INFRARED-SPECTROSCOPY OF SULFURIC-ACID WATER AEROSOLS - FREEZING CHARACTERISTICS

A low-temperature flow cell has been used in conjunction with a Fourie r transform infrared (FT-IR) spectrometer to study sulfuric acid/water aerosols. The aerosols were generated with a wide range of compositio n (28 to 85 wt %), including those characteristic of stratospheric sul fate aerosols, and studied over the temperature range from 240 K to 16 0 K. The particles exhibited deep supercooling, by as much as 100 K be low the freezing point in some cases. Freezing of water ice was observ ed in the more dilute (<40 wt % sulfuric acid) particles, in agreement with the predictions of Jensen et al. and recent observations by Bert ram et al. In contrast with theoretical predictions, however, the enti re particle often does not immediately freeze, at least on the timesca le of the present experiments (seconds to minutes). Freezing of the en tire particle is observed at lower temperatures, well below that chara cteristic of the polar stratosphere.