Relative humidity non-uniformities in Hygroscopic Tandem Differential Mobility Analyzer measurements

•We use a tracking model to predict the state of particles in RH-controlled DMAs.•The simulations corroborate earlier suggestions for artifacts in HTDMA measurements.•Such artifacts can lead to erroneous interpretations of HTDMA observations.•Based on the model results we provide suggestions for the operation of HTDMAs.

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.