JOURNAL OF GEOPHYSCAL RESEARCH, VOL. 103, NO. D24, PAGES 32,195-32,213, DECEMBER 27, 1998 Senstvty of multangle magng to aerosol optcal depth and to pure-partcle sze dstrbuton and composton over ocean Ralph Kahn, Pranab Banerjee, Duncan McDonald, and Davd J. Dner Jet Propulson Laboratory, Calforna nsttute of Technology, Pasadena Abstract. Multangle, multspectral remote sensng observatons, such as those antcpated from the Earth Observng System (EOS) Multangle magng Spectroradometer (MSR), can sgnfcantly mprove oublty to constran aerosol propertes from space. Smulatons over cloud-free, calm ocean condtons were studed for pure partcles wth natural ranges of optcal depth, partcle sze, and ndces of refracton. Accordng to the theoretcal smulatons we can retreve column optcal depth from measurements over calm ocean foll but the darkest partcles, wth typcal sze dstrbutons and compostons, to an uncertanty of at most 0.05 or 20%, whchever s larger, even f the partcle propertes are poorly known. For one common partcle type, soot, constrants on the optcal depth over dark ocean are very poor. The smulated measurements also allow us to dstngush sphercal from nonsphercal partcles, to separate two to four compostonal groups based on ndces of refracton, and to dentfy three to four dstnct sze groups between 0.1 and 2.0/xm characterstc radus at most lattudes. The technque s most senstve to partcle mcrophyscal propertes n the "accumulaton mode" szes, where partcle scatterng undergoes the transton from Raylegh to large-partcle regmes for the MSR wavelengths. On the bass of these results we expect to dstngush ar masses contanng dfferent aerosol types, routnely and globally, wth multangle remote sensng data. Such data complement n stu and feld data, whch can provde detaled nformaton about aerosol sze and composton locally. An extenson of ths study to mxtures of pure partcles s part of contnung work. 1. ntroducton Recent advances n modelng the Earth's clmate have brought us to a pont where the contrbutons made by aerosols to the global radaton budget sgnfcantly affect the results [e.g., Andreae, 1995; Charlson et al., 1992; Hansen et al., 1997; Penner et al., 1994]. Aerosols are thought to contrbute to drect radatve forcng n the atmosphere and, ndrectly, through ther nfluence as nucleaton stes for cloud partcles. Knowledge of both aerosol optcal depth and the mcrophyscal propertes of partcles s needed to adequately model aerosol effects. Currently, we must rely on satellte remote sensng to provde the spatal and temporal coverage requred for global montorng of atmospherc aerosols. However, the retreval of aerosol propertes by remote sensng s a notorously underdetermned problem, and the only operatonal, global-scale, satellte-based retreval of aerosols derves aerosol optcal depth from sngle-angle, monospectral data, usng assumed values for all the aerosol mcrophyscal propertes [Rao et al., 1989; Stowe et al., 1997]. Multangle, multspectral remote sensng observatons, such as those antcpated from the Earth Observng System (EOS) Multangle magng Spectroradometer (MSR), provde a type of nformaton about the characterstcs of aerosols never before obtaned from satelltes [Dner et al., 1991, 1998]. We plan to retreve aerosol optcal depth and aerosol "type," whch Copyrght 1998 by the Amercan Geophyscal Unon. Paper number 98JD01752. 0148-0227/98/98JD-01752509.00 32,195 represents a combnaton of ndex of refracton, sze dstrbuton, and shape constrants, globally, at 17.6-km spatal resoluton. MSR s scheduled for launch nto polar orbt on the EOS-AM (ante merdem) platform n late 1998 or early 1999. t wll measure the upwellng vsble radance from Earth n four spectral bands centered at 446, 558, 672, and 866 nm, at each of nne vew angles spread out n the forward and aft drectons along the flght path at 70.5 ø, 60.0 ø, 45.6 ø, 26.1 ø, and nadr. The maxmum spatal samplng rate s 275 m n the cross-track and along-track drectons, at all angles except nadr, where the cross-track value s 250 m. Ove perod of 7 mn, a 360-km-wde swath of Earth comes nto the vew of the cameras at each of the nne emsson angles, provdng a wde range of scatterng angle coverage for each surface locaton. The data wll be used to characterze aerosol optcal depth, aerosol type, surface albedo and bdrectonal reflectance, and cloud propertes. Complete coverage of lattude bands wll take 9 days at the equatond 2 days n polar regons; the nomnal msson lfetme s 6 years. Ths s the second n a seres of papers that explores our ablty to retreve nformaton about atmospherc aerosols from MSR. The frst paper [Kahn et al., 1997] asks how well we can dstngush sphercal from nonsphercal partcles havng mcrophyscal propertes commonly assocated wth mneral dust from the Sahara desert. On the bass of theoretcal sm- ulatons we showed that over calm ocean surfaces, and wth expected ranges of partcle optcal depth and sze dstrbuton, MSR can dstngush sphercal from nonsphercal partcles and can retreve column optcal depth for these nonsphercal partcles to an uncertanty of at most 0.05 or 20%, whchever s larger. At most lattudes, MSR can also dentfy three to four
32,196 KAHN ET AL.: AEROSOL PROPERTES FROM MULTANGLE MAGNG dstnct sze groups between 0.1 and 2.0 p,m characterstc radus. n ths paper we expand ths work to nclude natural ranges of optcal depth, partcle sze dstrbuton, and composton for common types of sphercal partcles. We concentrate on stuatons under whch the MSR senstvty to partcle propertes (except possbly absorpton) s lkely to be greatest: over calm ocean. These results provde a theoretcal upper bound on the senstvty of actual MSR retrevals foll aerosol propertes except partcle sngle scatterng albedo, whch may be better constraned over land surfaces wth spatally varyng contrast. For ths study we consdetmospheres contanng "pure" partcle types: aerosol populatons wth unform composton and wth aerosol szes characterzed by unmodal, lognormal dstrbuton functons. A future paper wll treat the MSR senstvty to mxtures of partcle types. 2. Our Approach to the Aerosol Senstvty Study Our overall approach s to desgnate one set of smulated reflectances as MSR "measurements," wth an atmosphere havng fxed aerosol optcal depth (ra), partcle characterstc radus (ra), real ndex of refracton (nr ), and magnary ndex of refracton (n ). We then test whether they can be dstngushed, wthn nstrument uncertanty, from a seres of "comparson" model reflectances. For the comparson models we systematcally vary the four comparson model parameters: aerosol optcal depth (re), characterstc radus (re), real ndex of refracton (nr ), and magnary ndex of refracton (n ). The goal s to determne the ranges of comparson model propertes that gve an acceptable match wth the measurements. Pror to launch of the MSR nstrument we rely on smulatons of top-of-atmosphere radaton to explore the senstvty of multangle observatons to aerosol propertes. The MSR Team has developed a radatve transfer code, based on the addng-doublng method [Hansen and Travs, 1974], to smulate reflectances as would be observed by MSR forbtrary (r ), real ndex of refracton, and magnary ndex of refracton, usng a standard Me scatterng code. The lognormal functon s gven as lognormal (r) = exp - 1 [n 211n (r) - n c)]21 r n ( r)2 The wdth parameter ((r) n ths functon s set to 2.5 for accumulaton, 2.0 for coarse, and 2.0 for nucleaton modes, representng farly broad, natural dstrbutons of partcles. 