Anomalous hygroscopic growth of fine particles of MgSO₄ aerosols investigated by FTIR/ATR spectroscopy

This paper demonstrates an approach for investigating the hygroscopic growth of MgSO₄ aerosols deposited on ZnSe substrate by using attenuated total reflection Fourier transform infrared (FTIR/ATR) spectroscopy. The experimental setup based upon a refitted standard FTIR/ATR accessory was adopted for the hygroscopic study of aerosols. It has been found that the "predeliquescence" of fine MgSO₄ aerosol particles with the mean spreading diameter around 500 nm occurred before roughly 15% RH. In contrast, the abrupt water absorption of coarse MgSO₄ particles with the mean diameter larger than about 10 μm was reported to occur at roughly 42% RH, which was reproduced in this study. Up to now, both theoretical and experimental investigations were rare and immature for fine particles, but the method we used in this study worked very well for the fine particles of MgSO₄ aerosols. It has been found that the possible reason for the "predeliquescence" fine particles of MgSO₄ aerosols is that the initial state or phase of fine particles is different from coarse ones after desiccation. This research demonstrates that the approach based on the ATR technique is very convenient, accurate and requires only a little amount of lab supplies.