The formation and stability of co-crystalline NH₃ · C₂H₂ aerosol particles

Abstract

NH₃/C₂H₂ aerosols were formed in a bath gas cooling cell at cryogenic temperatures and studied using infrared spectroscopy. The infrared band shapes associated with these particles were quite different from those of the pure solids and vibrational shifts of up to 100 wavenumbers were observed. The phase of these particles was identified as a NH₃ · C₂H₂ co-crystal and was found to be thermodynamically stable with respect to the solids of pure NH₃ and C₂H₂. Experiments with core–shell structures demonstrated that the co-crystalline phase could also be formed through a solid state reaction of pure NH₃ and C₂H₂ in aerosol particles. The results of this work are relevant to planetary atmospheres where both NH₃ and C₂H₂ are present, as the co-crystalline phase can readily form in mixed particles of these substances and its optical properties cannot be understood by simply considering the pure substances.

Keywords:

• infrared spectroscopy
• molecular ices
• ice mixtures
• clouds in planetary atmospheres

Acknowledgements

We would like to thank Dr. Kerry J. Knox for help with preliminary experiments. Financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Canada Foundation for Innovation is gratefully acknowledged. This research has been enabled through the generous allocation of advanced computing resources by West-Grid and Compute/Calcul Canada. We acknowledge an NSERC graduate fellowship (T.C.P.) and an NSERC E. W. R. Steacie Memorial Fellowship (R.S.).