Measurements of size and composition of ... - Wiley Online Library

JOURNAL

OF GEOPHYSICAL

RESEARCH,

VOL. 94, NO. D14, PAGES 16,481-16,491, NOVEMBER

30, 1989

Measurements of Size and Composition of Particles in Polar Stratospheric Clouds From Infrared Solar Absorption Spectra S. KINNE

AND O. B. TOON

NASA Ames Research Center, Moffett Field, California

G. C. TOON AND C. B. FARMER NASA Jet Propulsion Laboratory, Pasadena, California E. V. BROWELL

AND M.P.

MCCORMICK

NASA Langley Research Center, Hampton, Virginia

The attenuation of solar radiation between 1.8- and 15-/am wavelength was measured with the airborne (DC-8) Jet Propulsion Laboratory Mark IV interferometer during the Airborne Antarctic Ozone Expedition in 1987. The measurementsnot only provide information about the abundance of stratospheric gases, but also about the optical depths of polar stratospheric clouds (PSCs) at wavelengths of negligible gas absorption. The spectral dependence of the PSC optical depth contains information about PSC particle size and particle composition. Thirty-three PSC cases were analyzed and categorized into two types. Type I clouds contain particles with radii of about 0.5/am and nitric acid concentrationsgreater than 40%. Type II clouds contain particles composed of water ice with radii of 6/am and larger. Cloud altitudeswere determinedfrom 1.064-/ambackscatteringobservations of the airborne (DC-8) Langley DIAL lidar system. Based on the PSC geometrical thickness, both mass and particle density were estimated. Type I clouds typically had visible wavelength optical

depthsof about0.008,massdensities of about20ppb,andabout2 particles/cm3. Theobserved type II clouds had optical depths of about 0.03, mass densities of about 400 ppb mass, and about 0.03

particles/cm 3. Thedetected PSCtypeI cloudsextended to altitudes of 21 km andwerenearlyin the ozone-depleted region of the polar stratosphere. Although PSC type II clouds under different temperature regimes may extend to similar high altitudes, the observed type II casesduring September were predominantly found at altitudes below 15 km. Some of these low-altitude clouds may have been associatedwith cirrus clouds that extended well below the DC-8 aircraft. Simultaneous spectroscopic measurements of nitric acid vapor within the polar vortex (Toon et al., this issue) display lower concentrationsin the presence of PSC type I clouds, the deficit being close to the amount of solid nitric acid inferred from the optical depths of these PSCs. This further supportsthe view that the type I PSCs contain

1.

frozen

nitric

acid.

INTRODUCTION

Polar stratospheric clouds (PSCs) have been observed from the ground for more than a century [Stanford and Davis, 1974], and during the last decade they have been observed extensively by polar-orbiting satellites [McCormick et al., 1982] and lidars [Poole and McCormick, 1988]. However, interest in the properties of these clouds has greatly increasedwith the discovery that PSCs might play a role in the chemistry which creates the ozone hole [Solomon et al., 1986;McElroy et al., 1986;Molina et al., 1987; Tolbert et al., 1987]. Several cloud characteristics need to be better understood,including the geographicand temporal extent of the clouds, the sizes and phases of the cloud particles, as well as their chemical composition. Until recently the clouds had only been observed remotely. Satellite and lidar observations [McCormick et al., 1982; McCormick and Trepte, 1987; Poole and McCormick, 1988] indicate that PSCs are widespread, generally optically

thin, persistent during the polar winter, and occur in two basic types. Theoretical and laboratory studies [Toon et al., 1986; Crutzen and Arnold, 1986; Wofsy et al., 1988] also suggest that two cloud types probably occur: clouds of half-micron sized particles composed of concentrated nitric acid solutions (PSC type I) and clouds of ice particles larger in size by at least 1 order of magnitude (PSC type II). During the Airborne Antarctic Ozone Expedition (AAOE), data were collected in PSCs with the ER-2 aircraft [Ferry et al., this issue; Fahey et al., 1989; Gandrud et al., 1989; Pueschel et al., 1989; Goodman et al., this issue]. The data support the cloud type I and cloud type II classification and confirm their suggestedparticle size, phase, and composition. The ER-2 data acquisition, however, was limited to a region equatorward of 72øS latitude, and most data were taken during conditions in which mountain waves were present [Gary, 1989]. Such wave clouds may have different properties than clouds formed under the slower cooling conditions

Copyright 1989 by the American Geophysical Union. Paper number 89JD00603. 0148-0227/89/89 JD-00603 $05.00

over the Antarctic

continent.

In order to complement and extend the ER-2 data, PSC observations will be presented based on infrared measurements of solar extinction made by the Jet Propulsion Labo16,481

16,482

KINNE ET AL.: PARTICLESIN SOLARSTRATOSPHERIC CLOUDS

aircraft, but such clouds were sometimes present and did affect the infrared

I""-e,

....

observations.

-.....................

3.

THEORY

DC-8

11km

"•

"--,'L

ANTARCTICA

\

Fig. 1. Experiment. Airborne attenuationmeasurementsof solar radiationthroughpolar stratosphericclouds(PSC) with the JPL MARK IV interferometer at infrared wavelengthsare usedto detect cloud vertical optical depths r. Typical solar zenith angles{9 were between

82 ø and 89 ø.

The interferometer's primary purpose is the detection of gas absorption lines in order to make abundance measurements of stratospheric gases. Spectroscopic measurements, however, also reveal the presence of particles in spectral regions of negligible gas absorption (windows). Stratospheric clouds above the aircraft reduce the measured intensity according to Beer's extinction law. Therefore, once an observation made with particles present (cloudy case) is matched with an observationwithout particles (clear case) under an identical solar zenith angle, then the cloud vertical optical depth at a particular window wavelength can be determined

r = [ln (I0) - In (I)]/X ratory (JPL) Mark IV interferometer [Farmer et al., 1987; Toon et al., this issue] on board the DC-8 aircraft. Although this instrument was designedand its operation optimized for the measurement of stratospheric trace gases, the wide simultaneousspectral coverage of the instrument offered the potential for determining cloud properties. The extent to which cloud optical depths and cloud particle sizes and compositionscan be derived from these spectroscopicmeasurements will be addressed in this paper. Below, the instrumentation and the theoretical aspects of the data analysis are explained first. Then the quality and usefulness of the available data are discussed. Finally, results for selected cases are presented. 2.

(1)

Here r representsthe vertical optical depth and I 0 and I are the measured window intensities under clear and cloudy conditions. An index indicating their wavelength dependence has been omitted.

X is the factor by which the line of sight optical depth exceeds that in the vertical. For a plane parallel cloud layer the value of X is defined by the ratio between the line of sight cloud path ds and the vertical cloud path dz.

X = ds/dz = [(a2- c2)V2_ (b2_ c2)V2]/(a - b)

(2)

with a = r e + zt, b - re + Zb, and c = (r e + za) sin {9, where zt, Zb, and za are the altitudesof cloud top, cloud bottom, and observer(aircraft), r e is the Earth's radius, and {9 is the solar zenith angle.

