Title: Trace Gas-Induced Brine and Disordered Interfacial Layers on Ice
Description: Brine and disordered interfacial layers (DILs) on ice are known to significantly alter the interaction between trace gases and ice surfaces. In order to better predict the impact of a warming climate on atmospheric composition, a quantitative physical understanding of the interaction between ice surfaces and key atmospheric trace gases is essential. In the series of complementary studies presented in this dissertation, theoretical and experimental techniques were employed to gain insight into the phenomenon of interfacial layers. A brine layer model developed based on equilibrium thermodynamics is the first model to have the unique capability of taking into account the non-ideality of the brine, as well as incorporate equilibrium partitioning among the gas, brine, and ice phases. The model was applied to study ice systems containing NaCl, HNO_3, and HCl. We found that for all three systems, accounting for the non-ideal behavior of the brine was important, especially at low temperatures.

Our findings also suggest that gas phase HNO_3 and HCl can induce the formation of brine layers on ice at temperatures close to the bulk melting point. In addition, we identified environmentally relevant regimes where brine is not predicted to exist, but the presence of DILs can still significantly impact air-ice chemical reactions. Next we examined the interaction of HNO_3 with water ice for partial pressures 2×10^-8 Torr to 1×10^-5 Torr and at temperatures from 216 to 256 K using (i) the surface-specific technique ellipsometry and (ii) a coated wall flow tube (CWFT) reactor, both coupled with chemical ionization mass spectrometry (CIMS) detection of HNO_3 in the gas phase. Our ellipsometry results show that exposure to HNO_3 induces surface disordering on ice at a range of environmentally relevant temperatures and HNO_3 partial pressures, particularly in the vicinity of the boundary between the ice and the HNO_3·3H_2O phases. The coated wall flow tube studies indicate that the nature of HNO_3 uptake changes from reversible adsorption to a continuous flux of HNO_3 into the bulk in the presence of a disordered interfacial layer.

These results have implications for atmospheric chemistry in the upper troposphere and in Polar regions. Ellipsometry and CWFT-CIMS techniques were also used to study HCHO and CH_3CHO in partial pressure and temperature conditions akin to polar snowpack interstitial space. Although these light aldehydes do not interact as strongly with the ice surface as does HNO_3, they were also found to induce surface disorder, especially at low temperatures. HCHO and CH_3CHO were also observed to cause the formation of opaque domains. For HCHO, the opaque domains were associated with high solute loading on the surface combined with temperature cycling. CWFT-CIMS studies of HCHO uptake by ice, show a hint of hydrate formation signature, though the result is not clear enough to be conclusive. Acetaldehyde ellipsometry studies revealed a hitherto unknown DIL surface transition at around 223±2 K, in agreement with earlier findings that partitioning behavior of CH_3CHO is different above and below this transition temperature.

Our findings regarding ice surface modification by HCHO and CH_3CHO alter the typical view of simple Langmuir adsorption and suggest that more complex interactions are occurring at the air-ice interface that can potentially impact exchange of trace gases between snow and the polar boundary layer.