The study of liquid-solid and glass transitions in finely divided aqueous systems Anatoli Bogdan Institute of Physical Chemistry, University of Innsbruck Austria and Department of Physics, University of Helsinki Finland

Contents Introduction - atmospheric aerosol - what is finely divided aqueous systems (FDAS)? - why the study of FDAS is important? Transitions for study - freezing, melting, and glass transition Methods - differential scanning calorimeter (DSC) - powder X-Ray diffraction (XRD) - optical microscope - Raman spectroscopy - Summary

What is aerosol? There is no strict definition of aerosol. Aerosol is a suspension of solid or liquid particles in a gaseous phase (air). It is important to remember that the term of aerosol includes both the particles and the suspending air.

Types of aerosol particles in the atmosphere - Bioaerosol – viruses, bacteria, fungi, spores, and pollen. - Cloud – a visible aerosol (liquid or solid) with defined boundaries. - Dust - solid particles formed by mechanical disintegration of a parent material. - Fume – solid particles produced by the condensation of vapours or gaseous combustion products. They are usually agglomerates of primary particles. - Mist and Fog – liquid aerosol particles formed by condensation. - Smog (derived from Smoke and Fog) – aerosol formed by the action of sunlight on hydrocarbons and oxides of nitrogen. - Smoke – aerosol particles resulting from incomplete combustion. These particles may be solid or liquid. Solid particles may be agglomerated like fume particles. - Spray – aerosol liquid droplets formed by the mechanical breakup of a liquid.

Mount Pinatubo, June 13, 1991 (Image courtesy of NOAA)

Aerosol mass concentration also ranges in very large limits from to 10 3 g/m 3. It is important to remember that Volume ratio or mass ratio in parts per million (ppm) is not used for aerosol, because two phases are involved. Numerically, aerosol concentration in ppm would be very low.

Aerosol particles impact on radiative properties of the atmosphere and visibility

Liquid aqueous aerosol droplets are the precursors of high altitude cold clouds: -upper tropospheric cirrus ice clouds -polar stratospheric clouds (PSCs)

The composition of liquid aerosol droplets determines the formation and microphysical properties of high-altitude cold clouds. In turn, the cloud microphysical properties determine the radiative properties of cirrus ice clouds and the rate of heterogeneous chemical reactions leading to the ozone destruction in the polar stratosphere and upper troposphere (UT).

Why it is important to know the rate of chemical reactions on these types of cold clouds? - the formation of ozone holes in the winter/spring time polar stratosphere - in the UT, ozone is important greenhouse gas

In situ observations of high-altitude clouds cannot give a reliable microphysical picture, let along the mechanism of formation: The existing aircraft instrumentations are usually optical particle detectors and counterflow virtual impactors. They collect particles, evaporate them, and measure the amount of evaporated water and the residual aerosol particles. The existing aircraft instrumentations are unable to properly characterize the composition and phase state of small (< 20 µm) cloud particles.

The elaborated laboratory measurements of the low temperature behavior of small (sub-micrometer and micrometer-scaled) aqueous droplets of size and composition relevant for the atmosphere can improve our understanding of the mechanisms of the formation and microphysics of the UT cold cirrus ice clouds and PSCs.

What do we need? The laboratory droplets should be similar to the atmospheric aerosol droplets and exist in similar environmental conditions - size - composition - cooling rates An important moment is to eliminate or reduce the contact of droplets with any surfaces which may impact on the freezing behavior of droplets.

What are finely divided aqueous systems (DFAS)?

Emulsions - suspensions of a large population of aqueous droplets in neutral homogeneous oil-phase.

Drop diameter is between ~ 0.1 and 5 μm An example of emulsified FDAS Emulsified 10 wt % HNO 3 solution

Emulsified 10 wt % H 2 SO 4 solution Drop diameter is usually between ~0.1 and 6 m m

The populations of micrometer-scaled aqueous droplets on hydrophobic surfaces can be produced by nebulizer.

Droplets are on hydrophobic surface. 50 μm

In our study, we use large bulk droplets (~mm-scaled) and micrometer-scaled droplets. Using these two types of droplets allows us to notice how the low-temperature behavior changes with the decreasing of the size of droplets.

In the emulsions, we can produce and then simultaneously study millions of droplets which size and composition are similar to those encountered in the upper troposphere.

Methods of the study: 1. Differential scanning calorimeter (DSC).

The advantages of DSC method: 1.Being very sensitive, DSC provides precise knowledge (thermal picture) of what is going on during the cooling/warming of aqueous solutions, i.e., reveals all phase transitions.

The example of DSC curves which show freezing and glass transition (on cooling) and glass transition, crystallization (freezing), and melting on warming the of bulk aqueous droplet of 25/3 wt % H 2 SO 4 /HNO 3

Large solute concentration: there are no freezing events on cooling and warming, only glass transition.

2. DSC allows to study how the addition of solute changes freezing temperature of droplets.

The addition of acid reduces freezing temperature. (Pure water and H 2 SO 4 /H 2 O droplets of the same weight).

3. DSC allows to notice the peculiarities of freezing behavior in pure water and aqueous droplets.

The addition of H 2 SO 4 does not change the character of freezing process, i.e., in both cases the freezing peak is similar (sharp).

4. DSC method clearly demonstrates how the size of droplets changes the freezing temperature of droplets.

The freezing temperature depends on the size of droplets (the case of pure water).

The freezing temperature of droplets depends on size (the case of H 2 SO 4 /H 2 O droplets).

Methods of the study: 2. powder X-Ray diffraction (XRD)

An example of the XRD spectrum of ice formed in H 2 O-in-oil emulsion

Methods of the study: 3. Optical microscope

SA-25 wt%

Methods of the study: 4. Raman spectroscopy

Raman spectroscopy allows to study the distribution of solute within aqueous droplets and the character of phase transition in them.

The most recent DSC measurements relevant for the atmosphere.

Example of phase separation during the freezing of emulsified solution H 2 SO 4 /H 2 O.

Example of freezing/melting curves obtained from emulsified ternary solutions H 2 SO 4 /HNO 3

Phase diagram shows how increasing solute (H 2 SO 4 ) concentration reduces freezing temperature until glass transition (vitrification) occurs.