2.1. Testng the Agreement Between Comparson Models and "Measurements" Over ocean, the MSR retreval makes use of up to 18 measurements: nne angles at each of the two longest MSR wavelengths (bands 3 and 4, centered at 672 and 866 nm, respectvely), where the water surface s darkest. We defne four test varables to decde whethe comparson model s consstent wth the measurements. Each s based on the )(2 statstcal formalsm [e.g., Bevngton and Robnson, 1992]. One test varable weghts the contrbutons from each observed reflectance accordng to the slant path through the atmosphere of the observaton: 2 = l Wk[9... (l,k)-pcomp(l k)] 2 Xabs S{wk) O'a2bs(/, k) ' (2) /=3 k=l where Pmeas s the smulated "measurement" of atmospherc equvalent reflectance and Pcomp s the smulated equvalent reflectance for the comparson model. (We defne equvalent reflectance as the radance multpled by rnd dvded by the exoatmospherc solar rradance at normal ncdence.) The ndces and k are the ndces for wavelength band and camera, N s the number of measurements ncluded n the calculaton, w k are weghts, chosen to be the nverse of the cosne of the emsson angle approprate to each camera k, {wk) s the average of weghts foll the measurements ncluded n the summaton, and rab s s the absolute calbraton uncertanty n choce of aerosol type and amount and varable surface reflec- the equvalent reflectance for MSR band and camera k. For tance propertes [Dner et al., 1997]. For the present study we MSR the nomnal value of rrabs/9 falls between 0.03 fo smulated MSR measurements ove Fresnel-reflectng flat target wth equvalent reflectance of 100% and 0.06 fon ocean surface n a cloud-free, Raylegh scatterng atmosphere equvalent reflectance of 5% n all channels [Dner et al., 1997]. wth a surface pressure of 1.013 bars and a standard mdlat- For the smulatons we model rrab as varyng lnearly wth tude temperature profle. (n the actual MSR retrevals over equvalent reflectance (Fgure 1). ocean, we model Sun glnt and whtecaps, whch depend on The test varable,¾abs 2 alone reduces 18 measurements to a near-surface wnd speed [Martonchk et al., 1998].) sngle statstc, whch emphaszes the absolute reflectance and A layer contanng partcles wth selected optcal depth, depends heavly on aerosol optcal depth for brght aerosols spectral sngle scatterng albedo, extncton coeffcent, and ove dark surface. However, there s more nformaton n the sngle scatterng phase functon s placed between the gas com- measurements whch we can use to mprove the retreval dsponent and the surface. (We assume here that the vertcal crmnaton ablty. dstrbuton of aerosols has a neglgble effect on the results. A second X 2 test varabl emphaszes the geometrc proper- Our tests wth partcles nsde and above the Raylegh scatter- tes of the scatterng, whch depend heavly on partcle sze and ng layer show that partcularly at the blue and green wave- shape. The camera-to-camera relatve uncertanty s small lengths and at the steepest emsson angles the effect may be compared to the absolute uncertanty. The X2eom test sgnfcant. However, atmospherc aerosols are usually concen- takes advantage of ths fact; each spectral measurement s trated near the surface, and the MSR dark water retrevals use dvded by the correspondng spectral measurement n the only the red and nfrared channels, for whch Raylegh scatnadr camera: terng s small. n the actual MSR retrevals we wll consder a "transported mneral dust" partcle type that models desert 4 9 Wk [ P p... (l,k) 2...(/,nadr) Pcom-, -a -r) dust advected hgh n the atmosphere.) Extncton and scatterng propertes for lognormal dstrbutons of sphercal partcles )('geom--s(wk) E k=l E O'geom(/, 2 k) /--3 k nadr are derved at selected values of partcle characterstc radus (3a) varable _ Pcomp(l,k) 12
_ KAHN ET AL.: AEROSOL PROPERTES FROM MULTANGLE MAGNG 32,197 o 030 0 o25 0 020 oo s OOlO Maxmum Absolute Error, nstrument Specfcaton o o Measured Absolute Error, Band 3 Red) E -- Measured Absolute Error, Band 4 Neor R) We nclude a maxmum devaton test varable that s the sngle largest term contrbutng to Xabs 2 (see equaton (2)): 2 [P... (/, k) - Pcomp(/, k)] 2 Xmaxdev- Max 2 (5) l,k O'abs(/, k) All the other test varables are averages of up to 18 measurements. The test varable m2maxdev makes greatest use of any band-specfc or scatterng-angle-specfc phenomenon, such as a ranbow o spectral absorpton feature, n dscrmnatng between the measurements and comparson models. o oo5 0 000 01 02 03 O0 04 05 06 07 08 09 1 0 Equvalent Reflectance Fgure 1. MSR absolute radometrc uncertanty assumed for ths study, based on nomnal nstrument requrements [Dner et al., 1997] and laboratory measurements [Bruegget al., 1998]. These curves are assumed to apply to all cameras. The relatve band-to-band and camera-to-camera uncertantes are assumed to be one thrd of the absolute uncertanty. Durng MSR retrevals, n-flght measured radometrc uncertanty for each camera wll be used drectly for calculatng the X 2 test varables. Here O geo m (a dmensonless quantty) s the uncertanty the camera-to-camera equvalent reflectance rato, derved from the expanson of errors fo rato of measurements [o-2(f(x, y))] = (df/dx)2o'x 2 + (df/dy)2o'y 2 [e.g., Bevngton may tself be dstrbuted as X2.) and Robnson, 1992])' O'c2am(/, k) trc2am(/, nadr) :, O' eom(l, k) = : 4, /9... (/, nadr) 9...(l nadr) + (3b) where O'cam(l, k) s the contrbuton of (band l, camera k) to the camera-to-camera relatve calbraton reflectance uncer- tanty; O'ca m s nomnally one thrd the correspondng value of Ohb s for the MSR nstrument [Dner et al., 1997]. Note that O-ca m ncludes the effects of systematc calbraton errors for ratos of equvalent reflectance between cameras, as well as random error due to nstrument nose, though the latter has been neglected n these smulatons, based on the hgh sgnalto-nose rato demonstrated durng MSR camera testng [Bruegget al., 1998]. Smlarly, we defne a spectral X 2 as 2 _ P mspec -- Wk /9 N(wk) k=l wth O'Sand(/, k)... (band 4, k) pcomp(band 4, 3, k) ]:... (band 3, k) o's2pec(/, k) p2 eas(band 3, k) (4a) O'}and(band 3, k) /92... (/, k) p4meas(band 3, k) (4b) where O'band(/, k) s the contrbuton of (band l, camera k) to the band-to-band relatve calbraton reflectance uncertanty; O'band s nomnally one thrd the correspondng value of O'ab s for the MSR nstrument. 2.2. Evaluatng the X 2 Test Varables Snce each X 2 varable s normalzed to the number of channels used, they are "reduced" X 2 quanttes, and a value less than obout unty mples that the comparson model s ndstngushable from the measurements. Values larger than about 1 mply that the comparson model s not lkely to be consstent wth the observatons. n more detal, X 2 < 1 means that the average dfference between the measured and comparson quanttes s less than the assocated measurement error. f the quantty n the numerator of a reduced X 2 varable defnton wth 17 degrees of freedom s sampled from a populaton of random varables, an upper bound of 1 corresponds formally to an average confdence of about 50% that we are not rejectng a comparson model when n fact t should be accepted; fon upper bound of 2, that confdence ncreases to almost 99% [Bevngton and Robnson, 1992]. (Ths s not strctly true for the "X 2" varables defned here. They are ac- tually the averages of correlated measurements from multple bands and cameras, so a gven upper bound s lkely to be a less strngent constrant. Each term contrbutng to these varables To llustrate the values of the test varables, we developed a color bar wth three segments (Plate 1): a logarthmc segment for values between 10 -s and 1 depcted n shades of blue, a logarthmc segment for values between 5 and 104 depcted n shades of red, and a lnear segment shown n lght green, yellow, and orange shades for the ntermedate values. Thus red shades n Plate 1 ndcate stuatons where the model s clearly dstngushable from the measurement, whereas blue shades ndcate that the model s ndstngushable from the measurement. Black s reserved for exact agreement between model and measurement, whch can occur n ths study because we are workng wth smulated observatons. Note that the color table has been desgned so that f Plate 1 s photocoped n black and whte, frst-order nformaton about the ablty to dstngush among models s preserved. 2.3. Organzng the Study of the Eght-Dmensonal Space There are four ndependent varables assocated wth aerosols from the "atmosphere"(r,, %, nr,, and n,) and another four representng the same aerosol propertes for the comparson models (rc, rc, nrc, and nc). We have defned four dependent varables to be used n comparng the measurements wth the models 2 (Xabs, ffgeom, ffspec, and X2maxdev). Ths creates an eght-dmensonal space, wth four scalar elements at each pont n the doman. On the bass of clmatologcal data we select natural ranges of aerosol optcal depth, partcle sze dstrbuton characterstc radus, and ndces of refracton to study. Smulatons are run fo grd of values n these four varables. We systematcally explore the propertes of ths space by selectng a subset of
32,198 KAHN ET AL.' AEROSOL PROPERTES FROM MULTANGLE MAGNG t o o o ' '-: ß, E o n j ß X o o td du o j og-o._. o.