INSTRUMENTATION

X is a function

The PSC properties presented in this paper are based on airborne (DC-8) measurementsfrom the JPL Mark IV interferometer [Farmer et al., 1987; Toon et al., this issue]. This instrument records high-resolution solar spectra over the

entire1.8-to 15-/•m(670-5500 cm-•) spectral regionsimul-

of the cloud altitude

above the observer and

the solar zenith angle, as Figure 2 illustrates. Only at small solar zenith angles ({9 < 85ø) may X be approximated by the inverseof the cosineof the solarangle(X = sec {9 - 1//• with /• = cos {9). For solar zenith angles in excess of 85ø the necessary information about cloud thickness and altitude

taneously. An InSb photodiode measured the short wavelengths(1.8-5.4/•m), while a HgCdTe photoconductormeasures the long wavelengths (5.2-15 /•m). During aircraft operation a maximum optical path difference of 30 cm was -1 employed, giving rise to a spectral resolutionof 0.020 cm

was derived

taneous interferometer

size.

from simultaneous

lidar measurements.

It is possible that a case which appears to be clear may contain particles in low concentrations.Such an error will lead to an underestimation of the cloud optical depth in cloudy cases.However, for the analysisof the cloud particle anda spectral pointspacing of 0.015cm-• . Sucha spectrum properties the spectral dependenceof the optical depth is took 54 s to acquire, during which time the aircraft typically more important than the absolute magnitudeof the optical moved 10 km. The interferometer viewed the sun through a depth. The spectraldependencecontainsinformation about 6-in-diameter wedged plate of ZeSe which replaced a cabin cloud particle size and composition. window. This restricted the field of view to a 10 ø cone The effect of particle size on the spectrumis illustrated in projecting from the left side of the fuselage, as illustrated in Figure 3. Over the spectral range of the interferometer, the Figure 1, and limited spectroscopicmeasurementsto solar extinction relative to that of 0.55-/•m wavelength was calculated using Mie theory with a wavelength independent zenith angles ({9) larger than 80ø. Also operated from the DC-8 platform was the Langley refractive index of (1.3, 0.5) and radii ranging from 0.25 to 4 DIAL lidar system [Browell et al., 1983]. It simultaneously /•m. Submicronsize particles rapidly loose their ability to transmits at four laser wavelengths (0.301, 0.311, 0.622, and attenuate toward longer infrared wavelengths, while parti1.064 /•m). The infrared lidar measurementswere used to cles with radii of several microns display an almost wavedetermine the altitudes at which clouds were present. These length independentextinction. Thus the slope of the extinccloud altitudes, however, may not represent those of simul- tion ratio servesas an indicator for the typical cloud particle observations, because the lidar

viewed in an upward direction, while the interferometer viewed the sun near the horizon. In addition, the lidar could not

observe

clouds

within

the

first

kilometer

above

the

The calculationspresentedin Figure 3 and also those later in this paper are valid for spheres.Fortunately, under partly absorbing conditions the optical depths of nonspherical

KINNE ET AL.' PARTICLESIN SOLAR STRATaSPHERICCLOUDS

PathlengthAmplification for 1.0kmCloud A/Cat 11.0km

16,483

Refractive Indices for Ice, Water, 10% 20% 40% 70% Nitric-Acid 1.65

1.25

•Cloud Bottom o! 12kmaltitude

0.85

14kmaltitude

% 18 km altitude

g 10-' ß• o

10-2

•oø

Wavelength [/zm ] 80.0

85.0

90.0

95.0

Solar ZenithAngle Fig. 2. Pathlength increase of direct solar radiation through a horizontally homogeneous layer for large solar zenith angles. The cosinecorrection(1/ta)is comparedwith actual pathlengthincreases for aircraft observations at 11-km altitude as they relate to 1km-thick clouds with bases at 12-, 14-, and 18-km altitude.

particles do not differ significantlyfrom those for spheresof equal surface area [Pollack and Cuzzi, 1980; Takano and Liou, 1989]. Thus the presented extinction ratios apply to nonspherical shapesas well. The constant refractive index of (1.3, 0.5) used in Figure 3 is unrealistic, as spectral variations related to particle phase and composition must be taken into account. Figure 4 displays the infrared refractive indices for ice [Warren, 1984], for liquid water [Downing and Williams, 1975], and for liquid nitric acid solutionswith concentrations of 10, 20, 40, and 70% by weight [Querry and Tyler, 1980]. Refractive indicesfor solid nitric acid particles have not been measured (PSC type I particles may be solids). However, absorption spectra for liquid solutions [Leuchs and Zundel, 1978] and corresponding solid nitric acid hydrates [McGraw et al.,

Sizes:r-.25/.50/1.0/2.0/4.0/8.0/zm --iz}- --o-

-•--

-+-

-x--

Fig. 4. Refractive indices. Infrared refractive indices are displayed for water [Downing and Williams, 1975] (solid line), ice [Warren, 1984] (dashed line), and nitric acid solutions with concentrations of 10, 20, 40, and 70% [Querry and Tyler, 1980] (solid lines with symbols).The differencesbetween ice and water are marked by the shaded areas. The upper half of the figure displays spectral differences in the real part; the lower half shows spectral variations for the imaginary part of the refractive index.

1965] are similar, particularly in the spectral range between 2- and 5-ttm wavelength. To illustrate the effects of particle composition on the infrared extinction ratio, calculations similar to those in

Figure 3 were performed usingrefractive indicesfor ice. The results are displayed in Figure 5. A comparison between Figures 3 and 5 shows that extinction variations due to refractive index changesare superimposedon the extinction slopes defined by the particle size. The variations correspondingto features in the refractive indices of ice and water are particularly large at 3- and 11-ttm wavelength. Extinction ratios for ice, water, and four nitric acid solutionsare compared in Figure 6. The results are based on the refractive indices of Figure 4 and displayed for particle radii of 0.5 and 4/am. Differences between water and ice are indicted by the shadedareas. Significantchangesare caused by an increasing nitric acid component. At most wavelengths, nitric acid has a larger imaginary index than water or

s- 1.5 RI- [L3,0.5]

---0--

Sizes:r-.25/.50/1.0/2.0/4.0/8.0/zm -la---O-

--•-

-+-

•

s - 1.5 RI-lICE ]

--O-

u• o

.>_ o

._

X

Wavelength [ /zm ] Fig. 3. Particle size effect on the infrared extinction. The infrared extinction is relative to that at 0.55 tam and is shown for six different particle sizes. In the Mie calculation, lognormal particle size distributionswith mode radii of 0.25, 0.5, 1, 2, 4 and 8 tamwere used. A geometric standard deviation of the size distribution of 1.5 and a constant refractive index [1.3, 0.5] were assumed.

'0

100

Wavelength [/xm ] Fig. 5. Infrared extinction ratios for six different ice sphere sizes. Apart from the use of refractive indices for ice, the calculations are identical to those in Figure 3.

16,484

KINNE ET AL.' PARTICLES IN SOLAR STRATOSPHERICCLOUDS

Ice,Wofer,10%20%40%70%Nifric-Acid r=4/.5/zm s=1.5 1.1

4.0

0.9

i

0.8 ,,

0.5

0.7

.. ......83 o

,

0.6 0.5 0.4

.

.WINDOW, i

,

•15.0

I

I

$$16.0

$•17.0

loo

Wavelength [/zm ]

•

Fig. 6. Combined particle size and composition effects on the infrared extinction ratio. Based on the refractive indices in Figure 4, extinction ratios were calculated for lognormal particle size distributions with mode radii of 0.5 and 4/xm and a geometric standard deviation

of 1.5. The differences

between

0.0

o

• •ß

-0.1 -0.2 -0.3

ice and water are indicated

by the shaded areas.

