Physical Chemistry of Microparticles :
Chemistry in very small volumes

In the beginning of the 90's, it became more and more clear, that heterogeneous chemical reactions between gaseous chlorine compounds and stratospheric particles formed during the polar winter were mainly responsible for the large ozone losses over Antarctica in polar springtime [1]. The heterogeneous processes take place on or inside the liquid phase of background aerosol particles (mainly liquid sulfuric acid) as well as on the so-called Polar stratospheric clouds (PSCs) which are formed at low temperatures in the polar regions. The latter consist of water ice and sulfuric and nitric acid. It is know from laboratory experiments that the heterogeneous reaction rates are largely dependent on the microphysical state of the condensed phase. However, it is difficult to predict the microphysical state of the particles from meteorological conditions only because the liquid particles are often far away from thermodynamic equilibrium.

We therefore designed an experiment to promote research on particles far from thermodynamic equilibrium. This work has been funded by the Kommission für Forschung und Nachwuchs of the Freie Universität Berlin. The main objective of the newly built apparatus was to measure the time scales of nucleation and crystallisation of stratospheric, supercooled liquid droplets, and to explore whether heterogeneous reaction rates of these liquids differ from earlier measurements on macroscopic samples. Large supercoolings, as they occur in the free atmosphere, are only possible in the laboratory if one avoids completely the wall contact of the liquid. We therefore used a the technique of single particle levitation in order to study single droplets.

Another motivation for the study of single aerosol particles is the possibility to have direct access to their optical properties which change significantly with the microphysical state. These results will be very valuable for a more quantitative evaluation of remote sensing aerosol measurements with the LIDAR technique (LIDAR = Light detection and ranging). Furthermore, exact scattering parameters are needed when modelling the radiative forcing of aerosol and cloud particles. There are currently large uncertainties concerning the climate forcing of areosols [2].

Experiments performed and principal results (see also papers published in 1999 and 2000): We constructed a 3 dimensional electrodynamic trap [3] in order to capture single, micrometric droplets (10-250 µm). The trap is situated in a chamber, where stratospheric conditions can be realised (low temperatures down to 150 K and low pressures). An injected droplet can be stored for an unlimited time. It is illuminated with a strong, linearly polarised He/Ne Laser (l = 632.8 nm). The scattered light is observed time and angle resolved with a CCD camera. Images are digitised with a programmable frame grabber and further analysed using Mie theory [4]. This yields diameter an refractive index of the droplet with high precision. The analysis of the polarisation of the scattered light permits to distinguish between liquid and frozen particles (for more details on the experiment see article 3).

In order to study the dynamics of the freezing process, droplets are injected into the cold trap. They are observed by light scattering until they begin to freeze indicated by the occurrence of depolarised, scattered light. This time is identified as being the time of freezing nucleation. The exact measure of diameter of each droplet observed enabled us to calculate homogeneous freezing nucleation rates J (in units of s^-1cm^-3) with a precision never reached so far. If the viscosity of the liquid under study is enhanced, like it is the case in sulfuric acid solutions, also the crystallisation dynamics can be investigated.

The measure of J in function of the temperature allows, with the aid of classical nucleation theory [5], to examine the energetics of critical germ formation. It is possible to derive the free energy of germ formation (DF_g) together with the free diffusion activation energy (DF_act). The latter energy is related to the transport and reorientation of molecules necessary to form a crystalline cluster inside the supercooled liquid (For the results obtained for water, see articles 2 and 3, for results on supercooled sulfuric acid solution see article 4, experiments on the freezing of ternary solution droplets H₂SO₄/H₂O/HNO₃ at stratospheric temperatures will be published soon). The obtained nucleation rates can further be used to predict the freezing times scales of polar stratospheric clouds and thus leading to a more quantitative understanding stratospheric chemistry.

Another part of the experiments performed with the new apparatus consisted in the detailed study of the dynamics of the HCl-uptake by single sulfuric acid solution droplets. Hydrogen chloride is a stratospheric trace gas constituent and plays a key-role in the heterogeneous chemistry of the PSC. The uptake of stratospheric trace gases like HCl, HBr or HNO₃ by PSC model systems had so far only been studied by using macroscopic samples fast droplet jets. In these experiments the gas-uptake is always monitored indirectly by observing the gas phase depletion of the reactant gas. Experiments in our work avoid disadvantages arising from the previous techniques as the HCl uptake of supercooled sulfuric acid is directly observed in the liquid phase for the first time. Parameters, such as liquid diffusion and Henry's Law coefficients could be determined in a straightforward manner under realistic stratospheric conditions. The results on the diffusion coefficients inside the supercooled liquid showed a breakdown of the Stokes/Einstein relation. This will lead to a better modelling of PSC chemistry in the future. Furthermore we could propose a new method to quantify accomodation coefficients of reactive gases on supercooled liquids (see article 5).

I also constructed a special TOF mass spectrometer equipped with a coolable droplet manipulator for the chemical analysis of the stored droplet itself. This system is able to transfer a single microdroplet from the trap into the high vacuum of the spectrometer where it is vaporised. The droplet vapour is then ionised by an electron beam to produce the ions necessary for mass spectrometric analysis. Preliminary studies were very promising and the system will be used for future studies.

[1] S. Solomon, Nature 347 (1990), 347.

[2] S.E. Schwartz, M.O. Andreae, Science 272 (1996), 1121.

[3] W. Paul, M. Raether, Z. Phys. 140 (1955), 262.

[4] G. Mie, Ann. d. Physik 25 (1908), 377.

[5] H.R. Pruppacher, J.D. Klett, Microphysics of clouds and precipitation, Reidel, Dordrecht 1978.