Laboratory studies of sulfate aerosols at stratospheric temperatures

olar stratospheric clouds (PSCs) have been implicated in polar ozone loss. Type II PSCs are water/ice particles, whereas type I PSCs are thought to contain nitric acid and water mixtures, but their exact composition and phase are not known. Lidar depolarization measurements indicate that the most common form of type I PSC is crystalline. However, it is not clear how crystalline PSCs form. Suggestions include nitric acid trihydrate (NAT) condensation on frozen sulfate aerosols and NAT nucleation out of ternary ;To study the low temperature behavior of stratospheric aerosols, a laboratory instrument was built which can monitor aerosols for periods of up to five hours. Sulfuric acid particles with median diameters near 0.5 micrometers were generated using a constant output atomizer source. The particles were then exposed to water and nitric acid vapors before being injected into the low temperature chamber. Multipass transmission FTIR spectroscopy was used to determine the phase and composition of the aerosols as a function of time.;Settling experiments with binary ;Experiments were also carried out to probe the freezing behavior of the ternary...