THE ICE NUCLEATION BEHAVIOR OF AMINO-ACID PARTICLES

The problem of whether ice nucleation takes place more readily from the vapor directly to the solid, or via an intermediate liquid phase has been studied for several of the more efficient amino-acid nucleators. It has been shown that the threshold temperatures observed in cloud chamber tests are in fact those of the material acting as freezing nuclei (i.e. via the liquid phase), and any discrepancies between such tests and trials with bulk water may be accounted for satisfactorily by partial destruction of the nucleus surface by the water. Investigations on ice formation about airborne particles and on macroscopic amino-acid crystals have shown that for certain of these substances a transition in behavior takes place around −20 °C. Below this temperature, ice formation no longer requires saturation conditions with respect to supercooled water and so the particles may be considered to act by converting the vapor directly to ice, and can, therefore, be designated sublimation nuclei.The major obstacle in the way of airborne particles acting as freezing nuclei has been the requirement that they act first as condensation centers. Under the conditions prevailing in supercooled clouds with vapor pressures equal to, or barely exceeding that of water saturation, condensation is unlikely on the somewhat hydrophobic surfaces of amino-acid particles. It has been shown, however, by using a radioactive tracer in small water droplets that droplet–particle collisions can occur. While not efficient, this process would permit a few particles in a cloud chamber experiment to act as freezing nuclei, thereby establishing the potential activity of the material itself.