Wovenumber[ 1/cm] Fig. 7. Window selection based on a comparison of measured intensities of direct sunlight for different solar angles under clear conditions. Intensities of the 2.7-/xm CO2 band for solar zenith angles of 83• (dotted line) and 89• (solid line) are compared in the upper graph. The lower graph showstheir difference, which clearly marks spectralregionsof CO: absoftion at 3315.70 and at 3317.35

ice. This reduces the spectral extinction slope for submicron-sized particles. Also nitric acid has an absorption feature at 8-/xm wavelength. The most significanteffect of adding nitric acid to submicron-sizedwater particles is the cm-• andH20 absoftionat 3317.20 cm-•. Theselected window 73 datapoints, degree of smoothingof the extinction ratio at 3-/xm wave- between3316.025and 3317.125cm-1 represents reflecting the high spectral resolution of the inte•erometer. The length. small differenceswithin the window are causedby inst•ment noise. Thus, based on the comparison between measured win- Thedipobse•edformeasured intensities at 3316.60 cm-1 isa solar dow optical depthsand calculatedextinction ratios, not only feature, and its absoftion depth is not affected by changes in the typical particle sizes but also possible nitric acid concentra- solar zenith an•e. tions can be determined.

This assumes that nitric acid is the

dominant nonwater component in PSC particles, as measurements with the ER-2 indicate [Fahey et al., 1989; Gandrud et al., 1989; Pueschel et al., 1989] and that effects due to traces of other components such as sulfuric acid are minor. The particle size determination, however, is limited by the spectral range of the interferometer. Only particles with radii that do not exceed 20/xm are detectable because all larger particles cause a neutral extinction ratio. Once the vertical optical depth has been determined, the cloud column mass and particle number for a given particle size can be found. Since the lidar providesinformationabout cloud thickness,even estimatesof massand particle number density are possible. 4.

DATA

ANALYSIS

Spectroscopic measurements taken with the Mark IV interferometer during DC-8 flights over Antarctica September 5, 8, 14, 19, 21, 24, and 26 as well as during the ferry flight over the Pacific Ocean on October 1 have been analyzed. The Pacific data were chosen for the determination of the infrared window regions becausethey were taken under clear conditionsat mid-latitudes, eliminatingthe possible appearanceof PSCs. Windows

were

identified

when

measured

intensities

re-

mained relatively constant despite changes in the solar zenith angle. Figure 7 illustrates a window selection in the 2.7-/xm CO•_band. The upper panel presentsintensitiesof airborne spectroscopicmeasurementsfor solar zenith angles of 83ø and 89ø; the lower panel displays their difference.

Large differencesindicate gaseousabsorption, i.e., in Figure 7 absorptionby H•_Oand especiallyCO•_to both sidesof the selected window.

Small differences

are related to instrument

noisethat greatly exceededlevels of ground-basedmeasurements [Farmer et al., 1987]. Twenty-four window regions selected 1.9- and 13-/xm

wavelength are listed in Table 1. Spectral window regions are as narrow as 0.1 cm- • and as wide as 8 cm-•. Given the

instrument's spectral gridspacing of 0.015cm-• , thiscorrespondsto 7 and 500 data points in each window. Most of the spectral window regions are at shorter wavelengths. Their spectral positions are also marked by solid circles on the abscissaof the lower graph of Figure 8. The majority of the almost 700 spectroscopicmeasurements during September DC-8 flights over Antarctica were taken under clear or almost clear conditions. Cloudy cases were identified by sudden intensity losses in the selected window regions. The detection of optical depths is based on logarithmic intensity differences (see (1)). Thus accurate window data for both the clear and the cloudy case are required. In order to minimize the effect of the instrument noise, windowaveraged intensities are used in the analysis. The clear and

cloudy typical standarddeviation are at least 2.5% each, for a total uncertainty of about 5%. For a solar zenith angle of 88ø, such an error correspondsto a vertical optical depth of about 0.002, while for an angle of 80ø it correspondsto a vertical optical depth of about 0.008. Figure 8 confirms the magnitude of this noise-related uncertainty. The spectra

KINNE ET AL..' PARTICLESIN SOLAR STRATaSPHERICCLOUDS

TABLE

1.

Summary of 24 Windows

?

16,485

Day:21.September 1987

._

Time11h +...

.>_

Wavelength, Total Data Band

/•m

Points

Spectral Range in Wave number,

cm-]

No. of Data Points in

Spectral Region

._ ,-: c•

_-:: 40.0,

:

4•.0

--:.:::.:--•42.0

•-'

_::.::.:.::::.•-':.;:.::-:....

4s.0

44.0

46.0

•.0

additional Time[ minutes ]

HgCdTe Photoconductor 1 2

13. 12.

7 112

3

10.

166

4 5 6

7

8.9 8.2 7.2

5.5

11 33 31

128

8

5.3

96

8

5.3

97

9

5.1

425

10 11

4.7 3.9

28 186

12

3.7

385

13

3.5

43

14 15

3.2 3.1

17 808

16

3.0

139

17

2.9

207

18

2.5

136

19

2.4

284

20 21 22

2.3 2.2 2.1

57 173 515

23 24

2.0 1.9

43 371

InSb

766.850-766.950 831.000-832.000 837.000-837.700 948.000-949.325 960.625-961.625 967.525-967.700 1128.475-1128.650 1214.000-1214.500 1371.500-1371.625 1391.750-1391.900 1409.500-1409.700 1786.150-1786.375 1827.175-1827.425 1849.650-1850.000 1854.750-1855.200 1857.500-1858.150 1881.400-1882.600 1899.775-1900.025

7 66 46 87 67 12 11 33 8 10 13 15 17 23 30 43 79 17

Photodiode 1881.400-1882.600 1899.775-1900.025 1968.625-1970.025 1980.000-1985.000 2140.150-2140.575 2563.400-2563.700 2591.950-2592.225 2612.850-2613.375 2615.825-2617.325 2623.800-2624.000 2658.750-2659.775 2689.350-2690.925 2699.300-2701.775 2731.850-2732.575 2838.250-2838.425 2890.775-2981.250 3117.600-3117.850 3188.875-3189.150 3211.450-3212.925 3222.950-3225.550 3245.775-3253.600 3285.000-3286.000 3316.025-3317.125 3395.050-3396.025 3399.075-3400.925 3429.600-3429.975 3912.150-3912.550 3946.000-3946.975 3966.425-3967.100 4052.350-4056.425 4115.300-4115.500 4326.075-4326.925 4513.250-4515.850 4670.500-4675.000 4695.000-4698.250 5041.900-5042.550 5165.000-5168.925 5172.000-5173.275 5200.075-5200.475

80 17 93 332 28 20 18 35 100 13 68 105 164 48 12 31 17 18 98 173 519 66 73 64 122 21 27 64 45 270 14 57 173 299 216 43 261 84 26

Windows are spectral regions where gas absorption above 10-km altitude can be neglected.

measured under clear conditions on September 21 between 1140 and 1146 UT, when the solar zenith angle was 88ø, produce noise equivalent optical depths of 0.002. Such low optical depths were frequently encountered at large wavelengths for clouds containing small particles. For such

ß

ß. ,: .":= ,

ß

: i

ß ß

ß

ß

I

,

ß ß ß

ß

ß

;J ;:: ::

;L_::; : I

: * : I

I_ • I: J !

