www.elsevier.comrlocateratmos
Laser depolarization studies of simulated
crystallized H O
2r
H SO clouds
2 4Shubhangi Tavker, P. Pradeep Kumar
)Department of Physics, UniÕersity of Pune, 411 007 India
Received 27 May 1999; accepted 22 December 1999
Abstract
Ž .
Laboratory experiments were carried out to investigate the linear depolarization ratios LDRs of crystallized H O2 rH SO acid clouds at angles of 452 4 8, 1358and 1578to the forward direction. LDR is the ratio of the returned energies in the planes of polarization perpendicular and parallel to
Ž .
that of the source Sassen, 1974 . Experiments were carried out for different concentrations of aqueous solution of sulfuric acid. A cloud of supercooled droplets is formed inside an experimen-tal chamber kept in a walk-in cold room by heating an aqueous solution of sulfuric acid, and the ice crystal formation is initiated by momentarily introducing a rod dipped in liquid nitrogen into the cloud. A 2-mW polarized He:Ne laser beam is directed through the cloud and a photomulti-plier tube measures the scattered intensity. It is found that the scattered energy is higher in the direction of 1578and 1358than that at 458. Microscopic examination of the crystals formed from acid concentrations above 50% shows dark fuzzy spots on the surface of the ice crystals suggesting that this could be a film of acid drops. This film also alters the shape of the crystals, and the crystal boundaries are no longer sharp and well defined like that observed for pure water clouds and this is found to influence the LDR.q2000 Elsevier Science B.V. All rights reserved.
Keywords: Depolarization ratio; Scattering; Clouds; Sulfuric acid; Contrails; Cirrus
1. Introduction
Cirrus clouds cover nearly 35% of the earth’s surface and influence the climate Ž
greatly through their effect on radiation budget Ramsnathan et al. 1983; Liou, 1986; .
Jensen et al., 1994a,b . Cirrus clouds are typically composed of ice particles with
)Corresponding author. Fax:q91-20-5651684rq91-20-5653899.
Ž . Ž .
E-mail addresses: [email protected] S. Tavker , [email protected] P.P. Kumar .
0169-8095r00r$ - see front matterq2000 Elsevier Science B.V. All rights reserved.
Ž .
number densities between about 0.001 and 1 cmy3 and are formed in the upper Ž
troposphere wherein atmospheric conditions range fromy308toy708C Laaksonen et .
al., 1997 . Recent studies show that cirrus clouds play an important role in the
Ž .
heterogeneous chemistry of upper troposphere Borrmann et al., 1996 . A cirrus cloud is believed to form during the cooling of an air mass containing hygroscopic particles ŽJensen et al., 1994a . The particles uptake water, become metastable with respect to ice,. and then nucleate ice to begin the cloud formation process. The nucleation of ice particles in the upper troposphere is not fully understood. Ice nucleation can occur either by homogeneous freezing of supercooled droplets or by heterogeneous freezing of droplets containing soot or crustal components. The background stratospheric and upper
Ž .
tropospheric aerosol consists of aqueous H SO2 4 droplets about 75% by weight
Ž . Ž .
according to Sheridan et al. 1994 and Pueschel et al. 1998 . Field observations support the fact that the nucleation process occurs by homogeneous freezing of ice in an aqueous
Ž
H SO solution droplet Sassen and Dodd, 1989; Jensen et al., 1994a; Heymsfield and2 4
.
Miloshevich, 1995 . It should be noted that these studies are for neutralized sulfuric acid, that is, ammonium sulfate particles. There are modelling, laboratory, and field studies to support the fact that H SO acid aerosols can remain supercooled to very low2 4
Ž .
temperatures Carslaw et al., 1994; Tabazadeh et al., 1994; Anthony et al., 1995 . Thus, nucleation of ice particles mostly occurs in an aqueous solution droplet when ice saturation is fairly exceeded in the atmosphere.
Due to their high altitudes, cirrus clouds are probably not affected by most ground-based human activities, but it is being recognized that the effects of high-altitude jet air-traffic has the potential for altering the properties and extent of cirrus clouds. Several international programmes have focused on understanding the impacts of aircraft on climate and atmospheric chemistry. Increasing emissions from jet aircraft will change the surface area, composition and number of particles in the lower stratosphere and upper troposphere and may modify atmospheric chemistry and the earth’s climate. Aircraft emissions are expected to modify the earth’s radiative forcing as a result of
Ž
several processes, viz. the emission of radiatively active gases e.g., CO2 and water
. Ž
vapour , the emission of chemical species that produce relatively active substances like .
