What Are the Sources of Unclassified Organosulfates in the Atmosphere?

Organosulfates (OSs) are ubiquitous components of atmospheric aerosols, contributing up to 30% of total aerosol mass, yet their sources remain poorly constrained. Recently, we have compiled 366 ambient OS molecular formulas from previous field studies and found that only about 36% can be assigned to known precursors or formation mechanisms. These mechanisms primarily explain the production of freshly formed OSs via photochemical and/or condensed-phase reactions associated with different precursors. Consequently, 64% of observed OS formulas remain unclassified. Concurrently, laboratory studies have revealed that certain OSs can undergo efficient multiphase transformation through heterogeneous and aqueous-phase reactions, yielding higher-generation OS products. Although such studies are still limited, the identified transformation OS products can account for about 17% of all reported ambient OS formulas and, notably, explain an additional 5% of previously unclassified OS signals. These findings imply that a potential fraction of ambient OSs may not be freshly formed but rather secondary or higher-generation products arising from chemical transformation. Current field analyses and atmospheric models focus mostly on precursor-specific OS formation pathways, largely overlooking the role of OS transformation. We therefore propose that transformation of freshly formed OSs is an important and previously underrecognized source of unclassified ambient OSs.