However, since bare soot particles cannot form cloud droplets at atmospherically relevant water supersaturations, small amounts of sulfuric acid or water-soluble oxidized organics must have condensed on those particles during aging, making them cloud active. Aging of soot in a remote marine free troposphere would be a slow process in current aerosol models, as a significant vapor saturation is required to overcome the Kelvin effect and initiate the condensation of trace gas chemicals on bare soot particles, which are represented as spheres. On the contrary, our recent experiments have shown that in the case of real soot, even subsaturated vapors can condense promptly on fractal particles, resulting in minor but detectable compaction. This has led us to hypothesize that concurrent capillary condensation of water-soluble trace chemicals and water vapor on fractal soot particles can result in their significant compaction and serve as a highly sensitive indicator of the transformation of initially hydrophobic soot to a hydrophilic state, and possibly to cloud condensation nuclei (CCN). To test this hypothesis, here we performed experiments where fractal soot particles were exposed to subsaturated vapors of several trace chemicals and then to progressively higher relative humidity (RH). To quantify the amounts of vapor transferred to the particle phase, we modeled the step-by-step condensation of the trace chemical and water in concave menisci of the fractal soot particles, as shown in Fig. . We found that the depressed vapor pressure of the condensate above the menisci promoted rapid capillary condensation even under subsaturated conditions. To assess the role of capillary condensation in the aging of soot in the atmosphere, we extended our single-component model towards multi-component vapors and employed this new multi-component capillary condensation model (MCCCM) in conjunction with the particle-resolved PartMC-MOSAIC aerosol model to explore a scenario representing the remote atmosphere.