10ø d'•

101

Wavelength [/•m ] Fig. 8. Accuracy of optical depth due to instrument noise. For a particle-free or almost particle-free measurementperiod on September 21, window transmissivities (as explained in Figure 8) relative to the 1146 UT measurement are displayed in the figure' s upper graph. The resulting optical depths represent the instrument noise and are displayed in the bottom part of the figure for all 24 selected spectral windows, which are marked by solid circles on the abscissa. Only the larger optical depths at the smallest wavelengths may actually reflect the •esence of some aerosol particles. While these measurements, obtained at a solar zenith angle of 88ø, limit the determination of vertical cloud optical depths to values exceeding 0.002, measurements under smaller zenith angles are even more restrictive. At an observation angle of 80ø, the optical depth noise limit increases to 0.008. In Figures 11-16 the effective noise level is compared with detected optical depths.

clouds, only results at the shorter wavelengths are reliable, and only .for measurements where solar zenith angles are larger than 87ø. Otherwise the influence of the instrument noise becomes so large that only concentrations of large particles are detectable. For the presentation of detected optical depthsin Figures 11-16 the critical value corresponding to the 5% noise error is marked. During the course of a given flight the optical gain of the interferometer was frequently corrected, by means of an adjustable iris, to maintain the signal level (and hence the signal to noise ratio) in the face of changes of the solar intensity due to clouds and solar zenith angle variations. While these gain changeshad no effect on the relative depth of molecular absorption features, the instrument's primary objective, they did greatly complicate the calculation of optical depths, a secondary objective not suggested until after the AAOE campaign. Therefore both the clear and the cloudy case, necessary to determine the cloud's optical depth, were taken within a measurement period of constant gain. Since these time periods rarely exceeded 15 min, changesin the solar zenith angle remained small (generally less than 0.1ø), and the pathlength factor X was assumedto be constant. The short time periods, however, made it occasionally difficult to find measurements under completely clear conditions. The selected clear case may already contain cloud particles but in a lower concentration compared with other measurements within a given time period. In this case the computed optical depths will be an underestimate. A bad clear case that contains only small size particles is easily identified by a strong increase in the extinction ratio with increasing wavelength. Such an example is discussedlater for observations on September 26. The detection of bad clear cases contain-

16,486

KINNE ET AL.' PARTICLES IN SOLARSTRATOSPHERIC CLOUDS

ing large particles is more difficult and has to rely on lidar data and in-flight observations.In this context, observations on September 5 are addressedlater. The instrument's responseat the edgesof the band passof the INSb photodiode at 2 and 5 /xm and of the HgCdTe photoconductorat 5 and 13/xm was much smallerthan at the centers. Optical depth errors indicated by solid circles in Figure 8 and alsoby vertical bars in Figures 11-16 are larger at these wavelengths.For both detectors, most of the more accuratemeasuredoptical depths, marked by solidcirclesin Figures 11-16 are found near the center of their spectral

measurementsdisplay a significantloss in transmissivity, indicatingthe presenceof clouds. Transmissivities at the fourth and eighth measurements displaya strongspectraldependencewith higherattenuation at shorter wavelengths. This spectral dependence is an indication of small particle sizes. The last three measurementsdisplay strongattenuationswith hardly any spectral dependence;thus large cloud particle sizes shouldbe expected.Prior to the analysisof the two cloud types via the fourth and eleventh measurements, available lidar data are addressed.

The window

range.

transmissivities

of the first nine measure-

mentsdisplay considerablevariations and are an indication of a strong vertical and horizontal PSC structure. This is consistent with the 1.064-/zm wavelength lidar data for In this section, measured PSC optical depths are pre- September21 that are presentedalong with those for the sented,andtheir spectraldependenceis analyzedin termsof other two selected PSC cases in Figure 10. Relative backcloud particle size and possible nitric acid concentration. scattering values as well as their ratios to background Several selectedPSC cases,most of them in late September, scatteringof air moleculesare displayed.The ratios make it are addressed below in detail. easier to identify PSCs at high altitudes. Because of the observed horizontal and vertical variability On September 21 between 1430 and 1440 UT, spectroscopicmeasurementswere taken near the SouthPole (88øW, of the cloudfield on September21 the lidar data for the time 87øS).The sun at this time was just above the horizon at a period of the spectroscopic measurements(1430-1440UT) solarzenith angleof 88.3ø. Twelve measurementswere taken are of little help in determiningcloud positions,due to the during that time period, and the results are displayed in lidar's different viewing angle. Instead, cloud positionsare Figure 9 in terms of window transmissivityfor the entire based on lidar data from a later time period (1545-1555 UT) infrared spectrumas well as for three spectralsubsections when the aircraft had reversed its course and was flying (2-3, 3-8, and 8-13 /xm). The averagedwindow intensities below the clouds that the interferometer had earlier ob5.

RESULTS

were scaled to those of the seventh measurement, which

served on the horizon. At 1550 UT the relative backscatter-

displays the greatest overall transmissionand thus was selected to represent the clear case in the optical depth calculations.The fourth, eighth, tenth, eleventh, and twelfth

ing in Figure 10 displaysa cloudparticle signalthat extends

Day:21.September1987

Time 14h+...

from the aircraft to almost 19-km altitude.

Since the back-

scatteringvalue is particularlystrongbetween12- and 14-km altitudethe large-particlecloud(tenth and eleventhmeasurement) was believed to be at this altitude with the smallparticle cloud (fourth and eighth measurement)on top between 15- and 18-km altitude.

For the small-particlecloud type, Figure 11 displaysthe measuredvertical optical depths of the fourth measurement and their associated error bars in all 24 selected windows.

The optical depthsdiminishquickly with increasingwavelength. Most accurate amongthese optical depths are the values whose measuredintensity variations remained small. These values are marked by solid circles. Beyond 5-/xm

2 -13 ,u,m

,'............ 12-5,/•,ml,,, 30.0

32.0

34.0

wavelengththe measuredopticaldepths,with valuessimilar to thosedetectedunder clear conditionsin Figure 8, are well below the horizontal line representingthe detected instrument noise. Thus only the measuredoptical depthsbetween 1.8 and 5/xm are meaningful.Also the large error bars at the longer wavelengthsare in the noise domain and are thus meaningless,and may be misleadingin later comparisonsto calculatedvalues. To avoid possible•misinterpretations, the

•

I,,, 36.0

I ..... 38.0

40.0

42.0

additional Time [ minutes ] Fig. 9. Window transmissivityon September21. Relative to the "clearest" case, transmissivities in spectral regions of no gas absorptionare displayedfor 12 observationtimesbetween1430and 1440UT at a solarzenith angleof 88.3ø.The smallattenuationduring the first 8 min displays a strong wavelength dependenceand indicates smaller particle sizes. The last three measurementswith strong attenuation have almost no wavelength dependence,and large particles should be expected. The fourth and eleventh measurementswere selectedto represent the small-particletype (PSC type I) and large-particletype (PSC type II), respectively.Measured optical depths for both types are presentedin Figures 11 and 14, respectively.

same results of the fourth measurement now for a linear

scalein the y axis are displayedin Figure 12. Also shownare optical depthsfor the eighth measurement. The slope of the optical depth between 2 and 5 indicatesparticle radii that are slightly less than 1.0 /xm, basedon a comparisonwith calculatedextinctionratios for variousparticle sizes. The strongvariationsof the optical depth near 3-/xmwavelength,which are observedfor pure water or ice particles of this size, are missing(see dotted linesin Figures 11 and 12). This can be explainedby a high nitric acid concentration.

