Inverse temperature dependence of Henry’s law coefficients for volatile organic compounds in supercooled water

Upon supercooling, water expels volatile organic compounds (VOC), and Henry’s law coefficients are increasing concomitant with decreasing temperature. This unexpected observation was found by measuring the VOC partitioning between supercooled water and gas phase in the temperature range from −5 °C to −15 °C for benzene, toluene, ethlybenzene, m-, p-, o-xylenes (BTEX), methyl tert-butyl ether (MTBE) and ethyl tert-butyl ether (ETBE). Aqueous standard solutions were analyzed using a static headspace method in combination with gas chromatography/mass spectrometry (GC/MS). Dimensionless Henry’s law coefficients (KAW) were calculated from measurements of the concentration of the VOCs in the headspace above the standard solutions at temperatures between −25 °C and 25 °C. The results show that the well known temperature dependence of Henry′s law coefficients at temperatures above 0 °C is inversed upon decreasing the temperature below 0 °C and formation of supercooled water while decreasing the temperature to −15 °C. Upon further decrease of the temperature to −25 °C freezing of the supercooled water occurs. KAW values increase from 0.092 (benzene), 0.099 (toluene), 0.098 (ethylbenzene), 0.117 (m/p-xylene), 0.076 (o-xylene), 0.012 (MTBE) and 0.014 (ETBE at 5 °C to 0.298 (benzene), 0.498 (toluene), 0.944 (ethylbenzene), 0.327 (m/p-xylene), 0.342 (o-xylene), 0.029 (MTBE) and 0.041 (ETBE) at −25 °C, respectively. Inversion of Henry coefficients upon cooling the aqueous solutions to temperatures below 0 °C is explained by the increasing formation of ice-like clusters in the water below 0 °C. The VOC are expelled from these clusters resulting in enhanced VOC concentrations in the gas phase upon supercooling. Formation of ice upon further cooling to −25 °C results in a further increase of the VOC concentrations in the gas phase above the ice. The findings have implications for the partitioning of VOC in clouds between the gas phase, supercooled water droplets, aerosol particles and ice.