Physical properties of iodate solutions and the deliquescence of crystalline I 2 O 5 and HIO 3

Abstract. Secondary aerosol produced from marine biogenic sources in algal-rich coastal locations will initially be composed of iodine oxide species, most likely I 2 O 5 , or its hydrated form HIO 3 , formed as a result of iodine gas-phase chemistry. At present, there is no quantitative hygroscopic data for these compounds and very little data available for iodate solutions (HIO 3 and I 2 O 5 share a common aqueous phase). With increased interest in the role of such aerosol in the marine atmosphere, we have conducted studies of (i) the deliquescence behaviour of crystalline HIO 3 and I 2 O 5 at 273–303 K, (ii) the efflorescence behaviour of aqueous iodate solution droplets, and (iii) properties (water activity, density, and viscosity) of subsaturated and saturated iodate solutions. The deliquescence of I 2 O 5 crystals at 293 K was observed to occur at a relative humidity (DRH) of 80.8±1.0%, whereas for HIO 3 , a DRH of 85.0±1.0% was measured. These values are consistent with measured water activity values for saturated I 2 O 5 and HIO 3 solutions at 293 K of 0.80±0.01 and 0.84±0.01 respectively. At all temperatures, DRH values for HIO 3 crystals were observed to be higher than for those of I 2 O 5 . The temperature-dependent DRH data, along with solubility and water activity data were used to evaluate the enthalpy of solution (ΔH sol ) for HIO 3 and I 2 O 5 . A ΔH sol value of 8.3±0.7 kJ mol −1 was determined for HIO 3 which is consistent with a literature value of 8.8 kJ mol −1 . For I 2 O 5 , we report for the first time its solubility at various temperatures and ΔH sol = 12.4±0.6 kJ mol −1 . The measured water activity values confirm that aqueous iodate solutions are strongly non-ideal, consistent with previous reports of complex ion formation and molecular aggregation.