Freezing nucleation of levitated single sulfuric acid/H2O micro-droplets.: A combined Raman- and Mie spectroscopic study

The phase behaviour of single sulfuric acid/H2O micro-droplets upon supercooling has been studied using optical levitation of such droplets in combination with Raman- and Mie spectroscopy for chemical and size analysis. It is found that the freezing nucleation behaviour depends on the sulfuric acid concentration of the droplets and hence the region of the phase diagram which is reached upon cooling. Whereas for diluted droplets (C-H2S04 < 30 wt%) instantaneous ice nucleation is observed, more concentrated droplets tend to supercool with no tendency to nucleate the thermodynamically stable hydrate phase sulfuric acid tetrahydrate (SAT) or sulfuric acid monohydrate (SAM). The upper limits of the nucleation rate in this regime were determined to be J less than or equal to 5 x 10(5) cm(-3) s(-1). (C) 2003 Elsevier B.V. All rights reserved.