Acidity-Dependent Atmospheric Organosulfate Structures and Spectra: Exploration of Protonation State Effects via Raman and Infrared Spectroscopies Combined with Density Functional Theory

Organosulfates formed from heterogeneous reactions of organic-derived oxidation products with sulfate ions can account for >15% of secondary organic aerosol (SOA) mass, primarily in submicron particles with long atmospheric lifetimes. However, fundamental understanding of organosulfate molecular structures is limited, particularly at atmospherically relevant acidities (pH = 0–6). Herein, for 2-methyltetrol sulfates (2-MTSs), an important group of isoprene-derived organosulfates, protonation state and vibrational modes were studied using Raman and infrared spectroscopy, as well as density functional theory (DFT) calculations of vibrational spectra for neutral (RO–SO3H) and anionic/deprotonated (RO–SO3 –) structures. The calculated sulfate group vibrations differ for the two protonation states due to their different sulfur–oxygen bond orders (1 or 2 versus 12/3 for the neutral and deprotonated forms, respectively). Only vibrations at 1060 and 1041 cm–1, which are associated with symmetric S–O stretches of the 2-MTS anion, were observed experimentally with Raman, while sulfate group vibrations for the neutral form (∼900, 1200, and 1400 cm–1) were not observed. Additional calculations of organosulfates formed from other SOA-precursor gases (α-pinene, β-caryophyllene, and toluene) identified similar symmetric vibrations between 1000 and 1100 cm–1 for RO–SO3 –, consistent with corresponding organosulfates formed during laboratory experiments. These results suggest that organosulfates are primarily deprotonated at atmospheric pH values, which have further implications for aerosol acidity, heterogeneous reactions, and continuing chemistry in atmospheric aerosols.