A new method based on the combination of a differential mobility analyser (DMA) and aerodynamic particle sizer (APS) is presented, to measure the aerodynamic hygroscopic growth factor of aerosol particles around one micrometer in size. The DMA is used as a filter, selecting a size fraction of the ambient aerosol. One half of the selected monodisperse aerosol is sized in dry state in a first APS, while the second half of the aerosol is humidified, and then sized in a second APS. By comparing the particle size distributions measured in the first and second APS, the hygroscopic growth factor can be derived as a function of relative humidity (RH). Hygroscopic growth factors can be measured at RHs up to 90%. Particle modes of different hygroscopicity can be distinguished, giving information on the mixing state and the fraction of more and less hygroscopic particles. The system was tested and characterised in the laboratory, using inorganic salt particles ((NH4)(2)SO4) of known hygroscopic growth behaviour. It agreed with growth curves given in the literature, within the measurement uncertainty. (C) 2008 Elsevier Ltd. All rights reserved.