Hygroscopic Tandem Differential Mobility Analyzers (HTDMAs) are widely used to measure the water uptake characteristics of aerosol particles. As has been shown experimentally in the past, potential differences in the relative humidity (RH) between the aerosol and the sheath flow of the second Differential Mobility Analyzer (DMA) can lead to erroneous estimates of the apparent hygroscopic behavior of the sampled particles. A prompt phase transition, for example, may become smeared and be erroneously interpreted as non-prompt. Using a particle-tracking model, here we simulate the trajectories and the state of the particles classified in a DMA with non-uniform RH and temperature profiles. Our simulations corroborate earlier observations proving that such an experimental artifact can induce particle growth within the second DMA. Given the importance of maintaining uniform RH and temperature inside the second DMA of HTDMA systems and the limitations of existing RH and temperature sensors, we further provide suggestions for their operation.@en

The hygroscopic properties of KBr, KCl, and KI nanoparticles having diameters from 8 to 60 nm were measured using a tandem Differential Mobility Analyzer. In all cases, the deliquescence and efflorescence relative humidity values increased with decreasing particle diameter. The associated growth factors also decreased with decreasing particle diameter, in agreement with predictions by Köhler theory. Overall, the theoretically predicted growth factors agreed well with the measurements, i.e., within ±3% uncertainty. For KCl particles having sizes down to 15 nm, however, a dynamic shape factor of 1.08, corresponding to non-spherical crystalline particles prior deliquescence, was inferred for agreement between measurements and theory. By comparison, KBr and KI within the same size range warranted shape factors of unity, equivalent to a sphere. These results contribute to an understanding of nanosize behavior widely relevant to material sciences as well as atmospheric aerosol particles over the oceans.@en