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 SO42− 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 (dp) 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.