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Atmospheric Research 53 2000 219–230 www.elsevier.comrlocateratmos
Laser depolarization studies of simulated crystallized H OrH SO clouds
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Shubhangi Tavker, P. Pradeep Kumar
Department of Physics, UniÕersity of Pune, 411 007 India Received 27 May 1999; accepted 22 December 1999
Abstract
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Laboratory experiments were carried out to investigate the linear depolarization ratios LDRs of crystallized H OrH SO acid clouds at angles of 458, 1358 and 1578 to the forward direction.
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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 1578 and 1358 than 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. q 2000 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: sutphysics.unipune.ernet.in S. Tavker , ppkphysics.unipune.ernet.in P.P. Kumar .
0169-8095r00r - see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž
. PII: S 0 1 6 9 - 8 0 9 5 0 0 0 0 0 3 6 - 3
number densities between about 0.001 and 1 cm
y3
and are formed in the upper Ž
troposphere wherein atmospheric conditions range from y308 to y708C 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 Ž
. Jensen 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
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tropospheric aerosol consists of aqueous H SO droplets
about 75 by weight
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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 and
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. 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 low
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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., CO
and water
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. 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 trigger
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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 SO rH O clusters in a
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jet engine exhaust plume were given by Frenzel and Arnold 1994 . In the upper troposphere and lower stratosphere, aircraft-derived H SO
may form new aerosol
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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
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Jensen 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 SO rH O droplets with radii less than 0.01 mm with number densities around
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2 y3
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1000 cm Laaksonen et al., 1997 . Recent upper tropospheric measurements by
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MPIK-Heidelberg Mohler and Arnold, 1992 indicate that the solid particles are coated
¨
by a supercooled liquid solution, which is formed by heterogeneous H SO rH O
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nucleation and condensation since gaseous sulfuric acid is continuously formed during daytime and highly supersaturated with respect to supercooled liquid H SO rH O. It is
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conceivable that the coating may also proceed by coagulation of the very small but numerous H SO rH O droplets with solid aerosol particles.
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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
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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
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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
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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
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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
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variation in the linear depolarization ratio LDR at various scattering angles with changing acid concentrations.
2. Experimental arrangement
