Freezing of sulfuric acid-water and nitric acid-water aerosols at polar stratospheric temperatures

The freezing of H2SO4-H2O and HNO 3-H2O particles of sub-micron diameter was measured using FTIR extinction spectroscopy and a low-temperature flow tube. Temperatures ranging from approximately 190 K to approximately 150 K were employed in the HNO3-H2O experiments, and temperatures from approximately 240 K to approximately 180 K were employed in the H2SO4-H 2O experiments.; In dilute H2SO4-H2O particles (<35 wt % H2SO4), ice formed at approximately 35 K below the corresponding solid-liquid equilibrium temperatures. The freezing temperatures of these particles are in reasonable agreement with theoretical calculations and other experimental data. The homogeneous nucleation rate associated with our freezing temperatures is between 1 x 109 cm -3 sec-1 and 4 x 1010 cm-3 sec-1. In contrast to dilute H2SO4-H2O aerosols, concentrated H2 SO4-H2O aerosols (>35 wt % H2SO 4) did not freeze. From the experimental results, an upper limit of 4 x 1010 cm-3sec-1 was calculated for the nucleation rate. This result agrees with other laboratory results on this system and the suggestion that stratospheric sulfate aerosols are liquid.; Nitric acid-water aerosols with compositions of 2:1 (moles H2O:moles HNO3) and 3:1 froze as NAD and NAT, respectively. The measured nucleation rate constants for the 2:1 particles are between (in units of cm -3 s-1 x 1011) 0.26 +/- 0.05 and 16 +/- 9 at temperatures between 178.8 and 175.8. The rates for 3:1 particles are between (in units of cm-3 s-1 x 1011) 0.38 +/- 0.18 and 97 +/- 63 at temperatures ranging from 167.2 K to 163.5 K. Freezing of non-stoichiometric nitric acid aerosols was also investigated. Aerosols with compositions ranging from 1.2 to 1.4 did not crystallize at any of the temperatures investigated. NAD formed in particles with compositions ranging from 1.4:1 to 2:1, and both NAD and NAT formed in aerosols with compositions ranging from 2.7:1 to 3:1. The highest freezing temperature in this composition range (1.2:1 to 3:1) corresponds to a composition of approximately 2:1. Based on this information and the nucleation rates determined from the 2:1 experiments, it is unlikely that concentrated nitric acid-water particles (1.2:1 to 3:1), if formed in the polar stratosphere, will freeze.