Development of the low-temperature hygroscopicity tandem differential mobility analyzer (Low-T HTDMA) and its application to (NH4)2SO4 and NaCl particles

Abstract

Influence of water vapor on aerosol particles depends on both relative humidity (RH) and temperature (T). Both temperature and RH should simultaneously be regulated to investigate physical properties of aerosol particles. We developed a low-temperature hygroscopicity tandem differential mobility analyzer (low-T HTDMA), which can be operated in the range of −20 °C ≤ T ≤ 22.5 °C (room temperature), and 0% < RH ≤ 90% (T > 0 °C) or slightly lower than ice saturation RH (T ≤ −10 °C). Temperature was controlled using a chest freezer, refrigerated liquid circulators, fans, and heat exchangers. The employment of heat exchangers allowed the operation of the instrument without immersing the whole setup into liquid coolant. Aerosol particles were humidified using a Shirasu porous glass (SPG) humidifier to facilitate efficient humidification. The performance of the low-T HTDMA was tested using (NH4)2SO4 and NaCl particles. The hygroscopic growth factors of these particles agreed well with previous studies. Deliquescence RH (DRH) of both (NH4)2SO4 and NaCl particles also agreed well with the literature. Compared with the exiting data, efflorescence RH (ERH) measured by the low-T HTDMA exhibited minimal temperature dependence. For instance, the ERH values were 42% (T = −20 °C), 42% (T = −10 °C), and 43% (T = 22.5 °C) for NaCl particles. We suggest that further independent laboratory studies will need to be conducted for experimentally constraining thermodynamic properties of atmospherically important chemical species for T < 0 °C.