The dashed line shows the calcu-

lated extinctionratio for particleswith radii of 0.8/xm and a

KINNE ET AL.' PARTICLESIN SOLARSTRATOSPHERIC CLOUDS

16,487

Date:9/21/87

LIDAR DATA

Time: 14-:33:14 GMT

150

3O0 450

1.0

um

relative

backscatter

900

ftItI''-' "l' '[]'11 'lIC '-

.

70% HN03

ß

'

I

Time

1.0

um

(GMT)

scattering

Wavelenglh [/zm ]

ratio

Fig. 11. Spectral dependency of the infrared optical depth for a small-particle PSC type I on September 21. Measured optical depths with their error bars are compared with calculated extinction ratios. The solid circles highlight the most accurate optical depths. The dashed line represents the best fit, based on calculations with refractive

indices

for a 70%

nitric

acid solution.

The

calculations

assume a lognormal particle size distribution with a mode radius of 0.8/am and a standarddeviation of 1.25. In contrast, calculationsfor the same size distribution

with refractive

indices

of water

ice are

different from the measured optical depths, particularly at 3-/am wavelength. Due to the noise level, optical depths at larger wavelengths are meaningless. ß

.

,

ß

.

,

T•_me (GMT)

5th

1õth

21 Sept. 1987

Fig. 10. Lidar data. For the selected cases on September 5, 19, and 21, airborne (DC-8) lidar measurementsat 1.064-/amwavelength are presented. Backscattering values are displayed in the upper graph, their ratios to the background scattering of air molecules in the lower graph. The ratios make it easier to detect PSCs at altitudes above 15 km. Even though lidar and interferometer measurements were made simultaneously, the lidar's overhead (© = 0ø) detection might not correlate with the interferometer's PSC event on the horizon, particularly on days like September 21, when PSCs display a considerable vertical and horizontal structure. The lidar data, however, provide a general picture of the cloud field and allow estimates

cloudcolumnmassof 1 x 10-6/cm2 andparticlenumberof 3 x 105/cm 2. For the cloudthickness, as detectedby lidar measurements, these values correspond to about 20-ppb

massanda particledensityof about1/cm3. For the large-particle cloud type, Figure 14 displays the measured vertical optical depths of the eleventh measurement

and

their

associated

error

bars

in

all

24

selected

windows. The detected optical depths are larger than 0.03

Date:9/21/87

of PSC altitudes.

I CE

70% nitricacidconcentration andcompares well with the

,

data.Effects ofothercomponents such assulfuric acidmay

' ,

t

Time: 14:.36:48 GMT

-'1•'•-•-',""•$ ' •,'?.-.r$ ...._•-

•'!

be similar. However, duetotheir smaller concentrations, only insignificanteffectson the extinction ratio are expected.

Time: 14:33:14 GMT ICE

Anotherindicationthat thesesmallparticlescontainnitric acid are measurementsof nitric acid vapor at the same time

withthesame instrument [Toon etal.,thisissue, Figure 15]. As soon as small-particle clouds are detected in the polar

stratosphere, the nitric acid vapor dropsby 3 x 10•5 molecules/cm 2. Asdiscussed later,thisamount isequivalent to the nitricacidthatis tiedup in the smallparticles. Optical depths of three other measurements in clouds composed of submicron-sized particles are displayed in Figure 13. Their extinction ratio, too, suggestsnitric acid concentrationsthat are larger than 40%. Based on the typical 0.8-/zm particle size, vertical optical depths of about 0.1 at visible wavelengths are estimated from Mie calculations. Optical depth and particle size yield approximate values for

lo0

Woveleng[h [/.zm ] Fig. 12. The lower graph shows the results of Figure 11, however, with optical depths displayed on a linear scale. The differences near 3 t•m between water ice and nitric acid composition are more pronounced, and there is less emphasis on meaningless optical depths below the noise level. The upper graph presents results from the eighth measurement.

16,488

KINNE ET AL.' PARTICLESIN SOLAR STRATOSPHERICCLOUDS

Date:9/5/87 Date:9/21/87

Time:13:53:12 GMT

Date:9/24/87

T•me:11:55:41 GIdT

Date.9/24/87

T•me'1416.56GMT

Time:14:18:49 GMT

T O

km

20

.......

i.....•,' i•,--b-•-....

lOo

ld

Wavelength [/•,m ]

0

10-5

Fig. 13. Three other small-particle PSC type I events. Results are compared, as in Figures 11 and 12, with calculated extinction ratios for 0.8-/•m-radius particles with a 70% nitric acid concentra-

I

Date:9/21/87

Time:14:59:29 GMT

¾ o

I

i

i

i

I

loø

Wavelength [/zm ]

tion.

and display almost no spectral dependency. The discontinuity at 5.3-/am wavelength is causedby the nonlinear response of the HgCdTe photoconductor, which leads to underestimation of optical depths. This effect, however, appeared only for strong attenuations, when optically dense clouds were observed at large solar zenith angles (19 > 88ø). To correct for this error, the optical depths at the larger wavelengths need to be increased by an amount indicated by difference in optical depths in the spectralregion where both detectors overlap (i.e., the overlap point at 5.3/am), and for the example of the eleventh measurement in Figure 14 by 0.01. For comparisonpurposes,calculatedoptical depthsare lowered by this value at the longer wavelengths. The measured data compare well with the calculatedextinction ratios of ice sphereswith radii of 15/am (dotted line) and 30/am

lO-3 km -1 )

Fig. 15. SAM II data comparisonfor a small-particle PSC type II case. Satellite extinction (31øW, 76øS)at 1.0-/•m wavelength is compared with infrared optical depths measured with the interferometer (26øW, 80øS measuring northward with 19 = 86.9ø). Also shown are calculated extinction ratios (dashed line) based on a lognormalmode radius of 6.0/•m, a geometricstandarddeviation of 1.25, and refractive indices of ice. The displayed optical depths

correspond to a 4-km-thickcloudwith 0.05 particles/cm 3. Their value of about 0.03 is smaller than the optical depth of 0.04 seen by the satellite for this cloud layer.

(dashedline). Due to the lack of spectral dependencefor the detected optical depths in Figure 14, even larger particle sizes may have been present. Other measurementsin largeparticle cloudsdisplay slight optical depth variations near 10 /am, which suggestsmaller particle radii. Optical depths in Figure 15 for the PSC observed on September 5 compare well with extinction ratios for 6-/am-radius size ice spheres (dashed line). Characteristic vertical optical depths of 0.3 and particle radii of 10/am for this large-particle cloud are equivalent to

a cloudcolumnmassof about2 x 10-5 g/cm2 andparticle number of about5 x 103/cm 2. Fortypicalcloudthickness of 2-4 km, as detected by lidar measurements, these values correspondto about 500-ppb mass and a particle density of about 0.02/cm3. Table 2 summarizes

Wavelenglh [/zm ] Fig. 14. Spectral dependencyof the infrared optical depth for a large-particle PSC type II observed on September 21. Measured optical depths with their error bars are compared with calculated extinction ratios. The solid circles highlight the most accurate optical depths. The dashed and dotted lines represent calculations for pure ice particles with radii of 30 and 15/zm, respectively.The calculationsare based on a lognormalparticle size distributionwith standard deviation of 1.1. Due to a lack of any spectral dependency for the measured optical depths, typical particle radii might be larger. The discontinuity near 5 /zm is caused by the nonlinear response of the detector at the larger wavelengths. This response tends to underestimate large optical depths.

