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

Abstract. This study reports the average carbon oxidation state (OSc) of secondary organic aerosol (SOA) particles formed from 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, high resolution time-of-flight Aerodyne Aerosol Mass Spectrometers (HR-ToF-AMS) and 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, ultra-high-performance liquid chromatography-high resolution mass spectrometry (UHPLC-HRMS), were employed to characterise molar atomic ratios (e.g. O/C, H/C and N/C) leading to estimation of the SOA particle average carbon oxidation state in mixtures of α-pinene/isoprene, o-cresol/isoprene, α-pinene/ o-cresol and α-pinene/o-cresol/isoprene. This paper firstly reports the detailed analysis of particle average carbon oxidation state during SOA formation in mixed anthropogenic and biogenic systems using two online and one offline mass spectrometry techniques simultaneously. Single precursor experiments at various initial concentrations were used as a reference. The oxidation state of nitrogen (OS_N) for CHON compounds was shown to non-negligibly influence the average OSc in individual precursor α-pinene and o-cresol experiments in FIGAERO-CIMS and UHPLCHRMS measurements. By definition, the average OSc accounting for OS_N, is lower than when the OS_N is not considered. SOA particle average oxidation state excluding consideration of OS_N obtained by the three techniques showed substantial discrepancies. That obtained from FIGAERO-CIMS was always found to be higher than from the other techniques, as a result of the negligible sensitivity of the FIGAERO-CIMS toward compounds without oxygen. Quantification of the SOA particle average OSc was challenging, but all three techniques showed similar trends across systems. In the single precursor experiments, the initial concentration of precursors influences the average OSc in the single α-pinene experiment, but not in the single o-cresol experiment. In binary precursor systems, the injected isoprene affected the average OSc in the presence of α-pinene but the influence was more modest in the presence of o-cresol. The OSc in the binary α-pinene/isoprene mixture was lower than in the α-pinene system although they have a similar trend in average OSc with SOA mass concentration. This suggests that the isoprene has the potential to decrease the average OSc by acting as an OH scavenger, resulting in suppression of the formation of low-volatility and highly oxygenated organic compounds in a mixed system. The OSc in the binary α-pinene/ o-cresol mixture was between those measured in the single precursor experiments, where that in the α-pinene experiment was lower than in the o-cresol experiment, suggesting contributions to the OSc from both precursors. In the ternary mixture, the OSc was not dominated by any single precursor, with substantial contributions from products uniquely found in the mixture. These results implies that interactions between VOC products should be considered, to enable the level of chemical aging or oxidation of organic compounds understanding in ambient atmosphere.