Interest in quantifying processes of ice formation in the atmosphere has led to the recent development of new laboratory techniques, including an aerosol flow tube (AFT) reactor employed for the study of the ice nucleation kinetics of suspended submicrometer aqueous particles. The AFT technique employs an infrared (IR) beam along the flow tube axis. Spectral changes between 700 and 6000 cm<sup>-1</sup> indicate the formation of ice at sufficiently cool temperatures. Apparent freezing temperatures are determined as a function of condensed-phase mole fraction composition. A typical aqueous chemical system is (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub>/H<sub>2</sub>O. The mole fraction composition of the condensed-phase of this aerosol is determined by the ratio of the integrated spectroscopic bands for H<sub>2</sub>O and SO<sub>4</sub><sup>2-</sup>. A key uncertainty in the AFT-IR technique is the freezing mechanism, and knowledge of the mechanism is essential to estimate homogeneous nucleation rates (<i>J</i>, cm<sup>-3</sup> s<sup>-1</sup>) from observed apparent freezing temperatures. The current work provides observational and modeling spectral evidence, based upon changes in the scattering component of the recorded IR extinction spectra with temperature, that observed ice freezing events at warmer temperatures arise from the following mechanism: relatively few particles in the aerosol freeze (e.g., 1 in 10<sup>6</sup>) and this primary event is followed by rapid scavenging of water vapor to grow the few ice particles into large ice particles observed in the IR spectra. Correspondingly, the remaining aqueous particles partially evaporate. In contrast, the spectral evidence provides support that a modified mechanism is operative at cooler temperatures: the ice freezing event consists of the freezing of a much larger fraction of the particles (e.g., 1 in 10) accompanied by a much less important vapor-phase mass transfer event.

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