Water uptake of internally mixed ammonium sulfate and dicarboxylic acid particles probed by infrared spectroscopy

► IR spectra of ammonium sulfate/dicarboxylic acids measured in an aerosol flow tube. ► Sulfate ion ν3 vibrational band near 1115 cm−1 broadens in mixed particles. ► Perturbations in particle spectra are due to intermolecular interactions. ► Predeliquescence water content correlates with the interactions in dry particles.