Trace gases on ice: A laboratory study of HCOOH, CH3COOH and HNO3 uptake on ice

The uptake on ice of the trace gases formic acid (HCOOH), acetic acid (CH3COOH) and nitric acid (HNO3) was measured at low concentrations and temperatures of the upper troposphere. The quantification of these uptakes is necessary for understanding the impact of redistribution of trace gases in the upper troposphere by uptake on (and re-emission from) ice particles. Uptakes were measured using a low pressure ice-coated wall flow reactor coupled to a chemical ionization mass spectrometer. The ice films were frozen from liquid water. Varying portions of the ice films could be exposed to the diluted trace gas. The uptakes were quantified by continuous measurements of gas phase concentrations downstream of the ice film. Data were analyzed using physico-chemical expressions for gas-surface interactions and computer modelling of the coupled transport in and between the gas, surface and ice phases in the reactor. A two-dimensional tracer transport model was developed for this purpose. The measurements were used for parameterizing the gas : ice-surface equilibria of HCOOH and CH3COOH. Enthalpies of adsorption were found to be (−51 ± 6) kJ mol−1 for HCOOH and (-55 ± 9) kJ mol-1 for CH3COOH. A relatively small long term contribution to the uptake was attributed to diffusive uptake into the bulk ice and/or grain boundaries/ veins. A sample of HNO3 uptake experiments on pure ice and pre-acidified ice was performed at concentrations of ca. 3•109 molecules cm-3. The experiments with non-acidified (pure) ice displayed a major irreversible contribution to the uptake which was attributed to irreversible/ reactive uptake. The uptakes on pre- acidified ice were successfully reproduced with the tracer transport model with a combination of reversible surface adsorption and reversible diffusive uptake.