FTIR-ATR chamber for observation of efflorescence and deliquescence processes of NaClO_4 aerosol particles on ZnSe substrate

摘要

NaClO_4 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 observe the structural changes of NaClO_4 aerosol particles at crystallization relative humidity (CRH) and deliquescence relative humidity (DRH). With the decrease of RH (relative humidity) from 96 percent to 24 percent 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 percent, 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 observed when the value of RH varies from 5 percent to 29 percent. Before the abrupt increase of the O-H stretching band at the DRH of about RH46 percent, a pre-deliquescence process is observed, i.e. in the RH range between 33 percent and 46 percent, 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 envelope, totally different from not only the FTIR-ATR spectra of ClO_4~- solutions either under supersaturated state or in diluted state, but also the characteristic of the residually interfacial 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 percent, two weak shoulders at 3384 and 3260 cm~(-1) can be resolved, and the solid NaClO_4 particles begin to be dissolved by increasing capillary cohesion water. The spectral characteristic of the v_3-ClO_4~- band also shows the transition from solid particles to mainly solvated ClO_4~- ions.