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The hygroscopicity of aerosol nanoparticles and related physical-chemical properties are crucial for atmospheric multiphase processes, physical chemistry, and materials science. One of the main problems of current research on aerosol hygroscopicity is that the most studies due to technical challenges lack measurements in the sub-10 nm size range, which is highly relevant for research on new particle formation and its initial growth. The goal of this thesis is to solve the technical problems and use the advanced nano-hygroscopicity tandem differential mobility analyzer (nano HTDMA) to investigate the size dependent hygroscopicity of aerosol nanoparticles with diameters down to 6 nm. (1) In the first part of this thesis, the detailed information on the design of a nano HTDMA system is presented. To enable high accuracy and precision in hygroscopicity measurements of aerosol nanoparticles, especially in the sub-10 nm size range, systematic and comprehensive calibration criteria of the nano-HTDMA have been developed and applied, including the calibration of nanoparticle sizing, sheath and aerosol flow rates, DMA voltage, relative humidity (RH) sensor, and temperature (T) sensor. After calibration, the nano-HTDMA system has been shown to have an accurate sizing and a small sizing offset between nano-DMAs (< 1.4 %) for aerosol nanoparticles with diameters down to 6 nm. Moreover, to maintain the RH-uniformities that prevent the pre-deliquescence and non-prompt phase transition of nanoparticles within nano-DMA2, the RH of sheath flow is kept as same as that of aerosol flow at inlet of nano-DMA2. Since temperature and RH are closely linked, the nano-DMA2 with its humidification system is placed in a well-insulated air conditioned chamber, which maintains a constant temperature. Using the nano HTDMA apparatus, we measure the hygroscopic behavior of aerosol nanoparticles of two inorganic substances (i.e., ammonium sulfate and sodium sulfate). We find a weak size dependence of deliquescence and efflorescence relative humidity (DRH and ERH, respectively) of ammonium sulfate nanoparticles but a strong size dependence of DRH and ERH of sodium sulfate nanoparticles down to 6 nm in size. (2) The second part of this thesis is about hygroscopic properties of organic nanoparticles with diameters down to 6 nm (i.e., levoglucosan and D-glucose) measured by a nano-HTDMA system. Levoglucosan is a biomass burning tracer compound and can contribute substantially (16.6–30.9% by mass) to the total organics in PM2.5. D-glucose, a hydrolysis product of cellulose and levoglucosan, is one of the major pyrolysis products of wood. Due to the partial evaporation of levoglucosan with diameters smaller than 20 nm in the nano-HTDMA system, we investigate the hygroscopicity of levoglucosan nanoparticles in the size range from 20 to 100 nm. A weak size dependence of hygroscopic growth factor is observed for levoglucosan and D-glucose nanoparticles with diameters down to 20 nm, while a strong size dependence of the hygroscopic growth factor is found for D-glucose nanoparticles with diameters from 6 to 20 nm. We further compare measurements for levoglucosan and glucose nanoparticles with modelling results of the Extended Aerosol Inorganics Model (E-AIM) and the ideal solution theory, respectively. The ideal solution theory well describes the hygroscopic growth factors of levoglucosan and D-glucose nanoparticles with diameters larger than 15 nm, while the E-AIM model prediction well describes measured growth factors of sub-15 nm D-glucose nanoparticles. (3) The goal of the third part of this work is to investigate the hygroscopicity of organic surrogate compounds from biomass burning and their interaction with inorganic ammonium sulfate aerosols using an HTDMA. The organic surrogate compounds represent a selection of some of the most abundant pyrolysis products of biomass burning. We find that levoglucosan and humic acid aerosol nanoparticles release water gradually in the range from 90 % down to 5% RH. However, 4- Hydroxybenzoic acid aerosol nanoparticles remain in the solid state and exhibit a small shrink in size in the whole dehumidification process. Predicted growth factors using the Aerosol Inorganic-Organic Mixtures Functional groups Activity Coefficients (AIOMFAC) model, the E-AIM, and a fitted hygroscopicity function are in general consistent with measured hygroscopic growth factors of levoglucosan, respectively. However, the use of the AIOMFAC and the E-AIM models without consideration of crystalline organic phases is not appropriate to describe the hygroscopicity of 4-hydroxybenzoic acid. Furthermore, we observe several effects of these organic components on the hygroscopicity behavior of mixtures containing ammonium sulfate in relation to the different mass fractions of organic compounds: (i) A shift of ERH of ammonium sulfate to the higher RH due to the presence of 25 wt % levoglucosan in the mixture. (ii) There is a phase transition at 25% RH for mixtures containing 50 wt % of 4-hydroxybenzoic acid compared to the ERH (i.e.,35 %) for organic-free AS nanoparticles, and a liquid-to-solid phase transition of 4- hydroxybenzoic acid in the mixed particles during dehydration process. (iii) The presence of humic acid components shows no significant effects on the efflorescence of AS in mixed aerosol nanoparticles. In addition, consideration of a solid-liquid phase transition of AS in both the AIOMFAC and the E-AIM models leads to a general agreement between models and measurements, as well as ERH of AS in the mixed system. The measured diameter growth factors of aerosol nanoparticles containing humic acid and ammonium sulfate are well predicted by Zdanovskii Stokes-Robinson (ZSR) relation. Lastly, the mixtures containing organic surrogates (i.e., levoglucosan, 4-hydroxybenzoic acid, and humic acid) and ammonium sulfate with increasing organics mass fractions is used to mimic, in a simplified manner, ambient conditions in the Amazon Basin during the wet and dry season. The measured hygroscopicity parameters (ĸdry and ĸwet) show relatively good agreement with field data in the dry and wet seasonal period in the Amazon Basin, respectively. This suggests that laboratory-generated mixtures of organic surrogate compounds with ammonium sulfate can be used to represent the chemical composition of ambient aerosols from the Amazon Basin for the purpose of RH-dependent hygroscopicity studies.
