Laboratory studies of thin films representative of atmospheric sulfate aerosol

Sulfate aerosols are present globally in both the upper troposphere and lower stratosphere. These aerosols are of great interest because they have a profound influence on Earth's radiation balance, heterogeneous chemistry, and cloud formation mechanisms throughout the atmosphere. The magnitude of these effects is ultimately determined by the size, phase, and chemical composition of the aerosols themselves. This thesis explores some of the questions that remain concerning the phase of these aerosols under atmospheric conditions and the effects of their chemical composition on heterogeneous chemistry and cloud formation mechanisms.; In the upper troposphere, cirrus clouds are thought to form via the homogeneous nucleation of ice out of dilute sulfate aerosols such as ammonium sulfate ((NH₄)₂SO₄). To investigate this, the low-temperature phase behavior of ammonium sulfate films has been studied using Fourier transform infrared (FTIR) spectroscopy. Experiments performed as a function of increasing relative humidity demonstrate that a phase transition from crystalline (NH ₄)₂SO₄ to a metastable aqueous solution can occur at temperatures below the eutectic at 254 K. However, on occasion, direct deposition of ice from the vapor phase was observed, possibly indicating selective heterogeneous nucleation.; In addition to serving as nuclei for cirrus clouds, sulfate aerosols can participate in heterogeneous reactions. The interaction of HNO₃ with ammonium sulfate has been investigated as a possible loss mechanism for gas-phase HNO₃ using a Knudsen cell reactor coupled with transmission FTIR spectroscopy. The results show that HNO₃ reacts with solid ammonium sulfate to produce ammonium nitrate and letovicite at 203 K. Furthermore, this reaction is enhanced as a function of relative humidity from 0 to 41%.; In the lower stratosphere, polar stratospheric clouds (PSCs) are important for springtime ozone depletion. The vapor deposition of ice on sulfuric acid tetrahydrate (SAT) has been studied as a possible formation mechanism for Type 2 PSCs. Results show that SAT is an efficient ice nucleus, requiring a supercooling of just 0.1–1.4 K below the ice frost point. Possible implications of this heterogeneous nucleation mechanism for PSC formation have been examined through use of a microphysical/photochemical model.