optical sky polarimetry and photometry at mauna loa observatory affected by stratospheric dust from...

Optical sky polarimetry and photometry at Mauna LoaObservatory affected by stratospheric dustfrom El Chichon

Walter G. Egan

The Dave vector model is shown to be a good representation of the atmospheric effects on the photometryand polarimetry of the sky above Mauna Loa Observatory (MLO) on 6 Nov. 1982 produced by the El Chi-chon veil. Observations and modeling were accomplished at 0.36, 0.400, and 0.500,um based on fundamen-tal aerosol data. The effects of nonsphericity of the aerosols are included as a previously validated approxi-mation in the input data. Extensive use was made of ground and air truth data available at MLO and theNational Weather Service. From the modeling, the vertical number density of the El Chichon aerosol is de-duced. The sky is found to be unusually bright from the aerosol scattering (radiance of 0.4 MW/m2/sr ata wavelength of 0.500 gm in the southerly and westerly directions at a sensor zenith angle of 600) and the cor-responding percent polarization low (3%). Associated meteorological observations indicate stratosphericheating.

1. IntroductionThe Mauna Loa Observatory on the island of Hawaii

is almost ideally situated for establishing base linemeasurements for aerosol monitoring. The site is farfrom landmasses, and local pollution is normally con-fined to the atmospheric layer below the trade windregime temperature inversion. Furthermore, there isa large complement of locally situated supporting in-strumentation such as atmospheric lidar probes,nephelometers, and meteorological sensors. Becauseof the normal clear atmospheric conditions, Mauna LoaObservatory is also an ideal site for the observation ofthe subsequent specific optical effects of the strato-spheric volcanic dust from the El Chichon eruption thatoccurred 4 Apr. 1982. The dust has persisted in thestratosphere but has moved irregularly to the north andsouth of the original range of latitudes.1

The worldwide monthly location of the volcanic veilhas been charted in considerable detail by Strong et al.,May 1982 to Mar. 1983.1 They used the differencebetween the apparent sea surface temperature sensedby the NOAA-7 AVHRR and the actual to determineareas of IR absorption and thus locate the veil andopacity. In May 1982, the veil was positioned mainly

The author is with Columbia University, Lamont-Doherty Geo-logical Observatory, Palisades, New York 10964.

Received 26 October 1983.0003-6935/84/071013-08$02.00/0.C 1984 Optical Society of America.

along the 15'N latitude. Small regions appeared as faras the 450N latitude in June-Oct. 1982. Also portionsappeared over the Equator in June-Sept. and Dec. 1982.By Nov. 1982, the veil was widely and nonuniformlydistributed in the Northern and Southern Hemisphereswith an especially high concentration in the regionabove and south of MLO.

During the period spanning the MLO observations(5-7 Nov. 1982), the observed vertical visual opticaldepths of the atmosphere above MLO were high (0.3)as contrasted with the normal value of 0.01. The tradewind regime (normally an east wind) has broken down(strong westerly winds), and the sky was unusuallybright.

There is a continuing interest in the effect of thisparticulate material in the atmosphere on short- andlong-term climatic trends. Although the El Chichonveil is gradually dispersing, the question also persistsas to whether, on a worldwide basis, aerosol particles ingeneral are increasing or decreasing in the long term.The associated question of whether the aerosols con-tribute to a net heating or cooling of the earth dependsupon their optical complex index of refraction, sizedistribution, and number density.

A particularly sensitive technique for detecting thepresence and approximate optical properties of aerosolsis achieved by measuring the polarization and pho-tometry of the light scattered and absorbed by them asa function of wavelength. The geometrical variationof the scattered radiation with illuminating and viewinggeometry furnishes additional factors for correlation.

1 April 1984 / Vol. 23, No. 7 / APPLIED OPTICS 1013

The optical scattering and absorption effects of aerosolsare significantly different from those of gaseous atmo-spheric constituents such as oxygen, nitrogen, andozone, and at the high altitude of the Mauna Loa Ob-servatory (3460 m), the effects normally are reduced.

