The humidified tandem differential mobility analyzer (HTDMA) measures hygroscopic growth of airborne particles, which is the ability of the particles to take up water at subsaturated relative humidity (RH) conditions. Hygroscopicity is an important property of atmospheric aerosols since it influences the particle interaction with incoming radiation as well as their role in cloud formation. Hygroscopicity of aerosol particles also affects human health since the deposition pattern in the respiratory tract depends on the hygroscopic state of the particles.

The central components of the HTDMA system are two differential mobility analyzers (DMA) set in tandem. In the first DMA, particles of a specific size is selected. The monodisperse aerosol is then humidified to a specific RH. The second DMA is used together with a CPC to perform a size distribution scan of the humidified monodisperse aerosol. The hygroscopic growth of the particles is characterized by a so-called growth factor that is the mean particle diameter of the wet aerosol divided by the mean particle diameter of the dry aerosol.