Atmospheric aerosols affect climate directly, by absorbing and scattering solar radiation, and indirectly, by altering cloud albedo and lifetime. Here, using a tandem differential mobility analyzer and a numerical model, we demonstrate that the restructuring of soot aggregates due to liquid coatings increases with the surface tension of the coating material. The agreement between the experimental and numerical modelling results further demonstrates that restructuring occurs upon coating condensation, not evaporation. These findings provide constraints on the direct effect of black carbon. In turn, using soot aggregates as novel nano-probes, we estimate the surface tension of secondary organic aerosol derived from the photo-oxidation of m-xylene to be 38 3 mN m(-1) under the present experimental conditions. Since surface tension influences nucleation and condensational growth, processes which lead to particles large enough to act as cloud condensation nuclei, our measurement provides an important constraint on the indirect effect of secondary organic aerosol.