Cross-Reaction Products from Mixed Volatile Organic Compound Oxidation: Evidence from Isotope-Labeled Toluene and α-Pinene Secondary Organic Aerosol

Cross-reactions between peroxy radicals (RO2) derived from different volatile organic compound (VOC) precursors have been proposed as an important pathway during atmospheric oxidation. However, direct molecular evidence has been limited. In this study, α-pinene and fully deuterated toluene (d8-toluene) were oxidized separately and as a mixture in a potential aerosol mass (PAM) flow reactor, and the resulting secondary organic aerosol (SOA) was characterized by a high-resolution mass spectrometer (ESI FT-ICR-MS). A constrained chemical mass balance (CMB) model attributed 82.9% of the mixed-SOA signal to single-precursor sources (66.5% α-pinene, 16.4% d8-toluene), leaving a 17.1% signal-based residual fraction unexplained by linear mixing. Among 2450 residual molecular formulas exclusive to the mixed-SOA, 1858 were identified as cross-reaction candidates, with carbon, oxygen, and double bond equivalents (DBE) distributions consistent with RO2-RO2 cross-reactions between toluene- and α-pinene-derived fragments. We also identified representative monomer-dimer pairs, where one monomer corresponded to a known α-pinene oxidation product, while the other matched a primary oxidation product of d8-toluene oxidation based on the Master Chemical Mechanism (MCM), providing strong molecular-level evidence for RO2-RO2 cross-reactions. Our findings demonstrate that the mixed VOCs generate unique SOA products that extend beyond simple additive chemistry, with implications for SOA yield parameterizations and chemical transport models.