Hygroscopic properties of internally mixed particles of ammonium sulfate and succinic acid studied by infrared spectroscopy.

Atmospheric aerosols typically consist of inorganic and organic material. Although the organic fraction can affect the behavior of mixed organic/inorganic particles, their physical properties are not well-understood. In this work, infrared spectra of internally mixed submicrometer particles of ammonium sulfate and succinic acid have been studied at ambient temperature in an aerosol flow cell. The spectra of dried particles show distinct features relative to the pure components, as a result of ion-molecule interactions between the inorganic and organic species. The hygroscopic behavior of the particles has been followed for several organic/inorganic mass ratios, showing that around equimolar composition, the mixed particles uptake water in a broad range of relative humidities (30-80%), substantially lower than the deliquescence relative humidity of the mixed system near 80%. Infrared spectra at predeliquescence relative humidities reveal that succinic acid is partially or completely in the liquid phase at much higher concentrations that those corresponding to a saturated solution of succinic acid. This behavior is proposed to arise from the ion-molecule interactions between the organic and inorganic components, which unstabilize the crystal structure relative to the pure solids and cause loss of translational order in the crystal, bringing about an increase in the Gibbs energy of the solid particles and allowing the uptake of water molecules prior to the deliquescence point. The obtained results show that water absorption prior to full deliquescence in this system has to be taken into account because it extends the range of relative humidities at which particles are partially or completely liquid.