Trace gases on ice

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The uptake of trace gases formic acid (HCOOH), acetic acid (CH3COOH), and nitric acid (HNO3) was measured at low concentrations and temperatures in the upper troposphere. Understanding these uptakes is crucial for assessing the impact of trace gas redistribution in the upper troposphere through uptake on and re-emission from ice particles. Measurements were conducted using a low-pressure ice-coated wall flow reactor linked to a chemical ionization mass spectrometer, with ice films created from liquid water. The uptakes were quantified by continuously measuring gas phase concentrations downstream of the ice film. Data analysis involved physico-chemical expressions for gas-surface interactions and a two-dimensional tracer transport model to simulate gas, surface, and ice phase interactions. The measurements helped parameterize gas-ice surface equilibria for HCOOH and CH3COOH, revealing enthalpies of adsorption of (−51 ± 6) kJ mol−1 and (−55 ± 9) kJ mol−1, respectively. A minor long-term contribution to uptake was linked to diffusion into the bulk ice or grain boundaries. HNO3 uptake experiments on pure and pre-acidified ice at approximately 3•10^9 molecules cm-3 showed a significant irreversible uptake on pure ice, attributed to reactive uptake, while pre-acidified ice exhibited reversible surface adsorption and diffusive uptake, successfully modeled by the tracer transport model.