Fig. 3.

Thermodynamic model predictions for an α-pinene-derived SOM particle. a Volume V of single and separated phases across a water activity range of 0.92–1. The dashed gray line shows the single-phase case predicted by Model I. Orange and blue lines show the shell and core phases, respectively, predicted by Model II when LLPS is present. Values are normalized by the volume V_dry of organic material when dry. The red dashed vertical line marks the predicted onset conditions of LLPS. The V/V_dry value of the shell phase drops off when water activity approaches unity because of the mixing of two phases into a single phase. Predicted volume fractions of hydrophilic, hydrophobic organic components, and water are shown in b for the surface and in c for the core of the particle. The discontinuity of hydrophobic SOM volume fraction shown in b represent onset of the LLPS. The volume fractions of water calculated based on the mass-based QCM measurements of hygroscopic growth are shown in c for comparison. d Predicted surface tension coefficient σ vs. the diameter growth factor d_m/d_m,dry. Predictions are made for dry diameters of 52 nm (blue) and 107 nm (green). Cartoons in a, d show the possible morphology of the particle. Blue and green colors represent water and organics, respectively. e Köhler curves of relative humidity vs. diameter growth factor predicted based on the surface tension shown in d. Measured supersaturations for 52 and 107 nm diameter particles are marked as horizontal lines for comparison. The thicknesses of the lines are scaled by the measurements' uncertainties