NO , which modifies the O concentration and the emission of substances that triggerx 3
Ž .
the generation of additional clouds e.g., contrails .
Sulfuric acid is expected to be one of the products of combustion in jet engines. The first laboratory experimental indications of the existence of H SO2 4rH O clusters in a2
Ž .
jet engine exhaust plume were given by Frenzel and Arnold 1994 . In the upper troposphere and lower stratosphere, aircraft-derived H SO2 4 may form new aerosol
Ž
particles, or condense on pre-existing particles both ambient as well as the abundant . Ž
small particles emitted in jet exhaust Karcher, 1995; Anderson et al., 1996; Hagen et .
al., 1996 . Contrail ice particles are supposed to be formed by the particles generated in a combustion process that may acquire a coating of sulfuric acid in the young plume ŽJensen et al.,1998 . The sulfate-coated soot particle can then take up water and. eventually act as a freezing nucleus as the plume continues to cool.
In relatively clean conditions, upper tropospheric aerosol particles are mostly very small H SO2 4rH O droplets with radii less than 0.012 mm with number densities around
y3 Ž .
Ž .
MPIK-Heidelberg Mohler and Arnold, 1992 indicate that the solid particles are coated
¨
by a supercooled liquid solution, which is formed by heterogeneous H SO2 4rH O2
nucleation and condensation since gaseous sulfuric acid is continuously formed during daytime and highly supersaturated with respect to supercooled liquid H SO2 4rH O. It is2
conceivable that the coating may also proceed by coagulation of the very small but numerous H SO2 4rH O droplets with solid aerosol particles.2
Ž .
Takano and Liou 1995 observe that fundamental scattering, absorption and polariza-tion data on the types of nonspherical ice crystals occurring in cirrus clouds are required for reliable modelling of the cloud radiative properties; for interpretation of the observed fluxes and heating rates; for incorporation in GCMs and mesoscale cloud models; and for development of remote sensing techniques to infer cloud optical depth, temperature and ice crystal particle size. Retrieval of cloud parameters from remote sensing devices and accurate calculations of precipitation growth also require precise knowledge of the ice particle habits. The use of wrong particle shapes in satellite retrievals of cloud optical thickness may result in an underestimation or overestimation of the optical
Ž .
thickness of clouds by a factor that can exceed three Mishchenko et al., 1996 . In calculating the mass of precipitation, the wrong use of ice particle shape may give an
Ž .
error in particle mass by a factor of 15 Mason, 1994 .
The focus of the present experiment was to look into the scattering and polarizing properties of water clouds and that of sulfuric acid clouds in the forward and backscatter
Ž .
region. As discussed by Sassen and Liou 1979 , there is a lack of rigorous theoretical solution to the scattering of light by particles with arbitrary geometry such as ice crystals. Mie scattering theory cannot be applied to the various crystal habits with
Ž .
favoured free fall orientations. Liou 1972 has proposed a theory for cylinders oriented randomly in a horizontal plane, which could be applied to the scattering behaviour of
Ž .
needles and similar habits. Takano and Liou 1995 have developed a new Monte Carlo geometric ray-tracing method for the computation of the scattering, absorption, and polarization properties of ice crystals with various irregular structure, including hollow columns, bullet rosettes, dendrites, and capped columns. The numerical methods for determining cloud composition are limited by the fact that the results therein are based on the parameterized optical properties of cirrus clouds.
Experimental findings must be relied on for the present to characterize the scattering properties of mixed phase clouds and assess the validity of approximate theoretical approaches to this problem. Laboratory experiments performed under defined controlled conditions can help in gaining insight into the depolarization processes that brings about
Ž .
variation in the linear depolarization ratio LDR at various scattering angles with changing acid concentrations.
2. Experimental arrangement
circumference of the flask at regular intervals of 458. In addition, there is a port at 1578 to facilitate observations closer to backscattering. Fig. 1 shows the schematic representa-tion of the experimental arrangement.