KAHN ET AL.: AEROSOL PROPERTES FROM MULTANGLE MAGNG 32,199 these values as the "atmosphere" cases and calculatng all four test varables that compare each of the "atmosphere" cases wth all the cases n the grd. We bult an nteractve system that plots X 2 values fony three ndependent varables. We can slce through ths volume and dsplay the color-bar values of any of the test varables, or for the maxmum of the four test varables, n a twodmensonal plot. For each pont n these plots we can call up the values of all the ndependent and dependent varables, as well as the calculated reflectance values for the assocated atmosphere and comparson model. We also created a summary tool that searches numercally through the space, dentfes the mnmum and maxmum values of any of the ndependent varables that meet user-specfed crtera on the test varables, and dsplays the results as bar charts. These tools are llustrated n secton 3. Wth them, we create general summares of the retreval senstvty and can dentfy the specfc equvalent reflectances that contrbute to these results. 3. Senstvty to Aerosol Optcal Depth, Characterstc Radus, and ndces of Refracton Plate 1. (opposte) Example of a comparson matrx. Ths shows '... results of tests t, e,... n an atmosphere contanng tropospherc sulfate-lke partcles and comparson models wth ranges of optcal depth and characterstc radus. For ths example, all partcles have ntal (dry) ndces of refracton set to nr = 1.53, n -- 0.0 and are hydrated to equlbrum wth 70% relatve humdty followng Hanel [1976]. The four panels correspond to atmospherc aerosol optcal depths of 0.05, 0.1, 0.5, and 1.0 at 0.55 p,m wavelength. n blue and black areas the largest value fony test varable s <1, ndcatng that the comparson models that are ndstngushable from the smulated MSR data. Red areas ndcate comparson models that are not consstent wth the MSR observatons. The calculatons presented here and elsewhere n ths papere for md- lattude cases: the cosne of the solar zenth angle (P-o) s 0.6, and the angle between the azmuth of the Sun and the nstrument vewng plane (AO) s 26.0 ø. Table 1. Parameter Space of Aerosol Propertes Used n Ths Study Number Mnmum Maxmum of Value Value Dvsons Aerosol optcal depth at 0.55 m 0.05 1.00 20 Real ndex of refracton 1.33 1.55 12 magnary ndex of refracton 0.0 0.50 20 Nucleaton mode characterstc 0.005 0.2 40 radus Accumulaton mode characterstc 0.05 2.00 40 radus Coarse mode characterstc radus 0.50 4.50 41 *For the nucleaton mode, there are 20 ncrements of optcal depth, rangng from 0.025 to 0.50. sults n the operatonal MSR retreval [Martonchk et al., 1998]. We also use the clmatology as a gude to nterpretng the senstvty calculatons. (The present study deals wth sphercal partcles. n the MSR retrevals and n our prevous senstvty study [Kahn et al., 1997], mneral dust partcles are treated as nonsphercal.) To capture n a senstvty study the range of lkely aerosol effects, we systematcally explore the propertes of three parameter spaces. They cover natural ranges of aerosol optcal 3.1. Constrants on Aerosol Optcal Depth depth, partcle sze dstrbuton characterstc radus, and nd- We begn wth the queston: Gven MSR data ove cloudces of refracton for "nucleaton," "accumulaton," and free, dark ocean surface, how well can we constran the optcal "coarse" mode partcles. Table 1 lsts the ranges foll three parameter spaces. Smulatons were run on a grd of lnearly spaced values n all dmensons except the magnary ndex of refracton, for whch the grd spacng s logarthmc. We concentrate on accumulaton mode partcles because they are expected to contrbute most to total aerosol optcal depth at vsble wavelengths. Nucleaton mode partcles are much less effcent at extngushng lght, and coarse mode depth (r,) of atmospherc aerosols? Plate 1 s an example of a comparson matrx, taken from wthn the accumulaton mode parameter space. n Plate 1, the atmospherc partcles have fxed mcrophyscal propertes typcal of sulfate partcles n the troposphere [e.g., Shettle and Fenn, 1979]: dry values of n - 1.53, n a = 0.0, and lognormal sze dstrbuton wth r, = 0.5 p,m and wdth of 1.86. The partcles are hydrated to equlbrum at 70% relatve hupartcles are usually rare. However, even for the accumulaton mdty, usng the model of Hanel [1976]. mode sze range, not all locatons wthn the parameter space are lkely to be flled by commonly occurrng atmospherc aerosols. The propertes of common aerosol types are summarzed n Table 2. There are fve basc compostons: sulfates, mneral dust, sea salt, bomass burnng partcles, and soot. Szes range from tny soot partcles to coarse mneral dust and sea salt. Wth the excepton of soot, partcles have low magnary ndex of refracton and hgh sngle scatterng albedo n the red and near-nfrared channels. Ths clmatology helps constran re- There are four panels n Plate 1. Each contans all the test results for one choce of atmospherc optcal depth, so wthn a panel, all the modeled propertes of the atmospherc aerosols are fxed. The four panels correspond to atmospherc aerosol optcal depths at 0.55 p,m wavelength of 0.05, 0.1, 0.5, and 1.0, respectvely. The optcal depths of the comparson models at 0.55 vary systematcally from 0.05 to 1.0 n ncrements of 0.05 across the bottom of each panel n Plate 1; the dry rad of the comparson models vary from 0.05 to 2.0 p,m n ncrements of 0.05 along the vertcal axs. Refractve ndces for the comparson model are the same as those for the atmosphere n the slce shown here. Each box wthn the panel s dvded nto fve felds, four showng the colors correspondng to each of the X 2 test varables, and the background, whch s colored wth the largest (most red) X 2 value among the four tests. So the box for each comparson model tells whether that model can be dstngushed from the atmosphere wth MSR data and whch test(s) provdes the most constranng nformaton. n Plate 1 the blue areas, whch ndcate comparson models ndstngushable from the atmosphere wth MSR data, are small and vertcally orented: there are acceptable comparson models wth a broad range of r c but a narrow range of r c on ths scale. Close examnaton of the surroundng boxes shows that Xmaxdev 2 and Xabs 2 are responsble for constranng r c so tghtly. Both tests rely on the absolute brghtness. Ths s ex-