the results of 33 PSC events that were

observed during September 1987 at latitudes between 70ø and 90øS. Estimates of cloud position, mass, and particle density are given. The accuracy of the density values depends on the accuracies of (1) the lidar-detected cloud thickness, (2) the typical particle size, and (3) the vertical optical depth. Since the lidar data may look at a different cloud than the interferometer, the cloud thickness could be off by a factor of 2 or more. Unless particles exceed radii of 15 /am, the sizes based on the extinction ratio are fairly accurate. Due to nonlinear relationships between radius, mass,and especiallyparticle number, radius uncertaintiesof 40% can cause another factor of 2 uncertainty in mass and number densities. An incorrect cloud altitude at large solar zenith angles (19 > 88ø), as well as a bad "clear" reference case that already contains cloud particles, will lead to incorrect optical depthsthat are inaccurateto about a factor of 2. Given these uncertainties, it is estimated that the presenteddensity values are probably correct within a factor

KINNE ET AL.: PARTICLESIN SOLARSTRATOSPHERIC CLOUDS

TABLE 2.

Summary of Selected PSC Measurements During AAOE 1987

Angle, deg

Longitude (øW) Versus Latitude (øS)

Altitude,* km

r0.55?

r2

1314:05 1314:05 1315:52 1417:55 1418:49 1419:42 1005:38 1347:51 1350:31 1353'12 1408:50 1410:38 1418:32

82.6 82.6 82.6 86.9 86.9 86.9 88.7 89.5 89.4 89.3 88.8 88.8 88.6

32/75 32/75 32/75 26/80 26/80 26/80 45/81 109/82 109/82 108/82 105/84 105/84 99/86

11-15 11-15 11-15 12-16 12-16 12-16 18-20 15-17 15-17 15-17 15-17 15-17 13-14

6 -2 6 -2 6 -2 4 -2 3 -2 3 -2 1 8 -3 8 -3 8 -3 1-2 1-2 3 -3

21 21 21 21 21

1422:07 1423:01 1423:55 1423:01 1433'14

88.5 88.5 88.5 88.5 88.4

98/86 97/86 97/86 97/86 90/87

13-14 12-14 12-14 12-14 15-18

21 21

1436:48 1438:35

88.3 88.3

89/87 88/87

15-18 12-14

1-2 3 -2

6 -2 6-2 6 -2 4-2 3-2 3-2 6-3 5-3 6-3 4-3 8-3 8-3 3-3 4-3 2-2 2-2 2-2 1-2 7-3 3-2

7-2 5-2 7-2 5-2 7-2 5-2 5-24-2 4-2 4-2 4-2 3-2 2-30 1-30 1-30 1-30 2-3 1-3 2-3 1-3 3-3 2-3 4-3 3-3 2-2 1-2 2-2 1-2 2-2 1-2 3-3 1-3 2-3 1-3 3-2 2-2

21

1439:29

88.3

87/87

12-14

4 -2

4-2

4-2

3-2

24 24 24 24 24 26 26 26 26

1125:12 1155:41 1416:36 1417:29 1418:23 1156:11 1201:23 1205:27 1207:15

88.3 85.7 87.5 87.4 87.4 88.5 88.1 87.7 87.5

78/72 76/74 113/79 113/79 113/79 90/72 89/73 89/74 89/74

17-18 15-18 16-17 16-17 16-17 13-14 12-14 12-14 12-14

1-2 2 -2 8 -3 8 -3 8 -3 1-2 2 -2 2 -2 2 -2

26õ 26õ

1253:12 1254:06

83.7 83.7

89/72 89/72

14-16 14-16

26 26

1314:06 1314:59

81.8 81.7

85/74 85/74

15-17 15-17

9-3 1-2 6-3 6-3 6-3 1-2 2-2 2-2 2-2 8-3 6-3 4-2 4-2

1-3 2-3 3-3 2-3 3-3 1-2 2-2 2-2 2-2 1-2 1-2 1-2 2-2

1-3 1-3 1-3 1-3 1-3 7-3 1-2 1-2 1-2 1-3 1-3 6-3 6-3

Day in September 1987

Time

5 5 5 5 5 5 19 21 21 21 21 21 21

16,489

Optical Depth

4-3 2-2 2-2 2-2

3 -2 3 -2

•5

Mass

•12

Radius,? /am

Particles

g cm-2 ppb Content cm-2 cm-3

6.0 6.0 6.0 6.0 6.0 6.0 0.5 0.8 0.8 0.8 0.5 0.5 6.0 6.0 6.0 6.0 6.0 0.8 0.8 15.0 30.05 15.0 30.05 0.5 0.8 0.8 0.8 0.8 6.0 6.0 6.0 6.0

2 -5 2 -5 2 -5 2 -5 1-5 1-5 4-7 7 -7 7 -7 7 -7 4-7 4 -7 1-6 2-6 8-6 8-6 8-6 1-6 8-7 3 -5 6 -4 4 -5 8 -4 5 -7 2 -6 7 -7 7 -7 7 -7 4 -6 8 -6 8 -6 8 -6

3 +2 3 +2 3 +2 2 +2 2 +2 2 +2 1+1 2 +1 2 +1 2 +1 1+1 1+ 1 5 +l 1+2 2 +2 2 +2 2 +2 2 +l 1+ 1 8 +2 2 +4 1+3 1+4 2 +l 3 +l 3 +l 3 +l 3 +l 2 +2 2 +2 2 +2 2 +2

ICE ICE ICE ICE ICE ICE NAS NAS NAS NAS NAS NAS ICE ICE ICE ICE ICE NAS NAS ICE ICE ICE ICE NAS NAS NAS NAS NAS ICE ICE ICE ICE

4 +4 4 +4 4 +4 3 +4 2 +4 2 +4 8 +5 2 +5 2 +5 2 +5 8 +5 8 +5 2 +3 2 +3 1 +4 1+4 1+4 3 +5 2 +5 2 +3 5 +2 3 +3 7 +2 8 +5 4 +5 2 +5 2 +5 2 +5 6 +3 1+4 1+4 1+4

9 -2 9 -2 9 -2 7 -2 5 -2 5 -2 4 1 1 1 4 4 2 -2 2-2 6-2 6-2 6-2 1 1 1-2 2 -3 1-2 3 -3 8 1 2 2 2 6-2 6-2 6-2 6-2

2.0 2.0

3 -6 3 -6

1+2 1+2

ICE ICE

2 +4 2 +4

1-1 1-1

Aircraft altitude is 10.6 km. NAS, high nitric acid concentration;ICE, predominantlyice. *Estimates from simultaneousoverhead(0ø angle) lidar measurements. ?Estimatesfrom the spectralslopeof the particle extinctionratio. $Possiblelarger particle size. õBad "clear"

reference.

