The evolution of carbon oxidation state during secondary organic aerosol formation from individual and mixed organic precursors

This study reports the average carbon oxidation state (OSC) of secondary organic aerosol (SOA) particles formed through the photo-oxidation of o-cresol, α-pinene, isoprene and their mixtures - representative anthropogenic and biogenic precursors - in the Manchester Aerosol Chamber. Three independent mass spectrometric techniques, including two online instruments, namely a high-resolution time-of-flight Aerodyne aerosol mass spectrometer (HR-ToF-AMS) and a Filter Inlet for Gases and AEROsols coupled to an Iodide high-resolution time-of-flight chemical ionisation mass spectrometer (FIGAERO-CIMS), and one offline technique, namely ultra-high-performance liquid chromatography high-resolution mass spectrometry (UHPLC-HRMS), were employed to characterise the chemical composition of SOA and to derive average OSC in mixtures of α-pinene-isoprene, o-cresol-isoprene, α-pinene-o-cresol and α-pinene-o-cresol-isoprene systems. This paper firstly reports the detailed analysis of particle average OSC during SOA formation in mixed anthropogenic and biogenic systems using two online mass spectrometry techniques and one offline mass spectrometry technique simultaneously. Across single-precursor experiments, OSC generally declined with increasing SOA mass, suggesting a shift from highly oxygenated, low-volatility products at early stages towards more semi-volatile, less oxidised compounds at higher particle mass loading. This behaviour was robust across different initial precursor concentrations, implying that SOA growth is dominated by partitioning dynamics rather than initial precursors' reactivity. Moreover, comparisons across analytical techniques demonstrate systematic differences, with FIGAERO-CIMS consistently reporting higher OSC, UHPLC-HRMS (negative mode) aligning more closely with FIGAERO-CIMS, and HR-ToF-AMS underestimating OSC due to its inability to resolve nitrogen-containing species. Furthermore, correcting for the oxidation state of nitrogen (OSN) significantly reduced OSC estimates in o-cresol experiments, likely reflecting the strong influence of CHON+ ion fragments. Mixed-precursor experiments reveal that the isoprene suppressed the formation of highly oxygenated α-pinene products through OH scavenging and RO2 competition, lowering OSC in mixed systems. In contrast, α-pinene-o-cresol mixtures showed elevated OSC, likely contributed by the cross-interaction between precursor-driven RO2 forming multifunctional accretion products. The ternary mixture evolved to intermediate OSC values between the single-precursor experiment, which could imply a balance between OH scavenging, RO2 competition and cross-interaction reaction.