32,200 KAHN ET AL.: AEROSOL PROPERTES FROM MULTANGLE MAGNG Table 2. MSR Clmatology of Pure Partcle Types Vary rl, r2, rc, n w0 RH, Wth Aerosol Type / m / m / m nr (Band) (0.67/ m) Shape % RH? Sulfate 1 (accumulaton) 0.007 0.7 0.2 1.86 n/a 1.53 0.0 (all) 1.0 spheres 0 yes Sulfate 2* (accumulaton) 0.05 2.0 0.45 1.30 n/a 1.43 0.0 (all) 1.0 spheres ambent no Mneral dust? (accumulaton) 0.05 2.0 0.47 2.60 n/a 1.53 0.0085(1) 0.91 prolate/oblate 0 no 0.0055(2) spherods 0.0045(3) 0.0012(4) Mneral dust? (coarse) 0.5 15.0 1.90 2.60 n/a 1.53 0.0085(1) 0.70 prolate/oblate 0 no 0.0055(2) spherods 0.0045(3) 0.0012(4) Sea salt (accumulaton) 0.05 1.0 0.35 2.51 n/a 1.50 0.0 (all) 1.0 spheres 0 yes Sea salt (coarse) 1.0 20.0 3.30 2.03 n/a 1.50 0.0 (all) 1.0 spheres 0 yes Soot 0.001 0.5 0.012 2.00 n/a 1.75 0.455(1) 0.17 spheres 0 no 0.440 (2) 0.435 (3) 0.430 (4) Bomass burnngs 0.007 2.0 0.13 1.80 n/a 1.43 0.0035 (all) 0.98 spheres 97 no Near-surface fog{} 0.5 50.0 n/a n/a 2.5 1.33 0.0 (all) 1.0 spheres 100 no Thn crrusô 10.0 500. n/a n/a n/a 1.31 0.0 (all) 1.0 fractal 100 no Here r and r 2 are the lowend upper radus lmts for the partcle sze dstrbuton. Partcle types havng r c and rr specfed are dstrbuted lognormally, wth characterstc radus r c and wdth rr; those havng a specfed are power law dstrbutons wth exponent a (n/a, not applcable); o 0 s the sngle scatterng albedo, gven at the effectve wavelength of the MSR red channel. RH s the relatve humdty for whch the partcle sze dstrbuton and ndces of refracton are lsted. The last column ndcates that the propertes of sulfate 1 and sea salt partcles are assumed to vary wth relatve humdty, usng the hydraton model of Hanel [1976]. The aerosol physcal data are abstracted from Shettle and Fenn [1979], dmlmeda et al. [1991], World Clmate Programme WCP-112 [1984], and other sources, except as ndcated. Optcal data for sphercal partcles are calculated usng Me theory. *Stratospherc aerosol model based on work by Wang et al. [1989].?Nonsphercal mneral dust models based on work by Mshchenko et al. [1997]. $Bomass burnng partcle model based on work by Remer et al. [1998]. {}Near-surface fog model based on work by Prupackend Klett [1978]. ôfractal thn crrus model based on work by Mshchenko et al. [1996]. pected, snce the absolute scene brghtness ncreases wth op- wthn 20% regardless of the physcal propertes of the parttcal depth for nonblack partcles ove black surface. cles, as long as n a s 0.0. Ths covers all natural cases nvolvng We have studed senstvty to optcal depth over the entre sulfate and sea salt partcles (Table 2). The lower lmt of 0.05 parameter spaces gven n Table 1. The results foccumula- s set by the assumed camera calbraton uncertanty, whch ton and coarse mode partcles are summarzed n Fgures 2a reaches a lmtng value for low reflectance levels (Fgure 1). and 2b, respectvely. These bar charts show the senstvty, as Calbraton uncertanty affects the senstvty through "cr" n ndcated by the range of acceptable comparson model optcal the X 2 test varable defntons. The choce of grd spacng for depth, along the vertcal axs foll comparson models n the ths study was made n consderaton of ths lmt. Durng accumulaton mode parameter space that gve acceptable MSR operatons, n-flght measured radometrc uncertanty matches to an atmosphere wth fxed partcle propertes. For for each camera wll be used drectly for calculatng the X 2 test Fgures 2a and 2b an acceptable comparson model s one for varables n the retrevals [Bruegget al., 1998]. whch all four X 2 test varables fall between 0 and 2. Accordng to Fgure 2, as na ncreases, senstvty to aerosol Each group of four bars corresponds to four choces of optcal depth degrades and becomes more dependent on atatmosphere partcle optcal depth (0.05, 0.1, 0.5, and 1.0 n mospherc aerosol optcal depth, partcle sze, and magnary MSR band 2) for fxed atmosphere partcle radus. The groups refractve ndex. Senstvty to ra also decreases as optcal are spread along the horzontal axs to ndcate dfferent depth ncreases, partcularly from 0.05 to 0.5. Part of the degchoces of atmosphere partcle radus. Each panel of Fgure 2 radaton comes from the ncreased contrbuton of multple represents a fxed value of atmosphere partcle magnary ndex scatterng to the sgnal as scene brghtness ncreases. For exof refracton. The real ndex of refracton fotmosphere par- ample, when r - 0.5 / m, nr = 1.47, and n = 0.13, the tcles s fxed at 1.47 foll these charts, but wthn ts natural multple scatterng contrbuton to the steepest forward lookrange of values ths quantty has very lttle effect on these ng MSR camera ncreases from about 20% of the sgnal to results. nearly half as r ncreases from 0.1 to 0.5. As ra ncreases So for each bar, all the atmospherc partcle propertes have further, the senstvty to optcal depth ncreases agan n some been fxed. The comparson model propertes, r c, nrc, and n c, cases. Here the multple scatterng contrbuton s not growng have been vared over the entre parameter space gven n as rapdly, whereas the total aerosol sgnal relatve to back- Table 1. The comparson models havng the largest and small- ground Raylegh scatterng contnues to ncrease. (The upper est values of rc that meet the test crtera, regardless of the boundary for each chart corresponds to the maxmum value n values of the other three comparson model parameters, de- the parameter space, so the hgh-sde uncertanty for these termne the uppend lower lmts of the bar. cases s not shown. Owng to the number of computatons Senstvty to optcal depth s as low as 0.05 and remans needed for ths study, t was mpractcal to expand the param-
KAHN ET AL.: AEROSOL PROPERTES FROM MULTANGLE MAGNG 32,201 0.80. 0.60. n a = 0 n a = 0.004 1.0 ' '! ' ' 0.80 _ 0.60 _ '! 0.40. 0.20. 0.40 _ 0.20. 0.35 0.5 1 2 08 1.5 0.2 0.5 l 0.35 0.8 1.5 1.0 n a = 0.0079 1.0 n a = 0.032 020 11: 0.0 0.1 0.2 0.35 0.60 05 1 2 0.8 1.5 o ø 0.40 g 0.20 0.0 '! 0.2 0.5 l 0.1 0.35 0.8 1.5 n a = 0.13 n a = 0.5 1.0 0.80 060 m o 0.80 040 0.20 0.20 ' : : O0! 0 2 0.5 1 2 0.2 0.5 1 2 0.35 0 8 1.5 0.1 0.35 0 8 5 Fgure 2a. Bar chart showng the ranges of aerosol optcal depth values (rc) for comparson models that gve acceptable matches to an atmosphere wth accumulaton mode partcles havng real ndex of refracton n - 1.47, magnary ndex of refracton a rangng from 0.0 to 0.50. Fon acceptable match, all four X 2 test varables must fall between 0 and 2. Bars are produced for eght choces of atmospherc partcle characterstc radus (ra) between 0.1 and 2, arrayed along the horzontal axes. For each ra a group of four bars s produced, correspondng to four choces of atmospherc optcal depth (ra). As shadng ncreases, the bars represent values of ra ncreasng from 0.05 to 0.1, 0.5, and 1.0. eter space beyond the lmts shown. The reflectance tends to asymptote as optcal depth ncreases beyond 1. For large optcal depth the hgh-sde uncertanty s comparable to the lowsde uncertanty but grows larger than the low-sde uncertanty as partcle sze and magnary ndex of refracton ncrease. Cases away from the lmtng values provde a sense for the szes of these trends.) We now examne n detal the senstvty of the multangle retreval to partcles that have nonzero n. When n = 0.0, most of the nformaton about optcal depth comes from systematc ncreases n brghtness as r ncreases. As Fgure 3 shows, when n > 0.0, the sngle scatterng phase functons n the backscat- terng drectons decrease wth ncreasng radus. Fo gven set of measured radances, we fnd agreement based on the X crtera for several comparson models, some wth smaller radus and lower optcal depth, others wth larger radus and hgher optcal depth. Ths creates the hgher uncertanty n rc found n Fgure 2. For the MSR retreval we are most nterested n clmato- logcally lkely aerosol types. Among these, bomass burnng partcles, mneral dust, and soot have nonzero n, and only soot n clmatologcally hgh concentratons s lkely to affect the qualty of the MSR global aerosol products. n the red and near-nfrared channels, n for bomass burn-