Ž
A solution of sulfuric acid and water in definite proportion 0%, 10%, 50% and 70% .
sulfuric acid by weight in water is heated at a controlled rate and the vapour is introduced into the chamber to form a cloud of supercooled droplets and vapour. The total system is closed with no leakage of air in or out. A PT-1000 sensor records the temperature of the cloud and the data is recorded in real time on a computer-based data acquisition system. When a steady-state temperature is reached, the cloud is seeded with a rod dipped in liquid nitrogen. This initiates ice crystal formation by homogeneous nucleation followed by other ice multiplication processes. The cloud temperatures at the time of seeding varied fromy158C toy178C. As the particles grow and dissipate, the falling ice crystals are collected on a formvar-coated microscope slide and are analyzed for crystal shape, size, number density and growth pattern. Each slide is inserted into the cloud for about 10 s, but at times, the slide remained inside the cloud for about 11 to 12 s. In order to remove this error of extra exposure time, the number density of ice crystals has been expressed as number of crystalsrmm2rs. Each cloud cycle lasts for about 5 min duration and four slides are collected per cloud cycle. It should be noted that the quoted solution strengths are not the actual strengths of the droplets andror crystals forming the cloud in the chamber. In order to avoid contamination of the chamber, we carried out pure water runs first followed by increasing acid strength of the solution. For every acid strength, a fresh solution was prepared and the entire chamber was cleaned.
Ž .
A laser beam from a 2-mW polarized He:Ne laser manufactured by Melles Griot is
Ž .
directed into the cloud and a Photo Multiplier Tube manufactured by Thorn EMI , which has an analyzer on its front, measures the scattered intensity. Experiments were
Table 1
Scattered intensity measurements for different combinations of laser and analyzer positions
Laser position Analyzer position Measured intensity
Parallel Parallel M1
Parallel Perpendicular M2
Perpendicular Parallel M3
Perpendicular Perpendicular M4
first carried out for a 458 angle and then later repeated at 1358 and 1578angles to the forward direction. There is a light trap fixed on the port opposite the laser to prevent any spurious backscattering signal. The plane of polarization of the incident and scattered beam is switched between parallel and perpendicular positions by alternately rotating the laser and the analyzer. The scattered intensity measurements for different combinations of laser and analyzer positions are given in Table 1. The LDR is determined from these
Ž .
measured intensities. For vertically polarized incident energy, LDR V is the ratio of Ž .
M2rM1 and for horizontally polarized incident energy, LDR H is the ratio of M3rM4.
3. Results
The LDR for sulfuric acid clouds of different concentrations is measured at angles of 458, 1358and 1578to the forward direction. The LDR values plotted in Fig. 2 are those observed during the first 3 min after seeding the cloud. During these 3 min, the cloud is almost in a glaciated state. Each data point on the graph in Fig. 2 represents an average
Table 2
Number density, size and LDRs observed at 1358for different cloud runs
Ž . Ž .
Percentage of Number density Size LDR V LDR H
2
of 25 experiments performed at that particular concentration and error bars represent the standard error. It is seen that the magnitude of LDR increases with the scattering angle. The magnitude of LDR observed at angles closer to backscattering is higher and agrees
Ž .
with the angular scattering pattern for mixed phase clouds Sassen and Liou, 1979 . It is seen that average values of LDR at 458and 1358does not change much as the strength
Ž .
of sulfuric acid increases. However, for 1578, the change is large, LDR V for water is Ž .
0.69, and it increases to 0.85 for 50% sulfuric acid. Similarly, LDR H for water is 0.73, and it increases to 0.92 for 50% sulfuric acid. Scattering experiments at 1578were done only for pure water and 50% sulfuric acid. Since the slide collection port was not at a convenient location while the system was set for observations at 1578, some of the slides are over exposed and the microphysical data associated with theses runs was not good. Due to this reason, we did not carry out experiments for 70% sulfuric acid at this angle. It is seen that the behavior of LDR at 1578 agrees with the polarization lidar studies
Ž .
of corona producing cirrus clouds. Sassen et al. 1995, 1998 have found that these high
Ž .
clouds produce relatively strong laser depolarization f0.5 to 0.8 , indicative of Ž .
complex shaped ice crystals. Sassen et al. 1995 presumed that these are radial particles that have a polycrystalline structure due to the effects of a liquid sulfuric acid coat, which is driven to the surface of the ice germ during droplet freezing under the observed
Ž .
environmental conditions. Sassen and Liou 1979 have observed that the depolarized component is not a simple function of scattering angle. Polarization measurements by
Ž .
Sassen and Liou 1979 demonstrate that the dominant depolarizing process is a function of cloud particle shape.
Table 2 gives the parameters measured for cloud runs with various concentrations of sulfuric acid with the PMT mounted at 1358. If we consider the cases for pure water and
Ž .
50% sulfuric acid, with crystals having the same size 30 mm , it is seen that LDR is higher in the case of pure water as compared to the 50% sulfuric acid cloud. If we consider the case for pure water and 70% sulfuric acid, having similar crystal number
Table 3
Number density, size and LDRs observed at 1358for water and 70% sulfuric acid cloud 4 min after seeding
Ž . Ž .
