High-humidity tandem differential mobility analyzer for accurate determination of aerosol hygroscopic growth, microstructure, and activity coefficients over a wide range of relative humidity
Abstract. Interactions with water are crucial for the properties,
transformation, and climate effects of atmospheric aerosols. Here we present
the high-humidity tandem differential hygroscopicity analyzer (HHTDMA) and a new
method to measure the hygroscopic growth of aerosol particles with in situ
restructuring to minimize the influence of particle shape. With this
approach, growth factors can be measured with an uncertainty of 0.3 %–0.9 %
over a relative humidity (RH) range of 2 %–99.6 % and with an RH
measurement accuracy better than 0.4 %. The HHTDMA instrument can be used in hydration, dehydration, and
restructuring modes of operation. The restructuring mode allows us to
investigate the effects of drying conditions on the initial microstructure
of aerosol particles and specifies the optimal parameters that provide their
rearrangements into compact structures with a nearly spherical shape. These
optimal parameters were used in hygroscopic growth experiments by
combining the restructuring mode with a conventional hydration or dehydration
mode. The tandem of two modes allowed us to measure the particle growth
factors with high precision as well as to determine the thickness of the
water adsorption layer on the surface of compact crystalline particles. To verify the HHTDMA instrument we compared the measured ammonium sulfate growth
factors with those obtained from the E-AIM-based Kohler model (E-AIM: Extended Aerosol Inorganics
Model). Averaged over
the range of 38 %–96 % RH, the mean relative deviations between
measurements and model results is less than 0.5 %. We demonstrate this precision by presenting data for glucose, for which bulk
thermodynamic coefficients are available. The HHTDMA-derived activity
coefficients of water and glucose were obtained for both dilute and
supersaturated solutions and are in good agreement with those reported in the
literature. The average deviation between the measured activity coefficients
and those obtained by the bulk method is less than 4 %. For dilute solution
in water with an activity range of 0.98–0.99, the hygroscopicity parameter of
glucose and the molal osmotic coefficient were obtained with an uncertainty of 0.4 % and 2.5 %, respectively.