32,202 KAHN ET AL.' AEROSOL PROPERTES FROM MULTANGLE MAGNG n a = 0 n a = 0.004 1.0 080 080 _ 0.60 060 _ 0.40 0.20 040.. 0.20.. 0.0 2.3 2.5 27 35 4.5 3 4 2 3 2.7 25 3 3.5 4 45 1.0 n a = 0.0079 n a = 0.032 0.80 060 080 _ 060 _ 040 _ 020 00! 23 27 2 25 3 3 5 4.5 4 23 25 27 3.5 n a = 0.13 10 n a = 0.5 0.80. 060 080 0.40 020 020 O0 O0 23 25 27 3 35 4 4.5 2.3 2.7 3 5 2 25 3 4 45 Fgure 2b. Same as Fgure 2a but for part of the coarse mode partcle parameter space. ng partcles n Table 2 s around 0.004. Foll accumulaton mode partcles and for optcal depths less than 0.5 the range of optcal depth focceptable comparson models falls wthn 20% of the atmospherc value. Ths covers the expected natural range of condtons except near source regons, whch are on land but may be close to a coast. f column optcal depths of these partcles reach 0.5 or f coarse-szed bomass burnng partcles form, the senstvty of MSR to optcal depth wll be dmnshed. (Note that the dfferences between the bars for - 2 /zm n correspondng charts of Fgures 2a and 2b arse because the ranges of comparson model partcle szes for the two cases are dfferent (Table 1).) For mneral dust, n n Table 2 for the red channel s also about 0.004, a value representatve of the optcally mportant dust component n many global studes [e.g., Tegen and Lacs, 1996]. However, a great varety of mneral dust partcle types are known to occur n nature [Sokolk and Toon, 1996]. Wth ncreasng na or %, senstvty to optcal depth degrades. n the clmatologcally unlkely stuaton that optcal depths of a few tenths or more of coarse mode dust are present over ocean, or n exceeds 0.008, the retreved optcal depth wll be uncertan to 25% or more. f no other nformaton s avalable, MSR results wll depend upon the assumptons we make about mneral dust propertes, partcularly regardng the magnary ndex of refracton. However, MSR data can dstngush sphercal from natural mxes of nonsphercal mneral-dust-lke partcles. Ths s an advantage over prevous remote sensng aerosol retrevals and s expected both to help dentfy mneral dust and to brng the optcal depth senstvty wthn 20% or 0.05, whchever s larger, for many natural stuatons [Kahn et al., 1997]. The poorest senstvty to optcal depth occurs when dark partcles (those havng large n) are found over the dark ocean surface. Soot partcles fall nto ths category, but they are smaller than the other partcle types n Table 2 and le below the sze ranges covered n Fgure 2. Fgure 4 explores ths case,
KAHN ET AL.' AEROSOL PROPERTES FROM MULTANGLE MAGNG 32,203 10000.00 1000.00 100.00 Symbol Wavelength r_effectve n nr RH 0.865 2.000 0.0000 1.47 0.00 G... 0.865 2.500 0.0000 1.47 0.00 A----- 0.865 5.500 0.0000 1.47 0.00 [3----El 0.865 4.500 0.0000 1.47 0.00 (3------C) 0.865 2.000 0.0040 1.47 0.00. (_.. 0.865 2.500 0.0040 1.47 0.00 0.865 3.500 0.0040 1.47 0.00... A 0.865 4.500 0.0040 1.47 0.00 E ------E 0.865 2.000 0.0316 1.47 0.00 (3- - -0 0.865 2.500 0.0316 1.47 0.00 )K------ 0.865 3.500 0.0316 1.47 0.00, - 0 0.865 4.500 0.0316 1.47 0.00 10.00 1.00 0.10 0.01 30 60 90 120 150 Scatterng Angle (o) Fgure 3. Partcle scatterng phase functons at the effectve wavelength of the MSR near-nfrared band for sngle-szed sphercal partcles n the coarse mode range, between 2.0 and 4.5/zm radus, and nr = 1.47. Three groups of phase functons are plotted, one wth n = 0.0, one wth n = 0.004, and one wth n = 0.032. These three groups correspond to the uppermost, mddle, and lowest four lnes, respectvely, when vewed at scatterng angles between 150 ø and 180 ø, the regon where much of the MSR optcal depth dscrmnaton made. 180 gvng bar charts for part of the nucleaton mode parameter 3.2. Constrants on Partcle Characterstc Radus space. t shows that even for tny partcles, multangle data are We now ask the queston: Gven MSR data ove cloudnot senstve to the optcal depth of very dark partcles ove free, dark ocean surface, how well can we constran the chardark surface. Ths s an nherent lmtaton of the technque. acterstc radus (ra) of a monomodal dstrbuton of atmo- On the spatal scale of 17.6 km that apples to the MSR spherc aerosols? aerosol product we expecthat optcal depths for pure soot wll ß -gure.)d a Lldl k, lldl t showng the ranges u, ' r. for " the. rarely exceed a few tenths over ocean. n addton, soot s most comparson models n the accumulaton mode parameter lkely to appear mxed wth other partcle types havng much space that gve acceptable matches to an atmosphere wth fxed lower n. Our senstvty to optcal depth for such mxtures may partcle propertes. Fgure 5b s a smlar set of charts for be hghend s the subject of contnung work. coarse mode szes. n these charts the real ndex of refracton On the bass of the results of ths secton, we expect the MSR aerosol optcal depth retreval senstvty over ocean to for the atmosphere partcles (nr ) vares from column to colfall wthn 0.05 or 20%, whchever s larger, for clmatologcally umn, and the magnary ndex of refracton (n ) vares from lkely aerosol stuatons. Wth the actual MSR data, we may row to row. Comparson model propertes, %, nr c, and n., are be able to decrease the lmtng X 2 test value below 2, whch free to vary over the entre parameter space gven n Table 1. would lead to a tghter optcal depth constrant. However, the An acceptable match s a case for whch all four X 2 test varmeasurements are nsenstve to soot or to dark mneral partcles wth optcal depth for ra greater than about 0.1. We plan to use n stu measurements whenever possble to mprove our assumptons about partcle propertes. ables fall between 0 and 2. Bars are produced for eght choces of atmospherc partcle radus (r ) n each mode. For each r, bars are produced for four choces of atmospherc optcal depth % (0.05, 0.1, 0.5 and 1.0) n MSR band 2.
