Hygroscopicity of isoprene-derived secondary organic aerosol mixture proxies: importance of diffusion and salting-in effects

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 aerosols (PM2.5) and within mixtures containing ammonium sulfate (AS). Despite its prevalence, the water uptake of 2-MT, 2-MTS, and their mixtures is not well understood. In this study, we determine the physicochemical properties (e.g., surface activity, diffusivity, phase morphology) of synthesized 2-MT and 2-MTS samples and their mixtures with AS. 2-MT and 2-MTS have been identified as surface active and viscous. Thus, dynamic surface tension (σs/a) measurements were taken to determine organic diffusion coefficients (Ds). The droplet growth of organic / AS mixtures was measured under subsaturated conditions using a humidified tandem differential mobility analyzer (H-TDMA) at 88.2 % RH ±1.5 %. Droplet activation was measured under supersaturated (>100 % RH) conditions using a cloud condensation nuclei counter (CCNC); supersaturation (SS) ranged from 0.3 %-1.4 %. Hygroscopicity in both regimes was parameterized by the single hygroscopicity parameter κ. This study demonstrates how diffusion and salting-in effects influence the water uptake of synthesized, isoprene-derived SOA mixtures. Results show that when mixed with AS, organic diffusion for 2-MTS / AS becomes an order of magnitude faster, while 2-MT diffusivity remains unchanged. Both 2-MT / AS and 2-MTS aerosols present a plateau in subsaturated κ values close to pure AS. However, under supersaturated conditions, 2-MTS / AS behaves ideally and well mixed and can be characterized by κ-Köhler theory. Isoprene-derived SOAs like 2-MT and 2-MTS samples are ubiquitous, and thus, the impact from biogenic sources and its non-ideal thermodynamic properties must be considered in aerosol-cloud interactions.