Abstract Aerosol nucleation accounts for the majority of secondary aerosols, yet it is unclear how biogenic and anthropogenic chemical precursors contribute to nucleation in mixed atmospheric environments. Here, we show laboratory experiments and quantum calculations, which demonstrate, for the first time to our knowledge, that highly oxygenated organosulfates (OOS) formed in the gas phase contribute to aerosol nucleation independently of sulfuric acid and oxygenated organic molecules. More than 200 different gas‐phase OOS were detected with the nitrate CI‐APi‐TOF from a mixture of α‐pinene, ozone, and SO2. The gas‐phase OOS concentrations were strongly correlated with sulfuric acid. Quantum chemical modeling simulations showed that OOS forms in the gas phase from α‐pinene diols and SO3 via a barrierless process. Nucleation rates increased much more rapidly with increasing α‐pinene concentrations in the mixed system than in the pure biogenic system, clearly demonstrating that OOS are effective nucleation precursors in mixed biogenic and anthropogenic systems.

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