Fig. 5. Photomicrograph of an individual representative ice crystal collected from a cloud formed from 70 sulfuric acid by weight in water. Size is ;55 mm. 1 scale divisions2.5 mm.
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 LN
2
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 .
boundary. Korolev et al. 1999 from aircraft measurements in the arctic, found that
idealized crystal shapes describe only 3 of the particles in samples of different kinds of clouds in the temperature range 08C to y458C. 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 water
2
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
Ž .
1998 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
