<p>Interactions between water and nanoparticles are relevant for atmospheric multiphase processes, physical chemistry, and materials science. Current knowledge of the hygroscopic and related physico-chemical properties of nanoparticles, however, is restricted by limitations of the available measurement techniques. Here, we present the design and performance of a nano-hygroscopicity tandem differential mobility analyzer (nano-HTDMA) apparatus that enables high accuracy and precision in hygroscopic growth measurements of aerosol nanoparticles with diameters less than 10 nm. Detailed methods of calibration and validation are provided. Beside maintaining accurate and stable sheath/aerosol flow rates (± 1 %), high accuracy of DMA voltage (± 0.1 %) in the range of ~0–50 V is crucial to achieve accurate sizing and small sizing offsets between the two DMAs (< 1.4 %). To maintain a stable relative humidity (RH), the humidification system and the second DMA are placed in a well-insulated and air conditioner housing (± 0.1 K). We also tested and discussed different ways of preventing pre-deliquescence in the second DMA. Our measurement results for ammonium sulfate nanoparticles are in good agreement with Biskos et al. (2006b), with no significant size-effect on the deliquescence and efflorescence relative humidity (DRH, ERH) at diameters down to 6 nm. For sodium sulfate nanoparticles, however, we find a pronounced size-dependence of DRH and ERH between 20 and 6 nm nanoparticles.</p>