Distinct Hygroscopic Growth and Phase-State Evolution of Glucaric Acid and Mucic Acid Aerosols under Highly Alkaline Conditions

School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 100081, China

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Abstract

The hygroscopicity and phase state of organic aerosols can be strongly modified by alkaline neutralization, while the role of molecular stereochemistry remains unclear. In this work, optical microscopy and confocal Raman spectroscopy were used to investigate the hygroscopic growth and phase-state evolution of glucaric acid and mucic acid aerosols at pH 12.60 during dehydration-rehydration cycles. Glucaric acid particles retained droplet-like morphology throughout the humidity cycle, whereas mucic acid particles transformed into irregular solids at RH = 36.4% and did not fully recover their original spherical shape upon rehydration. The hygroscopic growth factor showed continuous change with weak hysteresis upon a RH cycle, while mucic acid exhibited pronounced efflorescence-deliquescence hysteresis. Raman spectra provided the further evidence for particle phase. The C-H stretching modes of glucaric acid exhibit disorder arrangement of hydrocarbon chain at low RH, indicating the gel. Whereas mucic acid evolved toward a more ordered solid- like spectral profile after drying. These results demonstrate that the two stereoisomeric polyhydroxy dicarboxylic acids follow fundamentally different phase-evolution pathways under alkaline conditions, highlighting the combined effects of stereochemistry and neutralization chemistry on aerosol hygroscopicity and phase state.