Novel pathway of SO ₂ oxidation in the atmosphere: reactions with monoterpene ozonolysis intermediates and secondary organic aerosol

Ozonolysis of monoterpenes is an important source of atmospheric biogenic secondary organic aerosol (BSOA). While enhanced BSOA formation has been associated with sulfate-rich conditions, the underlying mechanisms remain poorly understood. In this work, the interactions between SO ₂ and reactive intermediates from monoterpene ozonolysis were investigated under different humidity conditions (10 % vs. 50 %). Chamber experiments were conducted with ozonolysis of α-pinene or limonene in the presence of SO ₂. Limonene SOA formation was enhanced in the presence of SO ₂, while no significant changes in SOA yields were observed during α-pinene ozonolysis. Under dry conditions, SO ₂ primarily reacted with stabilized Criegee intermediates (sCIs) produced from ozonolysis, but at 50 % RH heterogeneous uptake of SO ₂ onto organic aerosol was found to be the dominant sink of SO ₂, likely owing to reactions between SO ₂ and organic peroxides. This SO ₂ loss mechanism to organic peroxides in SOA has not previously been identified in experimental chamber studies. Organosulfates were detected and identified using an electrospray ionization–ion mobility spectrometry–high-resolution time-of-flight mass spectrometer (ESI-IMS-TOF) when SO ₂ was present in the experiments. Our results demonstrate the synergistic effects between BSOA formation and SO ₂ oxidation through sCI chemistry and SO ₂ uptake onto organic aerosol and illustrate the importance of considering the chemistry of organic and sulfur-containing compounds holistically to properly account for their reactive sinks.