Triple oxygen isotope constraints on organic aerosol oxidation pathways

Organic aerosols (OA) are a major source of uncertainty in assessing aerosol impacts on atmospheric composition and climate. Atmospheric oxidation governs OA formation and aging, yet the relative contributions of different oxidation pathways are under constrained by direct observations. Here, we present year long triple oxygen isotope (δ ^’18 O, ∆ ^’17 O) measurements targeting the organic matter in atmospheric OA samples. These isotopic data reveal that the OA contains mass independent oxygen and detail contributions from multiple oxidants involved in OA oxidation, including O ₂ (via radical initiated chemistry), O ₃ , and OH. Using an isotope-based mixing model with Monte Carlo resampling, we combine empirical and observational constraints on oxidant isotopic compositions to deliver quantitative, observation-based estimates of oxidant-specific contributions to atmospheric OA formation. The resulting oxidant contributions support a dominant role for radical initiated chemistry accompanied by O ₂ derived oxygen incorporation, along with persistent, but minor, contributions from ozonolysis and OH addition reactions. These findings are broadly consistent with independent model predictions of OA formation pathways, demonstrating triple oxygen isotopes as a quantitative tracer for OA oxidation pathways and a new observational framework for independently evaluating and improving representations of OA atmospheric chemistry.