Laboratory studies of ternary H2SO4/HNO3/H2O particles: Implications for polar stratospheric cloud formation

It has recently been suggested that type Ib polar stratospheric clouds are composed of supercooled ternary‐solutions of sulfuric acid (H2SO4), nitric acid (HNO3), and water (H2O). We have studied the low‐temperature behavior of ternary‐solution aerosols to determine if they will undergo homogeneous freezing nucleation under polar stratospheric conditions. Ternary‐solution aerosols were injected into a low‐temperature chamber and observed for periods of up to 3 hours. Fourier transform infrared spectroscopy was used to determine the aerosol composition and phase as a function of time. We found that ternary‐solution aerosols with compositions similar to those expected in the polar stratosphere remained supercooled for the duration of our experiments. Homogeneous freezing of the particles was never observed, even after warming from 190 to 204 K. However, heterogeneous freezing was occasionally observed for particles adhering to the infrared optics within the chamber.