A theoretical model describing the motion of a small, fast rare gas atom as it passes over a liquid surface is developed and discussed in detail. A key feature of the model is its reliance on coarse-grained capillary wave and local mode descriptions of the liquid surface. Mathematically, the model is constructed with several concepts from probability and stochastic analysis. The model makes predictions that are quantitative agreement with neon-liquid surface scattering data collected by other research groups. These predictions include the dominance of single, rather than multiple, neon-liquid surface collision dynamics, an average of 60 % energy transfer from a neon atom upon colliding with a non-metallic surface, and an average of 25 % energy transfer upon colliding with a metallic surface. In addition to this work, two other investigations into the gas-liquid interface are discussed. The results of an experimental investigation into the thermodynamics of a gas flux through an aqueous surface are presented, and it is shown that a nitrous oxide flux is mostly due to the presence of a temperature gradient in the gas-liquid interface. Evidence for a reaction between a carbon dioxide flux and an ammonia monolayer on an aqueous surface to produce ammonium carbamate is also found. The second of these is an investigation into the mechanism of bromine production from deliquesced sodium bromide aerosol in the presence of ozone, and involves a sensitivity and uncertainty analysis of the computer aerosol kinetics model MAGIC. It is shown that under dark, non-photolytic conditions, bromine production can be accounted for almost exclusively by a reaction between gas-phase ozone and surface-bound bromide ions. Under photolytic conditions, bromine production instead involves a complicated interplay between various gas-phase and aqueous-phase reactions.