NaClO4 aerosol particles with diameter of 5―20 microns are deposited on the ZnSe substrate. An FTIR-ATR chamber on the ZnSe substrate is set up to ob-serve the structural changes of NaClO4 aerosol particles at crystallization relative humidity (CRH) and deliquescence relative humidity (DRH). With the decrease of RH (relative humidity) from 96% to 24% in the efflorescence process, the absorbance of O-H stretching band of the aerosol solutions continuously decreases. A sudden decrease of the water peak is observed at RH20%, where solid particles form. Very small amount of residual interfacial water, with two weak peaks at 3602 and 3533 cm-1, can still be resolved in the FTIR-ATR spectra of the solid particles. In the deliquescence process of the same sample, little spectral changes are ob-served when the value of RH varies from 5% to 29%. Before the abrupt increase of the O-H stretching band at the DRH of about RH46%, a pre-deliquescence process is observed, i.e. in the RH range between 33% and 46%, there is really a slow absorbance increase for the peak area of O-H stretching band. The O-H stretching band shows an arciform O-H en-velope, totally different from not only the FTIR-ATR spectra of ClO4? solutions either under supersaturated state or in diluted state, but also the characteristic of the residually in-terfacial water discussed in the efflorescence process. Such kind of water is considered as pure water, indicating that small amount of water aggregates in the microspaces of solid aerosol particles due to the capillary cohesion effect. When the RH arrives at RH44%, two weak shoulders at 3384 and 3260 cm?1 can be resolved, and the solid NaClO4 particles begin to be dissolved by increasing capillary cohesion water. The spectral characteristic of the ν3-ClO4? band also shows the transition from solid particles to mainly solvated ClO4? ions.
Single supersaturated NaClO4 aerosol droplets on a quartz substrate were observed by a high-speed video camera for their morphologic changes with decreasing relative humidity (RH). The supersatu- rated droplets were found to form anhydrous NaClO4 at ~10% RH. Three stages were roughly observed in this process, i.e., the main stem appearance, primary and secondary branches growth, and the deep dehydration. The main stem grew steadily and finished in 120 ms. Affected by both crystal precipitation and water evaporation, primary branches grew for 1200 ms, accompanied by a slower growth of sec- ondary branches. The deep dehydration was more complicated, and lasted for a longer time.
