MECHANISMS AND TEMPERATURES FOR THE FREEZING OF SULFURIC-ACID AEROSOLS MEASURED BY FTIR EXTINCTION SPECTROSCOPY

We have measured the freezing curve of liquid H2SO4/H2O aerosol drople ts having average radii of approximately 0.2 mu m. We form the aerosol by the reaction of SO3 with H2O and flow it through a temperature-con trolled flow,tube equipped with reentrant windows, through which we ma ke observations by FTIR extinction spectroscopy. At the freezing point , a microcrystallite of pure ice (H2O(s)) nucleates in the aerosol dro plet, and this causes a small change in the spectrum near 3250 cm(-1). By recording the temperatures at which the crystallites appear for di fferent acid concentrations, we are able to map out the freezing curve . In the following account, we describe the experimental technique and report the freezing curve for the concentration range up to 35 wt % H 2SO4, which corresponds to the first eutectic point on the phase diagr am of the bulk material. We find that the aerosol supercools by about 35 K below the temperature at which the corresponding bulk material fr eezes. Our data show that the overall freezing mechanism is similar to that of the bulk solution: after nucleation, the crystallite grows wi th decreasing temperature, causing the remaining acid to become more c oncentrated due to the removal of H2O until eventually a eutectic mixt ure forms.