Diffusion and Salting in Effects on Isoprene-Derived Secondary Organic Aerosol Mixture Proxy Hygroscopicity
NAHIN FERDOUSI-ROKIB, Stephanie Jacoby, N. Cazimir Armstrong, Alana Dodero, Martin Ahn, Ergine R. Remy, Zhenfa Zhang, Avram Gold, Joseph Woo, Yue Zhang, Jason Surratt, Akua Asa-Awuku, University of Maryland, College Park
Abstract Number: 99
Working Group: Aerosols, Clouds and Climate
Abstract
Isoprene-derived secondary organic aerosol (SOA) constituents, such as the 2-methyltetrols (2-MT) and 2-methyltetrol sulfates (2-MTS), have been readily detected in atmospheric fine aerosols (PM2.5). Isoprene-derived SOA compounds exist within aerosol mixtures containing inorganic salts, such as ammonium sulfate (AS). Despite its atmospheric significance, the water uptake of 2-MT, 2-MTS, and their mixtures are not well understood. In this study, we determine the physicochemical properties of 2-MT, 2-MTS, and their mixtures with AS. 2-MT and 2-MTS have been previously identified as viscous; thus, dynamic surface tension (σs/a) measurements were taken for both compounds to determine their organic diffusion coefficients (Ds). The droplet growth of the synthesized organics and AS mixtures was measured under subsaturated conditions (< 100% RH) using a humidified tandem differential mobility analyzer (H-TDMA) at a relative humidity (RH) of 88.2% ± 1.5%. Aerosol activation and droplet growth was also measured under supersaturated (> 100% RH) conditions using a cloud condensation nuclei counter (CCNC); supersaturation (SS) ranged from 0.3-1.4%. Both subsaturated and supersaturated hygroscopicity were parameterized by the single hygroscopicity parameter κ. Furthermore, aerosol viscosity and phase morphology were analyzed using atomic force microscopy (AFM) measurements. This study demonstrates how diffusion and salting-in effects influence the water uptake of SOA-proxy mixtures. The presence of inorganic compounds, such as AS, can enhance organic diffusion, leading to ideally well-mixed aerosol hygroscopic behavior that can be described by traditional Köhler theory. As a result, this work shows that for atmospherically ubiquitous isoprene-derived SOA like 2-MT and 2-MTS, the impact of its non-ideal thermodynamic properties must be considered in aerosol-cloud interactions.