Evaluation of depth profiling using laser resonant desorption as a method to measure diffusion coefficients in ice

Area of Science:

• Physical Chemistry
• Materials Science
• Atmospheric Science

Background:

• Gas diffusion in ice is crucial for atmospheric and cryospheric processes.
• Limited data exist on diffusion coefficients in ice, particularly for gases like HCl.
• A previous study reported unusually high diffusion coefficients for HCl hydrate in ice.

Purpose of the Study:

• To investigate the high diffusion coefficient of HCl hydrate in ice reported by Livingston et al.
• To determine if the experimental conditions created amorphous mixtures rather than simple diffusion in ice.
• To assess the applicability of the reported diffusion data to atmospheric and cryospheric science.

Main Methods:

• Re-evaluation of experimental data and literature values for gas diffusion in ice.
• Infrared spectroscopy to analyze solid HCl:H2O mixtures.
• Analysis of potential artifact formation during ice doping and measurement.

Main Results:

• The reported diffusion coefficient for HCl hydrate in ice (5 x 10(-11) cm2/s at 190 K) is orders of magnitude higher than expected.
• New infrared data confirm the formation of amorphous HCl:H2O solid mixtures at 190 K.
• High HCl concentrations likely formed amorphous mixtures, leading to fast HCl diffusion, possibly enhanced by ice defects.

Conclusions:

• The high diffusion values likely result from diffusion in amorphous HCl:H2O mixtures, not pure ice.
• The experimental method may have created conditions not representative of natural ice environments.
• The reported diffusion data are unlikely to be applicable to atmospheric and cryospheric science.