32,204 KAHN ET AL.' AEROSOL PROPERTES FROM MULTANGLE MAGNG 0.5 0.40 n a = 0 n a = 0.004 0.5 0.30 0.20 0.0 1.33! 1.41 1.55 1.47 0.10 0.20 0.0 1.33 1.41 1.55 1.47 0.5 0.20 0.10. n a = 0.0079 0.40 _ 0.30.20 0.10 n a = 0.032.: 0.0 1.33 1.41 1.47 1.55 1.33 1.41 1.47 1.55 0.5 n a = 0.13 n a = 0.5 0.5 o 0.40 z 0.30 o 0.20 ' o n 0.10 e- ' 0.0 0.40 0.30 0.20 0.10 1.33 1.41 1.55 1.41 1.47 1.33 1 47 1.55 ra Fgure 4. Bar chart showng the ranges of aerosol optcal depth values (r c) for comparson models that gve acceptable matches to atmosphere partcles coverng part of the nucleaton mode parameter space that ncludes expected soot partcle szes and ndces of refracton. Atmosphere partcle characterstc radus s fxed at 0.01/ m. The magnary ndex of refracton na ranges from 0.0 to 0.50. Fon acceptable match, all four X 2 test varables must fall between 0 and 2. Bars are produced for four choces of real ndex of refracton (nra) between 1.33 and 1.55, arrayed along the horzontal axes. For each nr, a group of four bars s produced, correspondng to four choces of atmospherc optcal depth (%). As shadng ncreases, the bars represent values of % ncreasng from 0.025 to 0.05, 0.25, and 0.5. At each ra, the range of acceptable rc generally decreases as % ncreases. Ths means we have better constraned aerosol characterstc radus retrevals wth ncreasng %. We expectsuch behavor: by ncreasng the amount of aerosol, the contrbuton of aerosols to the measured radance s ncreased, relatve to the contrbuton from Raylegh scatterng gas. Some exceptons occur when ncreased nstrument radometrc uncertanty, caused by the greater scene brghtness (Fgure 1), s enough to offset the ncreased aerosol sgnal. Especally when % s greater than 0.05, the constrant on r c s tght foccumulaton mode atmospherc partcle dstrbu- tons wth characterstc radus smaller than about 0.8/ m. For larger partcles, r c s poorly constraned by smulated MSR data over ocean. Ths behavor s traced to the partcle scatterng phase functon, whch goes through a transton from "small"(raylegh) to "large"(geometrc) regmes as partcle sze ncreases. At mdlattudes and hgh lattudes, the nstru- ment sample scatterng angles rangng from about 60 ø to 160 ø (Fgure 6). Over ths range of scatterng angles the partcle sngle scatterng phase functons change rapdly as % ncreases, untl an upper lmt s reached (Fgure 7). For n = 0.0, the transton occurs around r = 0.8 / m; for nonzero n wthn
KAHN ET AL.: AEROSOL PROPERTES FROM MULTANGLE MAGNG 32,205 o d c ' o E :'J elqe;deooe o ebuej -- -- o c:5 o rq -- -- d d øj elqe;deooe.o ebuej øj elqe;deooe.o ebuej J elqel. deoo jo efuej
32,206 KAHN ET AL.: AEROSOL PROPERTES FROM MULTANGLE MAGNG : :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::,q oj alqeldaooe 1o aõuej oj alqedaooe 1o aõum :'J alqezdaooe.o aõuej øj elqe,deooe jo eõuej... : - -r-... ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::. - ß. ß. -.. :::::::::::::::::::::::::::::::::::::::::::::::::::::::::. ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: ' 1111 [1 :.. -.- -. -m rq :'J 01qeldoooe jo oõuej :'J 01qeldoooe jo oõuej :'J 01qeldoooe jo oõuej :'J elqel. deooe jo eõuej oj elqedeooe lo eõum
_ KAHN ET AL.' AEROSOL PROPERTES FROM MULTANGLE MAGNG 32,207 180 150 120 o 90.c- 60 < _ A 5C - Sun Symbol Lattude /% angle (ø) A A Hgh 0.20 78.5 O o Mddle 0.60 53.1 ----E] Low 0.95 18.2 0 Df Cf Bf Af /z 0 334 0 500 0 700 0 898 V ew angle (o) 70 5 60 0 45.6 26 1 An Ao Be Co Do Camera 1 000 0 898 0 700 0 500 0 354 0 0 26 1 45 6 60 0 70 5 Fgure 6. Typcal ranges of scatterng angles covered by the nne MSR cameras n the nomnal EOS-AM orbt at hgh, mddle, and low lattudes. the natural range the transton occurs at r, approachng 0.5 On the bass of Fgure 5, MSR should be able to dstngush three to four groups of characterstc radus across the natural range of partcle sze, even f lttle s known a pror about composton, as long as the optcal depth s greater than about 0.05. wust of ths en u¾ ty : ;.:.. occurs,u, c... pdl tlk, :_,_ lcb wth r,. between 0.1 and about 1 /zm. The senstvty to characterstc radus ncreases for hgher optcal depth and s greatest for partcles wth low magnary ndex of refracton. 3.3. Constrants on Aerosol Composton Fgures 8 and 9 llustrate the senstvty of MSR to real and magnary ndces of refracton, respectvely. These parameters descrbe the composton of aerosols n our retrevals. As wth partcle sze, senstvty ncreases wth atmospherc optcal depth and decreases for darker partcles wthn the parameter space. For n = 0, oublty to constran nr depends on MSR senstvty to ndex of refracton ncreases greatly for ncreasng %. Senstvty to nr s dmnshed for the smallest partcles (% - 0.1), whch fall n the Raylegh scatterng regme (Fgure 7). Oublty to dscrmnate nr s best for "medum" (accumulaton mode) aerosols, and decreases for both small and large partcles. Overall, one to two groups of nr values can be dstngushed over the natural range spannng nr: 1.33 to nr - 1.55, when % s about LJ..... 111U nmnber of groups ncreases to two to tleco nr values when r, s about 0.5, as long as the partcles are not strongly absorbng. However, the data become nsenstve to the real part of the ndex of refracton for darker partcles (magnary ndex larger than about 0.01). Senstvty to the magnary part of the ndex of refracton tself follows a smlar pattern, though the smulatons suggest that when % s 0.5 or greater, three to four groups of values between n = 0.0 and about 0.5 can be dstngushed. For optcal depths of a few tenths, more typcal of global ocean condtons, one or two groups of n can be separated n the data. 3.4. Addtonal Consderatons The MSR nstrument samples a broad range of scatterng angles, between about 60 ø and 160 ø, n mdlattudes and hgh