There have been many observations in clear condi-tions of the polarization and photometry of the skyabove Mauna Loa Observatory,2-5 particularly in thezenith direction during sunrise or sunset, to determineatmospheric turbidity. The Coulson2-5 measurementswere made at wavelengths from 0.365 to 0.8gAm (beyondthe Fesenkov6) at one-thirteenth of the optical depthsof Fesenkov.6 The maximum sky polarization wasfound by Coulson2 to be dependent upon the surfacealbedo and was of the order of 80%; for a low surfacealbedo at a wavelength of 0.80 gim, the polarizationapproaches 85%.

11. Atmospheric ModelA computational program that allows calculation of

the photometric and polarimetric sky radiance startingwith the fundamental optical and physical propertiesof the constituent aerosols has been developed byDave.78 The program consists of five separate sections,each involving a separate computer run.

The program computes the intensity, degree of po-larization, direction of polarization, and ellipticity ofpolarization of the scattered radiation emerging fromselected levels of a plane-parallel nonhomogeneous at-mosphere. The model allows the insertion of an arbi-trary aerosol number density and the addition of mo-lecular absorbers such as ozone. The lower bound isassumed to be a Lambert-type reflecting ground of agiven reflectivity; or a Fresnel reflecting ground, witha given optical complex index of refraction, may besubstituted for the Lambert reflecting surface; or theStokes parameters of an actual surface may be insertedinto the program. The input parameters for the Dave78

models are:(1) optical complex index of refraction of the

aerosol;(2) size range of the aerosols;(3) wavelength of radiation;(4) number density distribution of the aerosols;(6) vertical distribution profile for the aerosols;(7) Raleigh atomic and molecular scattering;(8) ozone absorption;(9) other molecular absorption as necessary;(10) solar zenith angle o;(11) sensing zenith angle 0;(12) sensing azimuth angle 0;(13) sensing altitudes;(14) (a) diffuse surface albedo or (b) photometric/

polarimetric Stokes parameters of surface.A considerable number of factors are thus required,

starting with the fundamental optical properties of theaerosols; experiment yields the appropriate valueswhere available. The appropriate selection of inputparameters is indicated for the computer output pre-sented in this paper.

Ill. Observational ProgramsThe verification of the applicability of the Dave

model to the specific optical phenomenon of the ElChichon veil requires measurements of the photometricand polarimetric properties of the sky at a variety ofwavelengths and the characterization of the appropriatephysical and optical properties of the constituentaerosols. The necessary observations were begun atsunrise at MLO on 6 Nov. 1982 and were continued until1 p.m. local time. The MLO temperature and relativehumidity at 7 a.m. local time were 50C (410F) and 12%,respectively, becoming 13'C (55.5 0F) and 55% at noon.A previously described spectropolarimeter/photometerwas used in the program.9 Observations were made atwavelengths of 0.36, 0.400, and 0.500 um at solar ele-vation angles between 10 and 540, sensor zenith anglesof 90, 85, 80, 70,45, and 100 at four compass azimuthaldirections of 0, 90, 180, 2700.

The interference filters for wavelength selection of0.36, 0.40, and 0.50 gm had bandwidths of 0.023, 0.015,and 0.013 gm, respectively (FWHM). Photometric andpolarimetric calibrations were made from a solar illu-minated diffuse white Nextel painted panel that wasconcurrently optically sensed with a Soligor spot pho-tometer configured to permit spectral measurementsat 0.400-, 0.433-, 0.500-, 0.533-, 0.600-, 0.633-, 0.7-, and1.0-gm wavelengths. Furthermore, the directlytransmitted incident solar radiation was measured atthese wavelengths as well as at 1.4 and 1.5 Am with anEppley model NIP Normal Incidence Photometer. Thediffuse plus directly transmitted incident radiation wasmeasured with an Eppley model 8-48 Black and WhitePyranometer. In addition, a Volz photometer was usedto determine optical depths at wavelengths of 0.342,0.380,0.500,0.868,0.946, and 1.67 gm and also to permita determination of the total precipitable water.

The temperature, humidity, wind speed, and direc-tion were also recorded as well as the scattering coeffi-cients from the four-wavelength GMCC nephelometer.Data on the height of the volcanic aerosol layer wereobtained from the lidar.

