Abstract:

mbient aerosol particles derived from environmental sources (e.g., sea spray aerosol) and biological sources (e.g., respiratory aerosol) are commonly complex, aqueous organic-inorganic mixtures. Accurately predicting the properties of these particles is important for understanding air quality, climate, and public health. Despite the importance of organic-inorganic aerosol, their physical properties remain uncertain. A common assumption is that due to the hygroscopicity of the inorganic component, aqueous organic-inorganic aerosol particles largely exist with liquid-like properties. In contrast to that assumption, we demonstrate that cooperative ion-molecule interactions can give rise to long-range, semi-solid supramolecular networks within aerosol particles. Specifically, using a dual-balance quadrupole electrodynamic balance, we demonstrate that organic-inorganic aerosol composed of atmospherically- and biologically-relevant oxygenated organics (including monosaccharide derivatives and amino acids, ~180-310 Da) and divalent inorganic ions (Ca2+, Mg2+, and SO42-) undergo a humidity-dependent gel phase transition under conditions common of the tropospheric outdoor and indoor environment (RH<65-80%, dependent on organic molecule and divalent ion). This gel phase transition alters the physical properties of organic-inorganic aerosol such that they are not well-described as liquids. Rather, they exhibit solid-like properties. For some organic-inorganic mixtures, the observed gel transition is unique to the highly-concentrated conditions that are accessible in aerosol particles and is not predicted from bulk considerations nor from the viscosity of the individual components. These observations are likely enabled by supramolecular interactions that are not readily observed under bulk conditions. Thus, rates of diffusion and hygroscopic properties of these gels are expected to be different than would have been predicted from knowledge of aerosol composition alone. These observations therefore suggest that supramolecular aerosol chemistry could be broadly influencing aerosol properties. Discussion will focus on utilizing laboratory observations to predict gel transitions in ambient aerosol, and how supramolecular aerosol chemistry may be influencing processes relevant to air quality, climate, public health, and disease transmission.