32,208 KAHN ET AL.: AEROSOL PROPERTES FROM MULTANGLE MAGNG 100.0 Symbol Wavelength r_effectve n nr RH.,,, ß.,, 0.865 0.100 <>....(> 0.865 0.300 A----- --A 0.865 0.500 [3"- -El 0.865 0.700 G----o-C) 0.865 0.900 0.865 1.100 <) 0.865 1.300 A-... A 0.865 1.500 0.0000 1.47 0.00 0.0000 1.47 0.00 0.0000 1.47 0.00 0.0000 1.47 0.00 0.0000 1.47 0.00 0.0000 1.47 0.00 0.0000 1.47 0.00 0.0000 1.47 0.00 10.0 1.0 0 30 60 150 180 Fgure 7. Partcle scatterng phase functons at the effectve wavelength of the MSR near-nfrared band for sngle-szed sphercal partcles wth nr = 1.47, n = 0.0, and rad rangng from 0.1 to 1.50/ m. Ths range covers the transton from small partcle (Raylegh) scatterng to large partcle scatterng. lattudes (Fgure 6). The analyss presented n ths paper s for mdlattude geometry. However, at low lattudes the range of scatterng angles s dmnshed. Ths reduces the retreval senstvty to partcle propertes. Senstvty to aerosol propertes s also reduced f clouds are present or f the surface s not black n red and near-nfrared wavelengths due to whtecaps, Sun glnt, or plankton n the water. For MSR retrevals, we elmnate data from any cameras contamnated by Sun glnt, snce modelng Sun glnt radance would ntroduce large uncertanrently n operatonal use, comes from the known, varyng geometrc path through the atmosphere and the range of scatterng angles observed. Over ocean, MSR retreval results are based on assumed surface reflectances and assumptons that the atmosphere s cloud-free and the partcles are horzontally homogeneous over the samplng regon, whch s 17.6 km at the surface, ncreasng to about 100 km at the tropopause for the nomnal nstrument vewng geometry. On the bass of our study of smulated data for pure parttes. We model whtecaps, whch nvolves makng assumptons cles, MSR retrevals over dark ocean wll provde nformaton about how they behave [e.g., Martonchk et al., 1998]. All these phenomena are expected at some tmes and places n the about atmospherc aerosol optcal depth, and some constrants on aerosol mcrophyscal propertes: MSR data. The goal of the present study s to explore the senstvty of MSR to partcle propertes usng smulated data, 4.1. Aerosol Optcal Depth under the best observng condtons we antcpate. By analyzng actual MSR data, obtaned under real observng condtons, We can retreve column optcal depth from measurements we wll assess how actual retreval senstvtes compare to the over calm ocean foll but the darkest partcles, wth typcal dealzed cases consdered here. sze dstrbutons and compostons, to an uncertanty of at most 0.05 or 20%, whchever s larger, even f the partcle 4. Conclusons propertes are poorly known. As partcle magnary ndex of refracton ncreases, senstvty to optcal depth degrades and The enhanced ablty of multangle magng to constran becomes dependent on partcle sze and optcal depth. For aerosol propertes, as compared to sngle-angle methods cur- optcal depths <0.5 and for partcle szes and ndces of refrac-
KAHN ET AL.: AEROSOL PROPERTES FROM MULTANGLE MAGNG 32,209 = 0.1, n a = 0 = 0.35, n a = 0 = 0.5, n a = 0.55 1 55 1.55 = 1, n a = 0 1.51 1.46 1.42 1.37 33 1 33.42 42.33 '- 1 33 _1!! 1.41.55 1.41 41 1.55 1.55 1.33 1.47 33 1.47 47 n n n 1.46 ' 1.42! 1.37 ' 1.33 1.41 1.55 1.33 1.47 n = 0.1, n a = 0.004 = 0.35, n a = 0.004 = 0.5, n a = 0.004 55 55 55 u u u ß ]1.42 142 g 1.42 137!l '}.,:.._ 1.37 [ 1.37 1 33 33 1 33 1.41 1.55 1.41 1.55 1.41 1.55 1 33 1.47 1 33 1.47 1.33 1.47 1 55 1.51 _ 1.46 42 1.37 1.33 = 1, n a = 0.004 1.41 1.33 1.47 n 1.55 = 0.1, n a = 0.0079 = 0.35, n a = 0.0079 1 55 55 1 55 = 0.5, n a = 0.0079 1.55 = 1, n a = 0.0079 142 O 1.42 146... 1.46 1.33 1.33 41 55 1.41 55 1.33 1.47 1.33 1.47 n n 1.51. 1.42 1.46 1.37!] ' l. 1.33 141 1.55 133 147 n 1.51 146 1.42.37 _ 33 41 55 1.33 1.47 n = 0.1, n a = 0.032 1 55 0.35, n a = 0.032 1 55 = 0.5, n a = 0.032 = 1, n a = 0.032 51 1.51. 46.42 37 ß o ø 142 ' :tll ll 1 42 1.37.33 '- 33 1.33 141 1.55 1.41 1 55 1 41 1 47 47 1.33 1.47 1.33 1 33 1 33 n n n 141 1 47 n 1.55 Fgure 81. Bar chart showng the ranges of partcle real ndex of refracton (nr c) for comparson models that gve acceptable matches to an atmosphere wth accumulaton mode partcles havng selected values of characterstc radus and magnary ndex of refracton. Fon acceptable match, all four X 2 test varables must fall between 0 and 2. Bars are produced for four choces of atmospherc partcle real ndex of refracton (nra). For each nra a group of four bars s produced, correspondng to four choces of atmospherc optcal depth (%). As shadng ncreases, the bars represent values of % ncreasng from 0.05 to 0.1, 0.5, and 1.0. ton expected foll common partcle types except soot (Table 2), optcal depth senstvty s stll better than 20%. The measurements are less senstve to optcal depth for common mneral dust and bomass burnng partcles wth % of 0.5 or greatend are nsenstve to ra for soot. 4.2. Aerosol Sze Dstrbuton We expect MSR to be able to dstngush three to four groups of characterstc radus across the natural range (small, medum, and large) over mdlattude and hgh-lattude ocean, even f lttle s known a pror about composton, as long as the aerosol optcal depth s greater than about 0.05. Most of ths senstvty occurs for partcles wth r c between 0.1 and about 1 /xm, whch covers the range of partcle szes where the scatterng phase functons change from Raylegh scatterng behavor to curves wth well-developed forward and back scatterng peaks. The senstvty to characterstc radus ncreases for hgher optcal depth, snce there s more aerosol sgnal n these cases, and s greatest for less absorbng (low magnary ndex of refracton) partcles. 4.3. Aerosol Composton MSR senstvty to ndex of refracton ncreases strongly wth ncreasng optcal depth and decreases wth ncreasng n. We can dstngush about two or three groups of real ndex of refracton values between 1.33 and 1.55, as long as the optcal
