Hygroscopicity of internally mixed aerosol particles containing benzoic acid and inorganic salts

The hygroscopic behaviors of benzoic acid (BA) particles and internal mixtures of inorganic salts (sodium chloride and ammonium sulfate) and BA are investigated in the 10–90% RH using the hygroscopicity tandem differential mobility analyzer (H-TDMA) system. Different morphology of BA, NaCl–BA, and (NH4)2SO4–BA particles at representative RH is characterized by Transmission Electron Microscopy (TEM). The hygroscopic growth factors (GFs) of BA exhibit a significant reduction in the 75–85% RH, followed by a slight increase at 90% RH. Internally mixed NaCl–BA and (NH4)2SO4–BA particles display deliquescence transitions at 70% RH, followed by apparently hampered hygroscopic growth due to the presence of BA. According to the TEM results, the morphology of BA particles and mixed particles show marked change in humidification, which possibly can be attributed to the strong microstructural rearrangement of BA fraction. This significant restructuring is responsible for the GFs change of BA particles and the hygroscopic behaviors transformation of mixed NaCl–BA and (NH4)2SO4–BA particles. These results demonstrate that atmospheric BA could significantly influence the hygroscopic properties of inorganic aerosols.