Rawinsonde data on the temperature, dew point de-pression, wind speed, and direction were obtained fromthe National Weather Service at Hilo, Haw., as well asGOES West visible, Hawaiian Island area, and full earthdisk 11-gm IR imagery.

IV. ResultsDevelopment and presentation of the results involve

basically a threefold process: (1) describing the ob-servational data on the aerosols as input to the vectoratmospheric model; (2) exercising the model with ap-propriate representations of the underlying terrestrialsurface; and (3) comparing the model output with boththe observed optical depths of the atmosphere and thepolarimetric and photometric properties of the atmo-sphere (sky) as a function of wavelength. Following areasonable validation of the model with available ex-perimental data, an inference may be made as to the

1014 APPLIED OPTICS / Vol. 23, No. 7 / 1 April 1984

Table 1. Input Parameters for Vector Atmospheric Programs; Mt. St.Helen's Ash Collected at Spokane, Wash.

Wavelength (m) Index of refraction

0.36 1.435-iO.00022180.400 1.419-iO.0002000.500 1.422-iO.000176

75% Sulfuric Acid0.36 1.452-i(<10-8 )0.400 1.438-i(<10-8)0.500 1.432-i(<10-8)

Table II. Vertical Optical Depths of Atmosphere

Solar zenith angleXAum 10° 50° 360

0.342 0.584 0.667 0.7940.36 (model) 0.535 0.535 0.5350.380 0.441 0.565 0.6540.400 (model) 0.397 0.397 0.3970.500 0.243 0.356 0.4280.500 (model) 0.209 0.209 0.2090.868 0.111 0.133 0.1880.946 0.277 0.569 0.8411.67 0.0556 0.142 0.181

Optically calculatedprecipitable water 0.29 cm 0.48 cm 0.58 cm

RawinsondeWater vapor - - 0.40 cm

composition, quantity, and optical effects, if any, of theEl Chichon aerosol layer. The results now will be de-scribed in the order indicated.

The size range of the modeled aerosols is of radii be-tween 0.005 and 4 Am, and the size distribution will beassumed to be a modified gamma function as a repre-sentative of natural ambient aerosols.10 11 A subsidiarydistribution of small particles is included to account forasperities and edges on the naturally occurring aero-sols.10 Initially, in the modeling, the optical propertiesof the El Chichon ash were assumed to be close to thoseof the Mt. St. Helen's ash that remained aloft longenough to be collected at a large distance from theeruption in 1980 (Spokane, Wash.). The Mt. St. Hel-en's ash is colorless andesitic glass with the compositionand optical properties differing as the distance from thevolcano to the collection site is increased; the materialcollected at Spokane had a lower refractive and ab-sorption component of index than that collected atKelso, Wash. and near Mt. Ranier.12 These are theindices of refraction presented in Table I. However, thematerial in the El Chichon aerosol layer consists mainlyof sulfuric acid,13 14 and the real portion of the indicesof refraction of 75% sulfuric acid15 fortunately is quiteclose to andesitic glass (Table I); thus the scatteringeffect of the El Chichon aerosol will be almost the sameas that for andesitic glass, even though it is composedof sulfuric acid. The effect of the small absorption ofsulfuric acid compared with andesitic glass must beevaluated from the Dave atmospheric model.

Three wavelengths are selected to be checked in thevector model, and these correspond to the polarimet-ric/photometric wavelengths of observations: 0.36,0.400, and 0.500 gm (Table I). The atmospheric par-ticle size distribution was taken as the DiermendjianHaze H [a = 2, y = 1, b = 20, rc(g) = 0.100, a = 1.18E +07] to represent coastal or marine aerosols, as charac-teristic of the region around MLO. Augmentation wasbased on matching the model to the observed volumescattering coefficients at MLO for the Haze H model.The actual aerosol vertical number density profile asused in Parts IV or V of the Dave program is determinedby adjusting the Dave number density distributionabove the height of MLO to match the observed aerosolnumber density at MLO. The number density of con-densation nuclei during the observation period rangedfrom 158 to 224/cc; the number of the larger aerosols wasthen somewhat smaller (100/cc).

