Detailed Molecular Characterization of Limonene Secondary Organic Aerosol Under Varying Oxidation Conditions and Reaction Timescales at the SAPHIR Chamber during CHANEL campaign

Limonene is an abundant and highly reactive monoterpene that forms secondary organic aerosol (SOA) in the atmosphere. Limonene SOA from emerging anthropogenic sources, e.g., volatile chemical products, can deteriorate air quality in developed cities, yet the impacts may vary depending on its chemical properties. During the CHANEL campaign, atmospheric oxidation of limonene was simulated in the 270 m³ SAPHIR chamber to study the influence of reaction conditions and timescales on the molecular-level chemical composition of SOA. Different combinations of hydroxyl (OH^.), nitrate (NO₃^.) and ozone (O₃) oxidants were used in medium NO_x for investigating day- and night-time oxidation conditions with each experiment spanning 10 – 12 hours. The SOA was transmitted from the chamber directly to an Ionicon CHARON-FUSION time-of-flight mass spectrometer that was operated in both H₃O⁺ and ammonium (NH₄⁺) modes with a periodic ion-switching measurement protocol. A positive matrix factorization approach was implemented via Source Finder (SoFi) to constrain the relative prominence of organic species in aerosol composition at different stages of each experiment. SOA evolved over several hours, yet with oxygenated species (C_xH_yO_z) constituting 70 – 80% of the mass spectra that were dominated by compounds containing 5 – 10 carbon and 2 – 6 oxygen atoms. For daytime oxidation (OH+O₃), C₉H₁₄O₄ and C₉H₁₄O₅ species were highly prominent in SOA formed immediately after the first precursor injection. The C₉-species group also dominated peak SOA concentrations during night-time conditions. These were followed by O₅ and O₆-containing species that dominated the daytime tests after 4 – 5 hours of initial injection. After 7 – 8 hours, the molecular distribution looked considerably similar to that of SOA formed in O₃-only oxidation tests with delayed appearance of C₈H₁₂O₅ and C₈H₁₄O₅ species that were likely multigenerational oxidation products. These observations suggest that properties of limonene SOA may continue to evolve over several hours following emissions and could influence their environmental impacts.