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))(2)SO(4) is also measured for comparison. The measured hygroscopic growth factors (GFs) of (NH(4))(2)SO(4) agree well with the theoretical Kohler curve. Great discrepancies between the measured GFs and the theoretical values are observed for NH(4)Cl and NH(4)NO(3) due to their volatile properties. The evaporation of NH(4)Cl below the deliquescence RH (DRH) is significantly promoted while RH increases. Similar trend is also observed for NH(4)NO(3) particle less than 50 nm. The proposed mechanism suggests that the increase of RH alters the chemical equilibrium among NH(4)X((s)) (X represents Cl(-) or NO(3)(-)). NH(3(g)) and HX((g)), i.e., NH(4)X((s))reversible arrow NH(3) + HX((g)), by converting NH(3(g)) and HX((g)) into NH(3)center dot nH(2)O and HX center dot nH(2)O, which accelerates the evaporation of NH(4)X((s)). When RH is higher than the DRH, the GFs of NH(4)X 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))(2)SO(4), NaCl, Na(2)SO(4) and NaNO(3), the GFs of NH(4)X are more sensitive to Do than RH due to the unique volatility of NH(4)Cl and NH(4)NO(3) particles. (C) 2011 Elsevier Ltd. All rights reserved.