However, the vertical aerosol number density of ne-cessity was increased between 16- and 30-km altitudeswith a peak at 23 km to account for the El Chichonaerosol layer. The measured volume scattering coef-ficient at MLO as given by the GMCC four-wavelengthnephelometer for a wavelength of 0.455 gm ranged be-tween 3.33 X 10-7 and 3.96 X 10-7/m. The calculatedvalue, at 0.500,gm using the Dave program and the an-desitic glass optical properties, was 2.3 X 10-7/m,agreement to the observed at 0.455 gim thus beingwithin a factor of 2. The AFGL pressure, ozone, andH20 vertical profiles for the tropical atmosphere16 wereused for the remaining variables in the radiative transfermodel. The surfaces underlying the atmosphere wererepresented as diffuse clouds of 80% reflectivity, whichwas the condition in the valley areas below MLO. Theoutput of the vector programs will be presented on theobservational data graphs to follow.

The comparison observational data for the modelconsist of three groups: (1) the vertical optical depthof the atmosphere (measured times secant of the zenithangle) between 0.342 and 1.67 gim (Table II); (2) thedirectly transmitted and diffuse radiation propertiesof photometric and polarimetric observations of the skyat 0.36-, 0.400-, and 0.500-gm wavelengths in the car-dinal directions (north, south, east, and west) at viewingangles between horizontal and 800 elevation (Figs. 2-4).Figure 5 shows the final vertical aerosol profile thatproduced a reasonable match to the observationaldata.

In detail, Table II lists the observed vertical opticaldepths of the atmosphere above MLO at wavelengthsof 0.342, 0.380, 0.500,0.868,0.946, and 1.67,gm. Thesedata were acquired with a Volz photometer. The ver-tical optical depths were obtained by correcting thephotometer reading for the solar zenith angle at the timeof observation. The large optical depths were inagreement with NIP observations (also in agreementwithin 5% of the MLO Quartz NIP at the time of ob-servation). There is a general increase in optical depthwith increasing solar zenith angle; the amount of pre-cipitable water also increases (as determined by theratio of readings at X = 0.946 to that at X = 0.868). The


6B400

200

SOLAR ZENITH ANGLE

- ...-A--A--A-. -

0.4 0.5 0.6 0.7 0.8 0.9 1.0

x"

Fig. 1. Horizontal direct solar and total radiation at Mauna LoaObservatory on 6 Nov. 1982 as a function of wavelength and solar

zenith angle.

a6us

2:

0

data of Strong et al. 1 indicate the November localizedposition of a very significant portion of the El Chichonaerosol over and south of MLO, which would corrobo-rate the greater optical depths nearer vertical viewingof the sun. True the El Chichon cloud had dispersedby this time but very nonuniformly dispersed.' For theobservation at the solar zenith angle of 36°, at the timeof the Hilo radiosonde, the optically calculated pre-cipitable water above MLO (0.58 cm) was greater thanthat deduced from the radiosonde observations by 0.40cm. Since sulfuric acid in varying concentrations ab-sorbs in roughly the same spectral regions as water, al-though much more strongly, the large optical depth of0.841 is attributed to the aerosol veil. The normal vi-sual optical depth is <0.01 in the absence of the ElChichon aerosol veil, and the additional visual opticaldepth also is inferred to be caused by the nonuniformly

OBSERVED MODELNX-_ N

E U------- E WA---- W +

1.0 - _j

EMODEL

B as ~~~~"/EOBS

id 0.5 /x\WMoDEL

* NOBS s

0 10 20 30 40 50 60 70 80

SENSOR ELEVATION ANGLE, DEG

(a)0 10 20 30 40 50 60


(a)

OBSEflVED MODELNX-- N nSO S AE° ------- E *WA---- W 4-

10NMODEL

30 ' ho^; | * @ N jE~MODEL

-VN~N + ~ ~ ~~

20~ ~ 4.~~ . +> EOBS;;*...NOBS

10 \ f \< WOBS-'-, WMODE L o

* y SMODEL _0

0 10 20 30 40 50 60 70 80SENSOR ELEVATION ANGLE, DEG

(b)

70 80

OBSERVED MODELNX -__ N SO S AEO------- E WA---- Wx+

60 r-

40

N

R 30,

0

0

I,/-A A

> EOBS EMOD+ 0 l 0

10 20 30 40 50 60 70


(b)

80 90

Fig. 2. (a) Photometric; (b) polarimetric. Optical properties of skyabove Mauna Loa Observatory 6 Nov. 1982, X = 0.36 Am, and com-parison to Dave vector model; solar zenith angle of 400 3 in four

azimuthal directions. Lines denote observations.

