Observations of supersaturated MgSO₄ and NaClO₄ aerosols by FTIR/ATR spectroscopy

FTIR/ATR (attenuated total reflection) spectroscopy has been used to investigate supersaturated MgSO₄ and NaClO₄ aerosols. The experimental ATR spectra of MgSO₄ and NaClO₄ aerosols were found to show little disturbance from water vapor, and thus were always of high quality independent of relative humidity (RH). Some relatively weak absorbing bands, such as the v₁ and v₃ bands of SO₄^2- in supersaturated MgSO₄ aerosols and the v₁ band of ClO₄^- and the residual bands of the water O-H stretching envelope in supersaturated NaClO₄ aerosols, could be identified by their peak positions comparable with those obtained by other experimental methods. In contrast, the relatively strong absorbing v₃ band of ClO₄^- in supersaturated NaClO₄ aerosols could be distorted very seriously, so that quantitative analysis with regard to band shape and peak position should be avoided. The above observations for MgSO₄ and NaClO₄ aerosols by FTIR/ATR spectroscopy could be understood, primarily in terms of the effects on weak and strong absorbing bands of anomalous dispersion and penetration depth (d_p) dependence on wavelength (λ) inherent in FTIR/ATR technique. Though rather limited in straightforward spectral analysis, the experimental high quality ATR spectra obtained under given conditions may be used to derive the optical constants of supersaturated aerosols.