32,210 KAHN ET AL.' AEROSOL PROPERTES FROM MULTANGLE MAGNG = 2, n a = 0 = 2.5, n a = 0. 1 51 1 51 ø 1 46! 1.46 o ø ' 142. o ø 142 = 3.5, n a = 0 = 4.5, n a = 0 1 55 1.55 O 1.42 l! O 1.42 1.33 1 33 41 1 55 1.33 1.47 lt a = 2, n a = 0.004 1.55 1.55 1.41 1.55 1 33 1.47 lt a = 2.5, n a = 0.004 '- 1.33 L. '- 133 1 41 1 55 1 41 1 55 1 33 1 47 1.33 1 47 l = 3.5, n a = 0.004 = 4.5, n a = 0.004 1.55 1 55., ::!1... l 146 J 146 1.42 '... 142 '- 1.33 '- 1.33 1.41 1.55 1 33 1.47 lt a 1 41 1 55 1 33 1 47 lt a 1.41 1.55 1 41 1.55 1 33 1 47 1 33 1.47 n n = 2, n a = 0.0079 1 55 1.55 = 2.5, n a = 0.0079 = 3.5, n a = 0.0079 = 4.5, n a = 0.0079 1.55 1 55 46 ß 142... 137 133 141 1 55 133 1 47 1.41 1.55 1 33 1 47 lt a 1.46.: t.::.. :?l.!; :.!!: t. J 146.'" :...':':... 1 42 o 1 42.. 1 41 1 55 1 41 1 55 1 33 1 47 1.33 1 47 lt a lt a 155 = 2, n a = 0.032 1 55 = 2.5, n a = 0.032 = 3.5, n a = 0.032 = 4.5, n a = 0.032 155 1.55 ß - 1.51 146 O 142 37 '- 133 ß 142.' ß :... '!... t 1 42!l ll. ::117:... 42....!! j! ::.:!... : l'. O 1 137 ' } ' lll ' 1 137 1.33 1 33! 1 33 141 1 47 lt a 1 55 1 33 1.41 1.55 1.41 1 55 1 41 1 55 1 47 1 33 1 47 1 33 1.47 lt a n n Fgure 8b. Same as Fgure 8a but for part of the coarse mode partcle parameter space. depth s 0.5 or largend the partcles are not strongly absorbng. Only one to two groups of nr values can be dstngushed over the natural range when r, s about 0.1 or less. n addton, the data dscrmnate nr best for "medum" szed partcles and Our results suggest MSR wll be able to dstngush small, medum, and large partcle szes, "drty" or "clean" compostons, and "sphercal" and "nonsphercal" shapes, over the range of aerosol propertes commonly found n nature. Ths are nsenstve to the real part of the ndex of refracton for dark partcles. Senstvty to the magnary part of the ndex of refracton follows a smlar pattern, though the smulatons suggesthat three to four groups of values for n between 0.0 and about 0.5 can be dstngushed when r, s 0.5 or greater. The actual senstvty of MSR retrevals to partcle propertes over ocean wll depend n part on the n-flght nstrument calbraton uncertanty, whch sets the values of the r, and governs our choce of the upper lmt on X focceptng a comparson model as ndstngushable from the measureshould allow us to dstngush among the clmatologcally common partcle types (Table 2) f each type occurs unmxed n the vertcal column. We wll use ths mprovement over smply assumng all the physcal propertes of partcles, as s currently done for satellte aerosol montorng, to dentfy ar masses contanng dfferent aerosol types, as well as to provde more accurate retrevals of aerosol optcal depth. The strength of satellte retrevals s n ther spatal and temporal coverage, whch complements detaled composton and sze dstrbuton nformaton that may be obtaned locally ments. For ths paper we used prelaunch nstrument specfca- from n stu measurements. We plan to rely on ground-based tons for calbraton and nose levels. After launch we wll use and n stu nstrument data to derve detaled aerosol compothe tools and the approach developed here to assess the nflght senstvty of MSR. stons and sze dstrbutons n ar masses whenever possble. Ths approach s smlar to the way satellte-derved sea
: KAHN ET AL.' AEROSOL PROPERTES FROM MULTANGLE MAGNG 32,211 1 1 1,,,...!u elqelde:}:}e to o,! u elqe)de00e 0 elsuej oo o " o o :[u elqedeooe Jo efuej, E... 0 o m,, :- ='- ß ß. -* o o ::: :::s:::::::::::::::::: ::: :::s : :: :::: ::::::: : : ::: :: ::::::s:: : :::::s:: : ::: ::: ::: : ::: ::::::: ::: : ::: :::::::: '!u elqedeooe lo efuej --...!u elqeldeooe to o o o o 0 m o....,,,, : :.,,% : ' 'E d 'E :::::::::::::::::::::::::::::::::::::: o o u elqeldeooe to efuej o o o o o o!u elqelde e to efue ß d o ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::...!u elqelde:}:}e to
32,212 KAHN ET AL.: AEROSOL PROPERTES FROM MULTANGLE MAGNG d - g :!u elqeldeooe o eõuej!u elqelde33e jo eõum!u elqm, deooe jo eõuej :!u elqm, deooe,o oõuej :!u elqm, deooe,o oõuej :!u elqm, deooe,o oõuej,e.,!u alqe,daooe lo aõue,,!u alqe,daooe lo aõue, /,!u alqm, daooe o aõuej!u elqeldeooe jo eõuej!u elqe,deooe jo eõuej
KAHN ET AL.: AEROSOL PROPERTES FROM MULTANGLE MAGNG 32,213 surface temperature (SST) data are often used: the satellte data are assmlated wth n stu measurements from shps and buoys. Satellte data provde a way to "nterpolate" between feld measurements, gvng vastly more nformaton about global, tme-varyng spatal dstrbutons. Wth the help of addtonal constrants from n stu aerosol measurements we plan to use the MSR data products for studes of global aerosol budgets. n work currently underway, we are extendng our prelaunch assessment of MSR senstvty to deal wth mxes of commonly occurrng partcles. Acknowledgments. We thank our colleagues C. Bruegge and J. Martonchk for many dscussons on these topcs. Ths research s supported by the EOS-MSR nstrument nvestgaton and by the Clmate and Radaton Research and Analyss Program n the Earth Scences Dvson of the Natonal Aeronautcs and Space Admnstraton, under R. Curran. 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(e-mal: ralph.kahn@jpl.nasa.gov) (Receved December 16, 1997; revsed Aprl 29, 1998; accepted May 8, 1998.)