Fig. 3. (a) Photometric; (b) polarimetric. Optical properties of skyabove Mauna Loa Observatory 6 Nov. 1982, X = 0.400 pm, and com-parison to Dave vector model; solar zenith angle of 550 3 in four



.:

0

. . . . . . . . .J

nc

4

I

1

l

70rdistributed aerosol veil. There were no cirrus cloudsobserved during the morning measurements.

The inverse wavelength dependence of the opticaldepths would indicate that particles are smaller thanthe radiation wavelength (perhaps of average size of-0.1-gm radius).

The observed properties of the incident radiation areshown in Fig. 1 and Table III. In Fig. 1, the directlytransmitted radiation on a horizontal surface (solidlines) increases with solar elevation. These readings

601-

6

aI-I-

As

OBSERVED MODELNX _ . N SO S AEO------- E WA---- W +

WIg

1.01

E

I 0.5z:

a4r

z0

N

44S0o

K

50

40

30

20

10

A dSMODEL

WMODEL

20 30 40 50 60


(a)

OBSERVED MODEL

* NX..__ N SO S

EW------- E WA- - W +

ENMODEL

OEMODEL

N\ +WMODELA

5MODEL

70 80 90


(b)

Fig. 4. (a) Photometric; (b) polarimetric. Optical properties of skyabove Mauna Loa Observatory 6 Nov. 1982, = 0.500 pm, and com-parison to Dave vector model; solar zenith angle of 450 3 in four


K

K

K

I-,K_~

10i6

105 10-4 103 10 2 10.1 100 101 102 103 104

PARTICLE NUMBER DENSITY, NO./CC

Fig. 5. Vertical aerosol profile from the 32-layer Dave vector radi-ative transfer program.

were made using the Eppley pyrheliometer correctedfor solar zenith angle and appropriate narrowband in-terference filters. The total radiation of a horizontalreference standard (a white Nextel painted panel) is alsoshown in Fig. 1 (dashed line). These determinationswere made with calibrated photometers (Soligor andHoneywell). Measurements beyond 1-gtm wavelengthwere not possible because of lack of sensor response.The rise of the curves at 0.4gm is the result of Rayleighsky scattering.

The numerical values for the total observed irra-diance are given in Table III. The ratio of specular tototal is seen to increase for lower solar zenith anglesbecause of a decreased scattering path through thevolcanic aerosol and attendant lower scattering.

The photometric and polarimetric observations areshown in Figs. 2-4; the first figure of each group presentsthe photometry, and the second figure presents thepolarization. Measurements were made at wavelengthsof 0.36, 0.400, and 0.500 gm with both photometric andpolarimetric measurements being made simultaneouslyfor a given geometry and wavelength. The photometricresults will be discussed first and then the polari-metry.

Table Ill. Characteristics of Incident Radiation

Observations ModelSensor Surface Surface

elevation direct Total Ratio direct Total Ratioangle irradiance irradiance direct irradiance irradiance direct(deg) (W/m 2) (W/m 2) total (W/m 2) (W/m 2) total

10 93 201 0.460 79 184 0.42935 595 750 0.793 571 726 0.78755 779 847 0.920 904 1068 0.876


A. PhotometryFigure 2(a) presents the sky radiance at a wavelength

of 0.36 gm; the observations are shown as lines throughthe data points in the four cardinal magnetic compassdirections. The solar zenith angle was 40° t 30 duringmeasurements, but only one model, with a solar zenithangle of 40°, was used because of the small range of solarzenith angles. The model results are shown as indicatedin the legend. In general, the model results are belowthe observations by a factor of 2. The southerly di-rection has the highest radiance because of the southerlydirection of the winter sun. Although the models showthe same brightness at zenith, the south and west ob-servations are larger than those north and east at asensor elevation angle of 800; this appears to be the re-sult of a nonuniform sky near zenith and caused by af-ternoon thin cirrus clouds and time lapse between themeasurements. The 0.36-gm measurements were madeshortly after noon as clouds began to form.

