Glass formation and unusual hygroscopic growth of iodic acid solution droplets with relevance for iodine mediated particle formation in the marine boundary layer

摘要

Iodine oxide particles are known to nucleate in the marine boundary layerwhere gas phase molecular iodine and organoiodine species are produced bymacroalgae. These ultra-fine particles may then grow through the condensationof other materials to sizes where they may serve as cloud condensationnuclei. There has been some debate over the chemical identity of theinitially nucleated particles. In laboratory simulations, hygroscopicmeasurements have been used to infer that they are composed of insolubleI₂O₄, while elemental analysis of laboratory generated particlessuggests soluble I₂O₅ or its hydrated form iodic acid, HIO₃(I₂O₅·H₂O). In this paper we explore the response ofsuper-micron sized aqueous iodic acid solution droplets to varying humidityusing both Raman microscopy and single particle electrodynamic traps. Thesemeasurements reveal that the propensity of an iodic acid solution droplet tocrystallise is negligible on drying to ~0% relative humidity(RH). On applying mechanical pressure to these droplets they shatter in amanner consistent with an ultra-viscous liquid or a brittle glass. Waterretention in amorphous material at low RH is important for understanding thehygroscopic growth of aerosol particles and uptake of other condensablematerial. Subsequent water uptake between 10 and 20% RH causes theirviscosity to reduce sufficiently that the cracked droplets flow and merge.The persistence of iodic acid solution in an amorphous state, rather than acrystalline state, suggests they will more readily accommodate othercondensable material and are therefore more likely to grow to sizes wherethey may serve as cloud condensation nuclei. On increasing the humidity to~90% the mass of the droplets only increases by ~20%with a corresponding increase in radius of only 6%, which is remarkablysmall for a highly soluble material. We suggest that the small growth factorof aqueous iodic acid solution droplets is consistent with the small aerosolgrowth factors observed in previous experiments.