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 (NH 4) 2SO 4 is also measured for comparison. The measured hygroscopic growth factors (GFs) of (NH 4) 2SO 4 agree well with the theoretical Köhler curve. Great discrepancies between the measured GFs and the theoretical values are observed for NH 4Cl and NH 4NO 3 due to their volatile properties. The evaporation of NH 4Cl below the deliquescence RH (DRH) is significantly promoted while RH increases. Similar trend is also observed for NH 4NO 3 particle less than 50 nm. The proposed mechanism suggests that the increase of RH alters the chemical equilibrium among NH 4X (s) (X represents Cl − or NO 3 −), NH 3(g) and HX (g), i.e., NH 4 X ( s ) ⇆ NH 3 + HX ( g ) , by converting NH 3(g) and HX (g) into NH 3·nH 2O and HX·nH 2O, which accelerates the evaporation of NH 4X (s). When RH is higher than the DRH, the GFs of NH 4X increase with initial particle size ( D 0) throughout the investigated size range. In this study, the iso-GF curves are also drawn to illustrate the effects of D 0 and RH on the GFs. Different from (NH 4) 2SO 4, NaCl, Na 2SO 4 and NaNO 3, the GFs of NH 4X are more sensitive to D 0 than RH due to the unique volatility of NH 4Cl and NH 4NO 3 particles. ► The effect of particle size on the hygrocopicity of NH 4NO 3 and NH 4Cl is discussed. ► The effect of RH on the evaporation of NH 4NO 3 and NH 4Cl is investigated. ► The evaporation of NH 4Cl is significantly promoted while RH increases below the DRH. ► The hygroscopic growth factor of NH 4Cl and NH 4NO 3 is more sensitive to D 0 than RH.