Near the horizon (sensor elevation angle of 10°), theradiance increased from increased atmospheric scat-tering, being highest in the sun direction of south. Thesouth and west directions radiance at 0° elevation angleis low because of low reflectivity volcano fields that wereviewed by the sensor, whereas in the north and east thesensor viewed, in part, some high reflectance cumulusclouds in the adjoining valley.

In Fig. 3(a), the photometry for a wavelength of 0.400gim is shown; the solar zenith angle was 550 i 30, char-acterizing an early morning observation. Again theobservational data are represented by lines through thedata points and the model (for 550 solar zenith angle)as indicated in the legend. Here the easterly directionhas the highest radiance because of the sunrise in theeast. It is seen that near zenith (800 sensor elevationangle) all four observations lie close together indicatinga uniform sky for these observations; there is also closeagreement with the model. Again, toward the horizon(sensor elevation angle of 100), the increased scatteringcauses the sensed radiance to increase. The modelshows close agreement in the north, south, and westdirections.

The photometric observations at X = 0.500 gim arepresented in Fig. 4(a), again as solid lines through thedata points for a solar zenith angle of 450 i 3; themodel (solar zenith angle of 450) results are showndesignated in accordance with the symbols of the leg-end. The near-zenith observations (sensor elevationangle of 800) show some evidence of a slight sky nonu-niformity, with the southerly observed radiance beingdecreased because of distant cirrus clouds (evident inthe satellite imagery). Since the sun was in the south-erly direction, the observed radiance should have in-creased as the sensor elevation angle decreased. Thesky radiance at X = 0.500 gm is less than that at 0.36 gmin the north and east directions. The model at X =0.500 gim is quite good for all directions with the ex-ception of south at the lower sensor elevation angles.

B. PolarimetryThe polarimetric trends are more sensitively depen-

dent on the model than radiance. As indicated by theresults in the photometry section it appears that wehave a reasonable modeling approach to account for theaerosol size distribution that accounts for aerosol edgesand asperities of El Chichon veil. In Fig. 2(b) ( = 0.36AM), there is remarkable agreement between the ob-servations and the model compared to previous effortsfor altitudes near sea level.17 The west model producestwice the observed polarization, and there is a consid-erable difference between the near-zenith model po-larizations (800 sensor elevation angle). The modelpolarizations are high, which would indicate inadequatenear-zenith depolarization (scattering) in the modelcompared with the actual observational condition.This conclusion is borne out in the fact that, as notedin the corresponding photometry section, thin cirrusclouds were developing that served to increase atmo-spheric scattering near the zenith and thus reduce po-larization as shown. The polarization increases nearthe horizon, which is to be expected from the phasefunction behavior in Mie scattering angles (between 120and 180°), the polarization generally is large for non-absorbing or weakly absorbing spheres. For smallerscattering angles and for the usual distribution of par-ticle sizes, the polarization decreases. The north, south,and west polarizations are high at the horizon becausesurface polarization increases with the phase angle (theangle between the sun and sensor measured in the planecontaining the incident solar direction and the sensorviewing direction). The easterly direction has thesmaller phase angle.

The polarization for X = 0.400-gm wavelength ispresented in Fig. 3(b). The absolute agreement be-tween the model and the observations is not as good asfor A = 0.36 gim. The problem appears to lie in theadequacy of the plane parallel geometry model to rep-resent the actual physical conditions at high solar zenithangles, such as that existing at the time of observation(550 i 3°). Refractive effects begin to have a signifi-cant effect at 550.18 The lack of inclusion of horizontalatmospheric inhomogeneities and the curvature of theearth appear to be the major deficiencies in the model.However, the observational trends are followed in themodel for the most part.

