Characterizing the hygroscopic properties of aerosols: from binary aqueous systems to atmospheric aerosols

The quantification of the hygroscopic properties of atmospheric aerosols is important to understand several processes they are involved in, such as clouds formation, their interaction with solar radiation and the penetration of particles in the human respiratory track. In addition, the interaction of deposited aerosols with surfaces depends on their physical state, too; thus, characterizing their phase transitions as a function of their chemical composition is key to understanding the effects they have on materials (e.g. printed circuits, cultural heritage artifacts). In order to investigate the hygroscopic properties of aerosols, an electrical conductance method in an Aerosol Exposure Chamber was developed for the determination of the phase transitions of PM2.5 aerosol samples during relative humidity cycles. The obtained Deliquescence and Crystallization Relative Humidity (DRH and CRH) were put in relation with the ionic chemical composition of the analyzed samples: it was found that seasonal chemical variations result in seasonal trends for DRH and CRH, too. The implications of these results for Free-Cooled Data Centers, for the understanding of the role of particles in stone-decay processes of cultural heritage artifacts and for the common algorithms used in the remote sensing of particulate matter concentrations were evaluated. The ionic fraction characterisation was also used as an input for a state-of-the-art equilibrium aerosol model (E-AIM) to simulate the DRH of the samples. Some discrepancies were evidenced in the comparison of experimental and modelled values, because the hygroscopic properties of the organic components need to be included too. In order to effectively account for their contribute, current aerosol models need to be refined with accurate hygroscopicity measurements on organic compounds of increasing molecular complexity and their mixtures with common electrolytes. Such measurements are essential for understanding and modelling the microphysical properties that determine the partitioning of water between the gas and the aerosol phases in chemically complex systems. In this context, an experimental technique based on evaporation kinetics measurements in an Cylindrical Electrodynamic Balance was developed for the measurement of hygroscopic properties on single confined droplets from aqueous solutions with known chemical composition. To expand the range of applicability of a previously developed technique to water activities from 0.5 to values close to saturation (>0.99), well-characterized binary and ternary inorganic mixtures were considered. The obtained results were used to successfully validate this technique by comparing them with calculations from E-AIM model and to assess the sensitivity of this technique to small changes in chemical composition. The first class of atmospherically relevant compounds that was considered was aminium sulphates, which are the products of the neutralization reactions of sulphuric acid and short-chained alkylamines (methyl- and ethylamines). They have been detected in atmospheric aerosols up to hundreds of pg m-3, but their hygroscopic behaviour was less characterized than their inorganic equivalent, ammonium sulphate, even if they can promote cloud droplets formation and particle growth.