Measuredopticaldepthsat 2-, 5-, and 12-/amwavelength(r2, rs, r12) are listedfor selectedPSC events.The corresponding days, times,positions(latitudeandlongitude),and solarzenithanglesare given.Comparisonsof measuredand calculatedextinctionratiosallow an estimationof typicalparticleradii r, opticaldepthsat visiblewavelengths (r0.55),and subsequently, an estimationof massand particle numbercloudcolumnproperties.Usingthecloudthickness determination fromthelidardata,cloudmassandparticlenumberareexpressed in density values.

of 2, but under the worst possiblescenario,they could be off by 1 order of magnitude. It shouldalso be pointed out that a cloud with the larger

sizeparticlesmay containsubmicron-sized particlesas well. However, since infrared optical depthsfor the larger particles exceed those of the submicron sizes by 1 order of magnitude, the interferometer will only detect the larger particle size. A bad reference case containing small particles was detected on September 26 and is noted in Table 2. Small

particle sizes (r - 0.3 ttm) in the clear case and large particles (r - 2.0 ttm) in the cloudy case cause an inverse slope between 2- and 5-/xm wavelengthwith larger optical depth at the larger wavelengths as well as an inverse variation of the extinction ratio near 3-/xm wavelength. A bad referencecasecontaininglarge particlesmay have been present during measurementson September5. Lidar data in

Figure 10 display continuously high backscattering ratios with no indication of a clear situation. A clear situation, however, is still possible, since interferometer and lidar were sensing in different directions. If the condition of a bad reference case containinglarge particles exists, then calculated optical depths would certainly represent an underestimate of the actual cloud optical depths. SAM II satellite measurementsof the atmosphericextinction at 1-/amwavelengththat were taken in the vicinity of a large- and small-particle cloud event are compared with measured optical depths. For the comparison involving a

larger particle cloud, Figure 15 showsthat on September5 the SAM II extinction at (31øW, 76øS)is greatestbetween 12 and 16 km, as indicated by lidar data in Figure 10. The vertical optical depth of 0.04 agrees well with values of 0.03-0.06 determined by the interferometer. As suggested above, this close agreement also makes a poor clear case

16,490

KINNE ET AL.: PARTICLES IN SOLAR STRATOSPHERICCLOUDS

Date:9/19/87

Time: 10:5:38

GMT

TABLE 3.

Average Properties for PSC Type I and Type II

Clouds Based on Airborne

JPL Mark

IV Interferometer

Measurements During September 1987 Over Antarctica

PSC Type I

HNO3 concentration,% 1oSAM IIextinction

>40 0.5 0.005-0.02 5-50

Particle radius,/am Vertical optical Depth Mass, ppb

Numberof particles, cm-3

1-10

PSC Type II

>5* 0.01-0.067 100-2000

0.005-0.1

,l!,!,tt,...•,• It''l-• ."', .[ [. •O/ ?• 1,115. ''\• 1.•_3km 1 *Wave clouds may have smaller particle sizes (2-/am radius). ?Larger optical depths occurred but could not be measured by the interferometer due to low levels of sunlight.

type II, is composed of ice crystals with typical radii of at least 6 /am. The former conclusion comes from their large lOo mass, and the latter from the presence and behavior of Wavelength [/•m ] spectral features near 9 /am. However, considerably larger particle sizes, beyond the 15-/am-radiussize detection limit, Fig. 16. SAM II data comparisonfor a small-particle PSC type I case. Satellite extinction (21øW, 78øS) at 1.0-/am wavelength is are seenas well. During Septemberboth PSC types occurred compared with infrared optical depths measured with the interferat distinctly different altitudes. Type I clouds were often ometer (45øW, 81øSmeasuringnortheastward with O = 88.7ø). Also found at altitudes extending above 15 km, while type II shown are calculated extinction ratios (dashed line) based on a lognormalmode radius of 0.5/am, a geometricstandarddeviationof clouds were generally below that altitude. The restricted altitude of type II clouds may be related to specific temper1.25, and refractive indices from a 70% nitric acid solution. The displayed optical depths correspond to a 2-km-thick cloud with 4 ature regimes occurring at times when the interferometer particles/cm 3. Theextrapolated valueto 1-/zmwavelength of0.01is was operating and detecting clouds. On other flights, lidar larger than the total optical depth above 10-km altitude (0.007) data(E. V. Browell et al. private communication, 1989) show detected by the satellite. that observed type II clouds reached altitudes up to 20 km. On these flights the interferometer was not operating at the selection unlikely to be important. For the comparison time clouds were present. Average properties for both PSC involving a smaller particle cloud, Figure 16 shows the types are summarized in Table 3. The range for optical measured SAM II satellite extinction profile at (31øW, 76øS) depths, mass, and particle density is greater than variations and optical depths from spectroscopic measurements for for the 33 PSC cases given in Table 2 to account for a September 19. The ratio of the optical depths identifies a possibleerror of the order of 2. The values for particle size, number density, and nitric cloud with smaller sized particles and a high nitric acid acid abundancefor PSC type I observationsagree well with concentration. Again, the measured optical depths beyond 5 /am are meaningless.The lidar data for this particular casein other results from ER-2 data [Fahey et al., 1989; Ferry et al., Figure 8 display two signals:one between 11 and 14 km and this issue; Gandrud et al., 1989; Pueschel et al., 1989]. Few a slightly stronger one between 18- and 20-km altitude. The data exist to compare with type II results. Ice crystal sizes larger than 10/am were detectedfrom the ER-2 [Goodman et SAM II data confirm this bimodal cloud structure; however, al., this issue] on several occasions. the extinction at 19 km is much less than that at lower As discussedby Toon et al. [1989], the characteristics of altitudes. The SAM II total vertical optical depth at 1-/am type II clouds may vary considerably, depending upon the wavelength is 0.007 comparedwith a value of 0.010 basedon mode of cloud formation. For example, wave clouds may an extrapolation of infrared optical depth from spectroscopic measurements

containseveralmicron-sized ice particlesper cm3, while

with the interferometer.

Hence, despite the differences in time and location, the satellite data agree surprisinglywell with the interferometer and lidar measurements.

layered PSCs that formed in slowly cooling air massesmay

containa considerably smallernumberof particles percm3, but these particles may be larger than 10/am in radius. There are also clouds associated

6.

Measurements

CONCLUSION

of PSCs with the Mark

with cirrus at and below the DC-8

altitudes. From notes taken during the DC-8 flights, observations indicate that the type II clouds at 80øSon September IV

interferometer

and the DIAL lidar system during the austral springidentify two different types of PSCs. The results support a PSC classificationinto PSC type I and PSC type II, as suggested from ER-2 measurements at higher latitudes. The smallsized particle cloud, or PSC type I, has typical radii of 0.5 /am and a high nitric acid concentration. The former conclusion follows from the wavelength dependency of the extinction between 2 and 5/am, while the latter conclusionfollows from the near total absence of the 3-/am ice band and from nitric acid vapor deficienciesin the presence of these particles [Teen et al., 1989]. The larger particle cloud, or PSC

5 were

associated

with

cirrus

located

at and below

DC-8

altitudes, but there were no obvious cirrus at aircraft altitudes during the measurements taken at 75øS. The type II clouds on September 21 do not appear to have been associated with any cirrus at the altitude of the DC-8. The type II clouds observed at 81øS on September 26 may have been wave clouds, a number of which were observed visually on that day. In this regard it is interesting that rather small

(2-/am radius) ice crystals were seen. These correspondin size with the particles observedby Ferry et al. [this issue]on August 17 in a wave cloud. For future PSC detection with the interferometer, fewer