The polarimetric properties of the sky at =0.500-gm wavelength are shown in Fig. 4(b). Here themodel depicts higher polarization near the zenith(sensor elevation angle of 80°), indicative of increasedatmospheric scattering at the time of observation pos-sibly from thin cirrus clouds. At lower sensor elevationangles, the agreement between the model and the ob-servations becomes better, even near the horizon. Thenorth and east polarizations are higher than the southand west because of the higher scattering phase anglesof the former.


V. DiscussionThus it is seen that the Dave vector model reasonably

represents the atmospheric photometric and polari-metric properties when accurate input parameters areemployed. The inferred particle vertical numberdensity distribution for the El Chichon aerosol thatproduced the matches to the observations is shown inFig. 5 and the associated Rayleigh and Mie scatteringand absorption in Fig. 6. As indicated (shown in Fig.5), the number density at 23 km was increased to givethe best fit to all photometric and polarimetric obser-vations with an ordinary Braslau and Dave distributionbelow 16 km and above 30 km; also, as previously indi-cated, the overall distribution was adjusted to producea number density of 100/cc at the altitude of MLO.From the number density and size distribution, one mayobtain a rough index of the amount of volcanic ash in thestratosphere; a rough calculation shows it to be of theorder of the 4 X 106 tons previously calculated on aworldwide basis.19

As mentioned previously, the optical properties of theEl Chichon ash were taken to be either sulfuric acid orandesitic glass, both of which have nearly the same realindex of refraction. Fortunately, a small change in thereal index of refraction has negligible effect on thescattering' 0 but would contribute to the absorption byallowing more or less incident radiation to enter theaerosol particles and be absorbed (depending uponwhether the real index is lower or higher than thenominal assumed). The spectral region of higher ab-sorption of the andesetic glass (which may compose only5-10% of the dust veil) is not large enough to cause anappreciable effect on the aerosol scattering' 0 but wouldonly cause a slightly increased absorption. Thus thecalculated scattering properties of the aerosol cloud arevalid.

As seen in Fig. 6 at 0.500-gm wavelength, the aerosolveil with a maximum at 23-km altitude is a strong Mie

scatterer but only a weak absorber. There is no ques-tion that the large scattering from the El Chichon veilwill effect a redistribution of the incident solar radia-tion. The inferred particle size distribution peaks at0.1-gm radius at the lower end of that deduced by DeLuisi et al.2 0

An examination of the radiosonde temperature datashows that the El Chichon aerosol layer was heated inits location above the tropopause at 38-mbar level (22.4km) (see Fig. 7). The normal lapse rate, for 5-7 Nov.1981, is to be compared with the daytime 5-7 Nov. 1982.

x

l40 L

30

E

a0

20

10,

0

K

MIE SCATTERING(ABSCISSA x 10)

, ABSORPTION - I

4.RAYLEIGH SCATTERING

vF/ (ABSCISSA x 10 )

MLO ALTITUDE

0 100 200

EL CHICHONAEROSOL CLOUDRANGE

300

LAYER OPTICAL THICKNESSFig. 6. Plot of modeled Mie and Rayleigh scattering and absorption(at a wavelength of 0.500 gm) vs altitude showing scattering and ab-

sorption by the El Chichon aerosol cloud.

POTENTIAL TEMPERATURE

0C,

ALT(FT/M)

78244/23849

67507/20576

Fig. 7. U.S. Air Force Skew T, logp diagram of theatmosphere between the 100- and 20-mbar levelswith Hilo, Haw. radiosonde data for 5-7 Nov. 1981and 5-7 Nov. 1982; 5-7 Nov. 1981 (---), 5-7 Nov.

1982 (- ), 6 Nov. 1982, at night (---).

