Cirrus cloud mimics in the laboratory: An infrared spectroscopy study of thin films of mixed ice of water with organic acids and ammonia

The structures of formic and acetic acids deposited on a thin gold substrate held in vacuum at low temperatures and their related water-ice promoted chemistry have been investigated. The condensed water/guest films were taken to act as cirrus cloud "mimics." Such laboratory representations provide a necessary prelude to understanding how low temperature surfaces can affect chemical composition changes in the upper atmosphere. The systems were characterized by reflection-absorption infrared spectroscopy and temperature-programed desorption spectrometry. The interaction behavior of the binary acid ices was compared to that observed when ternary mixtures of water, formic acid, and ammonia were deposited. Differences in the chemistry were observed depending on deposition method: layering or mixing. The more atmospherically relevant codeposition approach showed that at low temperatures, amorphous formic acid can be ionized to its monodentate form by water ice within the bulk rather than on the surface. In contrast, the introduction of ammonia leads to full bidentate ionization on the ice surface. The thermal desorption profiles of codeposited films of water, ammonia, and formic acid indicate that desorption occurs in three stages. The first is a slow release of ammonia between 120 and 160 K, then the main water desorption event occurs with a maximum rate close to 180 K, followed by a final release of ammonia and formic acid at about 230 K originating from nonhydrous ammonium formate on the surface. The behavior of acetic acid is similar to formic acid but shows lesser propensity to ionize in bulk water ice.