The hygroscopicity of aerosol particles determines their size, concentration, chemical compositions and phase states, and thus affects the global climate, heterogeneous atmospheric chemistry and human health. In this study, an on-line and in-situ rapid scan attenuated total reflection Fourier transform infrared (ATR-FTIR) technique coupled with a linear relative humidity (RH) controlling system was utilized to obtain the IR spectra of aerosols under different RH. The mass growth factors (MGFs), deliquescence relative humidity (DRH) and efflorescence relative humidity (ERH) of (NH₄)₂SO₄, NH₄NO₃, and mixed (NH₄)₂SO₄/NH₄NO₃ aerosols were determined rapidly by measuring the peak areas of the bending vibration band of liquid water (~1 640 cm^-1). Comparisons between the measurements and the predictions from the E-AIM model showed good consistency, which verifies the rapid scan ATR-FTIR as a powerful tool for investigating hygroscopic behaviors and phase transitions of atmospheric aerosols. Furthermore, pure (NH₄)₂SO₄ and NH₄NO₃ particles were found to effloresce at 49% and 25% RH, respectively, while mixed (NH₄)₂SO₄/NH₄NO₃ aerosols with a mole ratio of 1∶1 and 1∶2 exhibited one-stage efflorescence transition beginning at 44% and 38% RH, respectively, upon dehydration. These results indicate that the presence of NH₄NO₃ can inhibit the crystallization of (NH₄)₂SO₄, and formed (NH₄)₂SO₄ seeds will act as heterogeneous nuclei to promote the efflorescence of NH₄NO₃ at higher RH. In addition, the double salt (NH₄)₂SO₄·2NH₄NO₃ was formed upon efflorescence of mixed particles. These findings are critical for understanding complex phase transitions of mixed inorganic aerosols and interpretation for RH dependency of heterogeneous reaction rates of atmospheric reactive species.