KINNE ET AL.'. PARTICLESIN SOLAR STRATOSPHERIC CLOUDS

gain changeswould provide more measurementsfrom which to choose

clear

observations

and should

make

the clear

reference more reliable. With longer periods between gain changes,it will be helpful to maintain a constant solar zenith angle. Also, a high solar elevation (O < 85ø)would allow the use of the cosine correction for the conversion from actual to

vertical pathlengths and lidar information about the cloud position would not be required to determine cloud vertical optical depths. However, the actual cloud pathlengthwould then be small, and with the current instrument noise it might prove impossibleto detect PSC type I clouds. Despite the wealth of data arising from the spectrometer'shigh spectral resolution,the smallPSC type I optical depthsbeyond5-/xm wavelength, even for solar zenith angles larger than 88ø, cannot currently be detected. An improved signal to noise ratio of the instrument and also a more sophisticateddata analysis are needed to provide reliable extinction ratios for submicron-sizedparticles at wavelengths between 5 and 13 /xm. This would allow a more accurate investigation of the chemical compositionand the particle phase. Presently, the identificationof the nitric acid componentvia comparisons to theoretical

calculations

suffers from the lack of infrared

optical properties.Thus laboratory measurementsof optical properties for nitric acid trihydrate would be useful. Acknowledgments. This work was supportedby NASA's Upper Atmosphere Theory Program managed by M. Prather and the AAOE programmanagedby R. Watson. S. Kinne was supportedby NRC through NASA's climate program, managedby R. Schiffer. REFERENCES

Browell, E. V., A. F. Carter, S. T. Shipley, R. J. Allen, C. F. Butler, M. N. Mayo, J. H. Siviter, Jr., and W. M. Hall, NASA multipurpose airborne DIAL system and measurementsof ozone and aerosol profiles, Appl. Opt., 22, 522-534, 1983. Crutzen, P. J., and F. Arnold, Nitric acid cloud formation in the cold Antarctic stratosphere--A major causefor the springtime "ozone hole," Nature, 324, 651-655, 1986. Downing, H. D., and D. Williams, Optical constantsof water in the infrared, J. Geophys. Res., 80, 1656-1661, 1975. Fahey, D. W., K. K. Kelly, G. V. Ferry, L. R. Poole, J. C. Wilson, D. M. Murphy, and K. R. Chan, In situ measurements of total reactive nitrogen, total water, and aerosolin a polar stratospheric cloud in the Antarctic, J. Geophys.Res., 94, 11,299-11,315, 1989. Farmer, C. B., G. C. Toon, P. W. Shaper, J. F. Blavier, and L. L. Lowes, Ground based measurementsof the composition of the antarctic atmosphereduringthe 1986springseason:Stratospheric trace gases, Nature, 329, 126, 1987. Ferry, G. V., E. Neish, M. Schultz, and R. F. Pueschel, Concen-

trations and size distributionsof Antarctic stratosphericaerosols, J. Geophys. Res., this issue. Gandrud, B. W., P. D. Sperry, L. Sanford, K. K. Kelly, G. V. Ferry, and K. R. Chan, Filter measurement results from the Airborne Antarctic Ozone experiment, J. Geophys. Res., 94, 11,285-11,297, 1989.

Gary, B. L., Observationalresultsusingthe microwavetemperature profiler during the Airborne Antarctic Ozone Experiment, J.

Leuchs, M., and G. Zundel, Easily polarizable hydrogen bonds in aqueous solutions of acids: Nitric acid, J. Phys. Chem., 82, 1632-1635, 1978. McCormick, M.P., H. M. Steele, P. Hamill, W. P. Chu, and T. J. Swissler, Polar stratospheric cloud sightings by SAM II, J. Atmos. Sci., 39, 1387-1397, 1982 McElroy, M. B., R. J. Salawitch, and S.C. Wolfsy, Antarctic O3--Chemical mechanisms for the spring decrease, Geophys. Res. Lett., 13, 1296, 1986. McGraw, G. E., D. L. Bernitt, and I. C. Hisatsune, Vibrational spectra of isotropic nitric acids, J. Chem. Phys., 42, 237-244, 1965.

Molina, M. J., T.-L. Tso, L. T. Molina, and F. C.-Y. Wang, Antarctic stratosphericchemistry of chlorine nitrate, hydrogen chloride and ice, Science, 238, 1253-1257, 1987. Pollack, J. B., and J. N. Cuzzi, Scatteringby non-sphericalparticles of size comparableto a wavelength--A new semiempiricaltheory and its application to tropospheric aerosols, J. Atmos. Sci., 42, 245-262, 1980. Poole, L. R., and M.P.

McCormick, Airborne lidar observations of

arctic polar stratospheric clouds: Indications of two distinct growth stages, Geophys. Res. Lett., 15, 21-23, 1988. Pueschel, R. F., K. G. Snetsinger, J. K. Goodman, O. B. Toon, G. V. Ferry, V. R. Overbeck, J. M. Livingston, S. Verna, W. Fong, W. L. Starr, and R. K. Chan, Condensed nitrate, sulfate, and chloridein Antarctic stratosphericaerosols,J. Geophys.Res., 94, 11,271-11,284, 1989.

Querry, M. R., and I. L. Tyler, Reflectanceand complex refractive indicesin the infrared for aqueousHNO3, J. Chem. Phys., 72(4), 2495-2522, 1980. Solomon, S., R. R. Garcia, F. S. Rowland, and D. J. Wuebbles, On the depletion of antarctic ozone, Nature, 321,755, 1986. Stanford, J. L., and J. S. Davis, A century of stratosphericcloud reports: 1870-1972, Bull. Am. Meteorol. Soc., 55, 213, 1974. Takano, Y., and K.-N. Liou, Solar radiative transfer in cirrus clouds, 1, Single scatteringand optical properties of hexagonalice crystals, J. Atmos. Sci., in press, 1989. Tolbert, M. A., M. J. Rossi, R. Malhotra, and D. M. Golden, Reaction of chlorine nitrate with hydrogen chloride and water at antarctic stratospheric temperatures, Science, 238, 1258-1260, 1987.

Toon, G. C., C. B. Farmer, L. L. Lowes, P. W. Schaper, J. F. Blavier, and R. H. Norton, Infrared aircraft measurements of stratospheric composition over Antarctica during September 1987, J. Geophys. Res., this issue. Toon, O. B., P. Hamill, R. P. Turco, and J. Pinto, Condensation of

HNO3 and HC1 in the winter polar stratospheres,Geophys. Res. Lett., 13, 1284-1287, 1986.

Toon, O. B., R. P. Turco, J. Jordan, J. Goodman, and G. Ferry, Physical processesin polar stratosphericice clouds, J. Geophys. Res., 94, 11,359-11,380,

1989.

Warren, S. G., Optical constants of ice from the ultraviolet to the microwave, Appl. Opt., 23, 1206-1225, 1984. Wofsy, S.C., M. J. Molina, R. J. Salawitch, L. E. Fox, and M. B.

McElroy, Interactions between HC1, NO x and H20 ice in the Antarctic stratosphere:Implicationsfor ozone, J. Geophys.Res., 93, 2442-2450, 1988.

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Goodman,J., O. B. Toon, R. F. Pueschel,K. G. Snetsinger,and S. Verna, Antarctic stratosphericice crystals, J. Geophys. Res., this issue.

16,491

(Received August 2, 1988; revised March 23, 1989; accepted March 23, 1989.)