 APPLIED OPTICS Vol 49 No 18 20 June 2010

9 DDaugeronJ-BRenardBGaubicherBCouteacuteEHadamcikF Gensdarmes G Basso and C Fournier ldquoScattering proper-ties of sands 1 Comparison between different techniques ofmeasurementsrdquo Appl Opt 45 8331ndash8337 (2006)

10 E Hadamcik J-B Renard A-C Levasseur-Regourd andJ-C Worms ldquoLaboratory measurements of the light scatteredby clouds of solid particles by imaging techniquerdquo in LightScattering Review 4 A A Kokhanovsky ed (SpringerPraxis2009) pp 31ndash70

11 B Hapke Theory of Reflectance and Emittance SpectroscopyVol 3 of Topics in Remote Sensing (Cambridge U Press 1993)

12 O Muntildeoz H Volten J W Hovenier Y G ShkuratovW J van der Zande and L B F M Waters ldquoExperimentaland computational study of light scattering by irregular dustparticles with extreme refractive indices hematite and ru-tilerdquo Astron Astrophys 446 525ndash535 (2006)

13 K Muinonen T Nousiainen H Lindqvist O Muntildeoz andG Videen ldquoLight scattering by Gaussian particles with inter-nal inclusions and roughened surfaces using ray opticsrdquo JQuant Spectrosc Radiat Transfer 110 1628ndash1639 (2009)

14 O Muntildeoz H Volten J W Hovenier K Muinonen G GuiradoF Moreno and L B F M Waters ldquoScattering matrix of largeSaharandust particles experiment and computationsrdquo J Geo-phys Res 112 D13215 (2006)

15 J-C Worms J-B Renard E Hadamcik N Brun-Huret andA-C Levasseur-Regourd ldquoLight scattering by dust particleswith the PROGRA2 instrumentmdashcomparative measurementsbetween clouds under microgravity and layers on the groundrdquoPlanet Space Sci 48 493ndash505 (2000)

16 Y G Shkuratov N V Opanasenko and M A Kreslavsky ldquoPo-larimetric and photometric properties of the Moon telescopeobservation and laboratory simulation 1 The negative polar-izationrdquo Icarus 95 283ndash299 (1992)

17 Y G Shkuratov and N V Opanasenko ldquoPolarimetric andphotometric properties of the Moon telescope observationand laboratory simulation 2 The positive polarizationrdquoIcarus 99 468ndash484 (1992)

18 S Bondarenko A Ovcharenko Y G Shkuratov G Videenand G Nelson ldquoParticle size effect on the opposition spikeand negative polarizationrdquo J Quant Spectrosc RadiatTransfer 101 394ndash403 (2006)

19 Y Shkuratov S Bondarenko A Ovcharenko C PietersTHiroiHVoltenOMuňoz andGVideen ldquoComparative stu-dies of the reflectance and degree of linear polarization of par-ticulates surfaces and independently scattering particlesrdquo JQuant Spectrosc Radiat Transfer 100 340ndash358 (2006)

20 F Kuik P Stammes and J W Hovenier ldquoExperimental deter-mination of scattering matrices of water droplets and quartzparticlesrdquo Appl Opt 30 4872ndash4881 (1991)

21 R A West L R Doose A M Eibl M G Tomasko andM I Mishchenko ldquoLaboratory measurements of mineral dustscattering phase function and linear polarizationrdquo J GeophysRes 102 16871ndash16881 (1997)

22 H Volten O Muntildeoz E Rol J F de Haan W VassenJ W Hovenier K Muinonen and T Nousiainen ldquoScatter-ing matrices of mineral aerosol particles at 4416nm and6328nmrdquo J Geophys Res 106 17375ndash17401 (2001)

23 J W Hovenier H Volten O Muntildeoz W J van der Zande andL B F M Waters ldquoLaboratory studies of scattering matricesfor randomly oriented particles potentials problems andperspectivesrdquo J Quant Spectrosc Radiat Transfer 79ndash80741ndash755 (2003)

24 E Hadamcik J-B Renard J-C Worms A-C Levasseur-Regourd and M Masson ldquoPolarization of light scattered byfluffy particles (PROGRA2 experiment)rdquo Icarus 155 497ndash508(2002)

25 J-C Worms J-B Renard E Hadamcik A-C Levasseur-Regourd and J-F Gayet ldquoResults of the PROGRA2 experi-ment an experimental study in microgravity of scattered po-larized light by dust particles with a large size parameterrdquoIcarus 142 281ndash297 (1999)

26 G A drsquoAlmeida P Koepke and E P Shettle AtmosphericAerosolmdashGlobal Climatology and Radiative Characteristics(Deepak 1991)

27 J Lasue Lunar and Planetary Institute 3600 Bay AreaBoulevard Houston Texas 77058 (personal communication)

28 J-B Renard J-C Worms T Lemaire E Hadamcik and NHuret ldquoLight scattering by dust particles in microgravity po-larization and brightness imaging with the new version of thePROGRA2 instrumentrdquo Appl Opt 41 609ndash618 (2002)

29 F Dulac LSCECEA CEA-Orme des Merisiers F-91191Gif-sur-Yvette Cedex France (personal communication)

30 D M Murphy D S Thomson and M J Mahoney ldquoIn situmeasurements of organics meteoritic material mercuryand other elements in aerosols at 5 to 19 kilometersrdquo Science282 1664ndash1669 (1998)

31 J Curtius R Weigel H-J Voumlssing H Wernli A WernerC-M Volk P Konopka M Krebsbach C Schiller A RoigerH Schlager V Dreiling and S Borrmann ldquoObservations ofmeteoric material and implications for aerosol nucleation inthe winter Arctic lower stratosphere derived from in situ par-ticlemeasurementsrdquoAtmosChemPhys5 3053ndash3069 (2005)

32 J-B Renard C Brogniez G Berthet Q BourgeoisB Gaubicher M Chartier J-Y Balois C VerwaerdeF Auriol P Francois D Daugeron and C EngrandldquoVertical distribution of the different types of aerosols in thestratosphere detection of solid particles and analysis of theirspatial variabilityrdquo J Geophys Res 113 D21303 (2008)

20 June 2010 Vol 49 No 18 APPLIED OPTICS 3559