In most cases, the particles assumed an approximately core-shell morphology. The outer phase can be identified as a less polar, hydrophobic organic phase while the inner phase is the more polar and hydrophilic of the two. As humidity increased in the flow cell, the inner phase usually swelled, showing that it was more hydrophilic and more polar. Higher polarity is generally expected to lead to higher surface tension, thus the higher polarity compounds are expected to comprise the inner phase to minimize the total surface energy of the system, assuming enough time for diffusion and equilibration within the particles. Others have shown that methanol-soluble BrC from biomass burning has a higher absorption of light than water-soluble BrC, suggesting that most of the colour comes from the less polar molecules. We should therefore expect the low-polarity hydrophobic outer phase to be darker in colour, which is indeed observed in the particles in Figure .