| 10 Jul 2025
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 (OSN) 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 OSN, is lower than when the OSN is not considered. SOA particle average oxidation state excluding consideration of OSN 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.
The paper summarizes average carbon oxidation states of SOA formed from photooxidation of single and mixture VOCs (o-cresol, a-pinene, and isoprene) in the presence of NOx. SOA were sampled online by HR-ToF-AMS and FiGAERO-I-CIMS and offline by HPLC-MS to determine the average oxidation state of carbon. Results from different experiments and instruments were then compared to investigate how the chemistry of SOA formation is influenced by the presence of another precursor. There are two other papers from the group which apparently have highlighted already what the common and unique SOA products are in these systems. I therefore don't know what the value of this paper is in addition to the more robust identified products in their previous work. As the authors highlight, HR-ToF-AMS cannot provide an accurate estimate of the average OSc since it cannot determine the N-content of aerosols well. FIGAERO and HPLC-MS are each sensitive to certain classes of compounds and so don't provide a complete picture of the oxidation products either. That said, I think there is value in comparing results from one instrument across the single/mixture systems to say something about the differences in chemistry. I therefore believe the manuscript can eventually be published, but it needs major revisions in my opinion. As is, there are simple conclusions that are hidden in the paper and lost in the mere numeric comparisons presented in Sections 3 and 4. Please see below my comments:
General comments:
Experimental section: The experimental section early on should mention that NOx was adjusted to keep the VOC/NOx ratio the same. What’s the predicted average OH in the expts?
Sections 3.1-3.3 read as sentences comparing average OSc estimates from one condition/instrument to the next and is not that valuable as written. For example, I didn’t get a sense of what to think about the different estimates or sometimes opposing trends in the estimates from the positive vs. negative ion modes of HPLC-MS. I think for this section to be useful, the data need to be better synthesized and a summary presented in a way different than just comparing pairs of average OSc values.
Section 4: I was hoping this section contains a more synthesized look at the data, but it’s a summarized set of comparisons again (without actually mentioning all the OSc values) and the discussion is more on limitations of the instruments (or preferential detection of certain compounds) rather than actual chemistry
Since there is evidence for nitrogen-containing ions to be formed in these systems and OS of nitrogen has a significant impact on the estimated average OSc, I don’t see the value in comparing OSc when OS of nitrogen is ignored, so in my mind, the data from HR-ToF-AMS should not be included in the paper and any other reference to average OSc disregarding OS of nitrogen should be removed.
Specific comments:
P3, L63: add some reference for recent research on multi-VOC systems
P3, L29-71: consider breaking this sentence to multiple or rephrase it for ease of readability
L 264: C in DeCarlo should be capitalized
L 265 Reference to Sueper doesn’t need the first name initial
L294: how long was sonication done?
L299: evaporation to dryness was carried out at a much warmer temperature than room. What’s the potential impact in driving off some of the more volatile SOA components?
L499: change significantly to significant
L639: remove either “in” or “for”
L770: delete “which”
L833: what do you mean by “at a higher magnitude”?
L928: delete “be”
L962 and 949: there are two contradictory statements on the average OSc of the mixture in these two statements