Concentration Time after seeding Number density Size LDR V LDR H
2 Ž .
per mm rs mm
Water fourth minute 11 25 0.296 0.412
Table 4
LDR at the first and fourth minute after seeding for two different runs having 70% sulfuric acid concentration
Ž . Ž .
Percentage of Time after seeding Crystal number density Size LDR V LDR H Run 2
Ž . Ž .
H SO2 4 % per mm rsec mm
70 first minute 22 15 0.345 0.319 Run A
70 fourth minute 22 20 0.306 0.310 Run B
Ž 2 .
density 13 crystalsrmm rs , it is seen that LDR for pure water is higher than that of the 70% sulfuric acid cloud. The LDR for 70% and for 50% acid concentrations are lower than pure water, which could be due to either the size factor or an acid coat on the crystal or could be due to both effects.
It is difficult to explain the tendency for the depolarization ratio to decrease with the increasing concentration of sulfuric acid at 1358scattering angle. It may be the case that LDR responds to increasingly mixed phase cloud conditions because the more concen-trated acid drops will shrink into haze particles, but would not disappear during seeding like pure water as the humidity drops. It is also possible that the acid coat could be Ž . changing the refractive index of the crystals which is influencing LDR. Kerker 1969 has studied the influence of refractive index on polarization ratio and found that for the same refractive index the behaviour of the polarization ratio is different at different angles.
Although it is possible to monitor the variation of LDR with time during a given cloud cycle, it is difficult to attribute the magnitude of variation in LDR to a particular factor. The number density of crystals is falling with time in a given cloud cycle, which causes LDR to reduce as the ratio of crystal to droplets changes. LDR variation in a cloud cycle, observed at a particular angle, will be a function of not just the
concentra-Fig. 3. Typical photomicrograph of crystalline ice particles collected from a cloud formed from pure water seeded with liquid nitrogen. Dominant crystal habit is plate and average size is;15mm. 1 scale divisions10
tion of sulfuric acid but will also depend on size, shape and number density of the
Ž .
crystals effectively the ratio of crystals to drops at that instant.
To study the effect of sulfuric acid concentration on LDR, we tried to isolate some of the runs having identical number densities and comparable sizes observed at the same time after seeding. LDR values for identical runs at 1358for water and 70% sulfuric acid are tabulated in Table 3. LDR values are smaller for 70% H SO crystals, which could2 4 be either due to the size factor or due to an acid coat. However, the size is not significantly different, and, therefore, it is possible that the effect due to an acid coat is dominant here. Table 4 shows the variation of LDR during the first and fourth minute after seeding for 70% sulfuric acid at 1358 scattering angles and gives values for two separate runs referred as Run A and Run B obtained under similar conditions of
Ž .
Fig. 4. a Photomicrograph of ice crystals collected from a cloud formed from 50% sulfuric acid by weight in
Ž .
water. Dominant crystal habit is plate and average size is ;12 mm. 1 scale divisions10 mm. b
Fig. 5. Photomicrograph of an individual representative ice crystal collected from a cloud formed from 70% sulfuric acid by weight in water. Size is;55mm. 1 scale divisions2.5mm.
concentration, temperature, number density and size. It is clearly seen that LDR for Run B is less than that of Run A. This is because the parameters for Run A are for the first minute when the crystals have sharp edges and that of Run B are for the fourth minute when the crystals have acquired a coat of acid film and the edges are rounded.
Fig. 3–5 shows photomicrographs of crystalline ice particles collected from a cloud formed from pure water, 50% and 70% sulfuric acid, respectively. For crystals formed from pure water, the crystal habit is well defined and the crystal boundaries are clear and sharp. For acid crystals, it is found that each crystal is surrounded by a halo of evaporated acid solution products, which resemble fuzzy dark spots and the crystal habit cannot be recognized. These evaporation products must be responsible for changing the refractive index and shape of the crystal, and thereby the scattering properties. When the cloud is seeded with liquid nitrogen, homogeneous nucleation is initiated by adiabatic cooling thereby causing water and acid to freeze together. As the cloud becomes warmer, the frozen particles start to evaporate and the sulfuric acid leaves the bulk ice Ž . and reaches the surface to form droplets, or a film of sulfuric acid. Sassen et al. 1989 have observed such spots while studying backscatter laser depolarizing properties during evaporation of clouds composed of sulfuric acid solution droplets, some treated with ammonia gas. It is also possible that the acid does not freeze, even when near the LN2
rod, only the water part freezes. When the vapour supply is turned off, the ice particle growth will tend to reduce the ice super-saturation, and it will take a while for evaporation to begin to occur. The end result, however, would be the same, an acid coated ice particle.
