Hygroscopicity and evaporation of ammonium chloride and ammonium nitrate: Relative humidity and size effects on the growth factor

The hygroscopicity and evaporation of ammonium chloride and ammonium nitrate in the size range of 40–200 nm are investigated from 20% to 86% RH using a self-assembled hygroscopic tandem differential mobility analyzer (H-TDMA) system. The hygroscopicity of 100 nm (NH4)2SO4 is also measured for comparison. The measured hygroscopic growth factors (GFs) of (NH4)2SO4 agree well with the theoretical Köhler curve. Great discrepancies between the measured GFs and the theoretical values are observed for NH4Cl and NH4NO3 due to their volatile properties. The evaporation of NH4Cl below the deliquescence RH (DRH) is significantly promoted while RH increases. Similar trend is also observed for NH4NO3 particle less than 50 nm. The proposed mechanism suggests that the increase of RH alters the chemical equilibrium among NH4X(s) (X represents Cl− or NO3−), NH3(g) and HX(g), i.e., NH 4 X ( s ) ⇆ NH 3 + HX ( g ) , by converting NH3(g) and HX(g) into NH3·nH2O and HX·nH2O, which accelerates the evaporation of NH4X(s). When RH is higher than the DRH, the GFs of NH4X increase with initial particle size (D0) throughout the investigated size range. In this study, the iso-GF curves are also drawn to illustrate the effects of D0 and RH on the GFs. Different from (NH4)2SO4, NaCl, Na2SO4 and NaNO3, the GFs of NH4X are more sensitive to D0 than RH due to the unique volatility of NH4Cl and NH4NO3 particles.