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

Secondary aerosol produced from marine biogenic sources in algal-rich coastal locations will initially be composed of iodine oxide species, most likely I sub(2)O sub(5), or its hydrated form HIO sub(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 sub(3) and I sub(2)O sub(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 sub(3) and I sub(2)O sub(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 sub(2)O sub(5) crystals at 293 K was observed to occur at a relative humidity (DRH) of 80.8 plus or minus 1.0%, whereas for HIO sub(3), a DRH of 85.0 plus or minus 1.0% was measured. These values are consistent with measured water activity values for saturated I sub(2)O sub(5) and HIO sub(3) solutions at 293 K of 0.80 plus or minus 0.01 and 0.84 plus or minus 0.01 respectively. At all temperatures, DRH values for HIO sub(3) crystals were observed to be higher than for those of I sub(2)O sub(5). The temperature-dependent DRH data, along with solubility and water activity data were used to evaluate the enthalpy of solution ( Delta H sub(sol)) for HIO sub(3) and I sub(2)O sub(5). A Delta H sub(sol) value of 8.3 plus or minus 0.7 kJ mol super(-1) was determined for HIO sub(3) which is consistent with a literature value of 8.8 kJ mol super(-1). For I sub(2)O sub(5), we report for the first time its solubility at various temperatures and Delta H sub(sol) = 12.4 plus or minus 0.6 kJ mol super(-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.