4. Discussion
We have observed that as the sulfuric acid concentration in a droplet cloud increases, the ice crystals tend to loose their original shape and they no longer have a sharp
Ž .
idealized crystal shapes describe only 3% of the particles in samples of different kinds of clouds in the temperature range 08C toy458C. The commonly observed irregularly
Ž shaped particles either consisted of faceted polycrystalline particles or sublimating solid
.
to vapour ice particles with smooth curving sides and edges.
From our observations of crystals formed from a high acid concentration, it appears that there is a film of acid drops forming on the surface. It could be possible that when nucleation is done using liquid nitrogen the acid gets frozen but later, as the temperature rises, the acid no longer remains in the crystal and comes out on the surface. It is also possible that acid does not freeze even when near the LN cooled rod, only the water2 part freezes. Even if the vapour supply is turned off the ice particle growth will lead to ice saturation, and it will take a while for evaporation to begin to occur. The end result,
Ž . however, would be the same, an acid coated ice particle. Chen and Crutzen 1994 have studied the possible influence of trace chemicals on growth and lifetime of ice crystals and have found that ice phase chemical reactions do not occur in bulk phase because of the strong bonding of the ice lattice. However, they can be very active on the surfaces as well as in the grain boundaries. Once the ice particle starts to evaporate, solute that was originally in the bulk ice will be released to the surface. The surface vapour pressure of the ice particle can thus be depressed by an amount that is dependent on the solute concentration and thickness of this surface coating. This coating prevents further evaporation of the ice crystal and their lifetime increases. After being coated with liquid, the shape of the crystal is not the same. The density, terminal fall velocity, growth rate and radiative properties, which is dependent on the shape of the crystal, gets modified
Ž .
and could be important for climate warming Korolev et al., 1999 .
The amount of depolarization, which is a function of radius and the coating on the surface of the crystal, is expected to change due to the change in shape of the ice crystal.
Ž . Ž .
It is seen that the variation of LDR V and LDR H is sensitive to the changes in the
Ž .
number density effectively the ratio of crystals to droplets , size, shape and also to the acid coat on the surface of crystals. In a recent theoretical study, Mishchenko and Sassen Ž1998 note that there is a strong depolarization dependence on the size of ice particles.
Ž . Ž .
typical of young contrails. Our observed values of LDR V and LDR H for water are in Ž .
agreement with those reported by Sassen and Liou 1979 . The behaviour of LDR at 1578, which is close to the backscattering angle, agrees with the observations of Sassen
Ž .
et al. 1995, 1998 taken at 1758 for corona producing cirrus clouds. The increase in LDR with sulfuric acid strength is attributed to the acid coat on the crystal surface resulting in complex ice particles. Because the occurrence of a liquid layer on tropo-spheric ice particles may enable important heterogeneous chemical processes, laboratory studies to evaluate the significance of such processes are important to derive the
Ž
parameterisations needed for global scale chemistry transport models Lelieveld and .
Voloshchuk, 1991 .
5. Conclusions
per cloud run to gather the microphysical properties, viz. size, shape and number density. It was observed that the crystals formed from sulfuric acid started to grow as regular crystals with clean well-defined edges but in the course of growth, an acid coat developed on the crystal surface. Due to this coat, the crystal edges are rounded off and crystal shapes become more spherical.
The experiments carried out at 1578 are closest to the backscattering angle in the present set up and therefore can be compared with the lidar observations. It is seen that LDR is higher for acid crystals at 1578, a result that agrees with the polarization lidar
Ž .
studies of corona-producing cirrus clouds reported by Sassen et al. 1995, 1998 . The magnitude of LDR at 1578 is also higher than that observed at 458 and 1358, which agrees with the angular scattering pattern for mixed phase cloud. It is seen that the
Ž . Ž .
variation of LDR V and LDR H is sensitive to the changes in the number density Žwhich is effectively the change in the crystal to droplet ratio , size, shape and also to. the acid coat on the surface of the crystals.
Acknowledgements
The authors acknowledge the funding received from the US Army for setting up the Cloud Physics Laboratory. They are thankful to Dr. Hugh Carlon and Dr. Jerry Comati of the US Army for their support. They thank Dr. Kenneth Sassen from University of Utah for his useful suggestions. The authors are also thankful to both reviewers for their
Ž . valuable comments. S.T. acknowledges the fellowship received from CSIR India for carrying out this research.
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