60504/18442

53083/16180

0 C' C, 0,


20

25

30

40 E

Lud

50

60

70

80

90

. E i

The effect of the El Chichon aerosol veil (peaking justbelow the 30-mbar level) is to cause a stratospherictemperature rise of 200C, enough to disrupt the normalstratospheric circulation pattern that initiates easterlytrade winds. As expected, at night (6 Nov. 1982) thetemperature decreases somewhat because of the lack ofsolar heating of the aerosol layer. The higher temper-atures caused the air mass to rise, concurrently liftingthe air above it and allowing the air mass below it toexpand upward. An approximate indication of therelative vertical air mass motion is the average verticaltransect times of the radiosonde baloons: the time totraverse from the 100- to 20-mbar levels was 54 seclonger on 4-7 Nov. 1981 than 5-7 Nov. 1982; the averagetotal time to rise from the ground to the 20-mbar levelwas correspondingly longer. The total time to rise tothe 20-mbar level is -100 min, resulting in an averagevelocity of -5 m/sec. Vertical stratospheric currentsof a few cm/sec exist2l and could be conjectured to causethe difference. It is interesting to note that the averageprecipitable water vapor derived from the radiosondeobservations was 3.76 cm on 4-7 Nov. 1981 and 3.08 cmon 5-7 Nov. 1982 (i.e., less precipitable water vaporunder the El Chichon veil).

VI. ConclusionsThe Dave vector model appears to be a good repre-

sentation of the atmospheric effects on the photometryand polarimetry of the sky above MLO on 6 Nov. 1982,produced by the El Chichon cloud veil. The validmodeling at 0.36, 0.400, and 0.500 gim, where strongaerosol and molecular scattering occurs, would indicatethe applicability of the model at longer wavelengthswhere less scattering occurs. However, the model doesnot include emission and thus is inapplicable beyond2.5-gim wavelength. The technique of using a subsid-iary distribution of aerosols to account for the effect ofaerosol nonsphericity appears adequate but is in needof theoretical validation. The Dave atmospheric model,as well as any atmospheric model, benefits greatly fromaccurate ground truth inputs. The Dave model, beingstructured to permit comparison of calculated withexperimental data, has a distinct advantage. Thusexperimentally observed volume scattering (and ab-sorption) coefficients may be compared with thosecalculated using what may be assumed to be correctaerosol indices of refraction and particle size distribu-tions; the correctness of these latter assumptions thenmay be validated before the subsequent atmosphericradiative transfer calculation is made.

The Dave model is limited in applicability to thesmaller solar zenith angles where the horizontal inho-mogeneity of the earth's atmosphere or earth curvaturemay be neglected. Also, even at smaller solar zenithangles, the model is applicable only to cloud-free con-ditions and higher altitudes.

The author wishes to thank K. Coulson and the ob-servatory staff for their cooperation in making the fa-cilities in MLO available for measurements; H. Tatumand staff of the Hilo, Haw. weather station for providing

extensive radiosonde and meteorological data; B.Bodhaine for data on the MLO aerosol GMCC nephe-lometer and condensation nuclei counter; T. DeFoorand J. DeLuisi for the MLO lidar data, E. Dutton for theMLO radiometer calibration data, and S. Oltmans forthe MLO ozone data.

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Seventeenth International Symposium on Remote Sensing ofthe Environment (Environmental Research Institute of Michi-gan, Ann Arbor, 1983); also Geofis. Int. 23, 3 (1984).

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3. G. A. Herbert, Ed., Geophysical Monitoring for Climatic Change8, U.S. Department of Commerce, NOAA, ERL, Boulder, Colo.(Dec. 1979).

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Polydispersions (American Elsevier, New York, 1965).12. W. G. Egan and J. E. A. Selby, "Atmospheric Radiation Transfer

Effects from Mt. St. Helens Eruption Ash," at 1980 InternationalRadiation Symposium, Ft. Collins, Colo.

13. J. M. Rosen and D. J. Hoffman, Trans. Am. Geophys. Union.64,197 (1983).

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S. Garing, Optical Properties of the Atmosphere, AF CRL-72-0497, Environmental Research Paper 411, Bedford, Mass.

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18. L. L. Smith, Grumman Research & Development Center; privatecommunication (1983).

19. N. M. Ashok, H. C. Bhatt, T. Chandrasekhar, and J. N. Desai,Nature London 300, 620 (1982).

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21. T. J. Dunkerton, J. Geophys. Res. 88, 10,831 (1983).


optical sky polarimetry and photometry at mauna loa observatory affected by stratospheric dust from...

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