Adsorption studies of acetone and 2,3-butanedione on ice surfaces between 193 and 223 K

摘要

Adsorption studies of acetone and of 2,3-butanedione on ice surfaces were performed using a new vertical coated wall flow tube coupled to a mass spectrometric detection.Adsorption of acetone on ice was found to be totally reversible for ice temperatures ranging from 193 to 223 K and for gas phase acetone concentrations varying between 5.4x10~(10) and 6.4x10~(13) molecules cm~(-3).Adsorption of 2,3-butanedione was also reversible at 213 and 223 K but partially irreversible at 193 and 203 K when its concentrations were larger than 1x10~(13) molecules cm~(-3).It was shown that,at 203 K,the surface coverage increases when the ice surface contains large and dense cracks but is independent of the presence of cracks at 223 K.The surface coverage also increases with decreasing temperature and with increasing acetone or 2,3-butanedione concentrations.The obtained experimental surface coverages were fitted according to the Langmuir and BET theories in order to determine the enthalpy of adsorption DELTAH_(ads) and the monolayer capacity N_M .The following values of N_M were derived (in units of 10~(14) molecule cm~(-2)):NM=1.3+-0.2 for acetone and N_M=1.2+-0.5 for 2,3-butanedione.The corresponding enthalpies of adsorption are (in kJ mol~(-1)):-49+-7 for acetone and -59+-8 for 2,3-butanedione.The results are discussed and compared with previous determinations for acetone.Finally,the obtained results are used to estimate the partitioning of acetone between the ice and gas phases in clouds of the upper troposphere.