Abstract
The phase of stratospheric aerosol particles is an important input parameter
for model calculations of ozone depletion over polar regions, since the
rates of heterogeneous chemical reactions are sensitive to the phase of
the involved particles. We have determined homogeneous and heterogeneous
nucleation rates of single levitated micro droplets of water, sulfuric
acid and ternary solutions of H2S4O, HNO3, and water using an electrodynamic
quadrupol trap housed inside a climate chamber. Concentrations of the sulfuric
acid solutions range between pure water and 57.7 wt.%. The ternary solutions
are of compositions as expected at stratospheric temperatures between 186
K and 196 K. Single droplets of a certain composition are injected directly
into the cold quadrupol trap and are stored until freezing occurs. The
freezing of many droplets at a certain temperature and concentration is
observed to determine the rate of homogeneous nucleation. The droplet size
is measured by angular resolved detection of the scattered light from a
linearly polarized Helium-Neon laser. The onset and the duration of the
freezing process are detected through a change in the depolarization ratio
of the scattered light. The new experimental technique allows to monitor
the onset of the nucleation over a long time range and avoids any contact
between the liquid and containment walls. All necessary parameters to derive
the nucleation rate can be determined with high accuracy: the temperature,
the radius of each investigated droplet, the time a droplet stays supercooled,
the composition of the droplet and the statistical distribution governing
the nucleation process. In consequence homogeneous nucleation rates can
be derived with an up to now unknown precision. This is shown for example
with pure water droplets. In the range of only one Kelvin (between 236
and 237 K) ten different nucleation rates could be determined for the
homogeneus nucleation of water. The nucleation rate changes in this range one and a half order of magnitude.
Furthermore it is possible to seed the droplets with ice germs to observe
heterogeneous nucleation. The droplets investigated here are larger than
the cloud particles in the stratosphere. Nevertheless, the derived nucleation
rates can be scaled to apply for atmospheric aerosol. Comparisons of our
results with stratospheric freezing events observed with in situ techniques
allow to characterize the polar stratospheric clouds (PSC). Under stratospheric
conditions, homogeneous nucleation could be only observed in dilute droplets
containing more then 77 wt. % water. In consequence, homogeneous nucleation
is the process which leads to the freezing of water rich clouds (PSC of
the type II). Nevertheless, heterogeneous nucleation is possible for droplets
of ternary solutions with higher acid content. A possible freezing mechanism
for the clouds having droplets of these compositions (PSC of type Ia) is
therefore the heterogeneous freezing process. |
