Two model systems are studied: dicarboxylic acid particles with gaseous amines and α-cedrene ozonolysis particles. Measurements by direct analysis in real-time mass spectrometry and high-resolution time-of-flight aerosol mass spectrometry show that the reaction of the amines with the acid particles is restricted to the surface layer, with an odd–even alternating pattern. Furthermore, in the α-cedrene study, DART-MS is able to differentiate isomers based on their volatility.