HOMOGENEOUS ICE NUCLEATION OBSERVED IN SINGLE LEVITATED MICRO DROPLETS

Homogeneous ice nucleation rates in single isolated micro droplets of water are determined under various atmospheric conditions inside an el ectrodynamic Paul-trap. The droplet size is measured by angle-resolved detection of the scattered light from a Helium-Neon laser. The freezi ng process is detected by a sudden change of the depolarization ratio which is an indication for the formation of nonspherical particles. Th e experimentally determined homogeneous ice nucleation rates J(LS) are J(LS) = (2.2 +/- 0.4). 10(8) cm(-3) s(-1) at T = (- 37.2 +/- 0.15)deg rees C and J(LS) = (5.6 +/- 1.0). 10(7) cm(-3) s(-1) at T = (- 37.7 +/ - 0.15)degrees C. These values are much more precise as determined by measurements inside a cloud expansion chamber. The method of single pa rticle observation eliminates complications arising from these nucleat ion rate measurements. The results underline the theory that one singl e ice germ of certain size is sufficient to induce the freezing of the whole droplet. The depolarization ratio of the scattered intensity in the parallel and perpendicular polarization plane I-s/I-p from the fr ozen particles exhibits a broad distribution with a mean value of I-s/ I-p = 0.3.