This work developed APCI-Orbitrap-MS and preliminarily characterized its performance. In this paper, the authors further utilize the APCI-Orbitrap-MS for field measurements at an urban background station and an agricultural field site. Based on the field results, they demonstrate the superior measurement capabilities of the APCI-Orbitrap-MS for characterizing organic aerosols. This represents a promising new technique for the online measurement of particulate organic components. While this work builds upon their prior research, aspects such as quantification and pyrolysis fraction of organic molecules require more detailed discussion. Importantly, the application of MS² may help address current concerns among researchers regarding the inability of online mass spectrometry to provide accurate molecular identification. Therefore, I recommend that this manuscript be accepted for publication after minor revision.

1. Lines 86-88: The authors state in their summary of existing online measurement techniques for particulate organic components that VIA experiences fragmentation at 250°C, emphasizing that the APCI-Orbitrap-MS mitigates this fragmentation. Figure 2 depicts the effect of temperature on signal intensity for different compounds. However, this figure alone does not demonstrate that fragmentation in the APCI-Orbitrap-MS is less severe than in VIA. Thermal decomposition/fragmentation remains a key challenge for the online measurement of organic components in particulates. The argument presented here would be substantially strengthened if the authors could include a direct comparison of thermal decomposition between the two instruments or provide a comparative analysis against relevant published data in the literature.
2. Figure 7: I have reservations about the normalization of units applied by the authors in this figure. This normalization obscures true concentration levels and makes it impossible to assess the original mass spectral intensity information. Consequently, the processed data significantly hinders its utility for meaningful interpretation and fails to support critical information assessment.
3. Line422-423: “4-nitrocatechol had the highest ionization efficiency”. This does not necessarily reflect ionization efficiency, but could also be affected by ion transmission efficiency.
4. Figure S7: The concentration of organic molecules is a critical limiting factor for online structural identification of compounds. The figure indicates that the maximum intensity of organic molecule signals reaches 1.8E7. This represents an exceptionally high signal level for online measurements. What causes such elevated signal intensities in organic molecules, and whether the authors implement optimizations to enhance instrument sensitivity?

Review of EGUsphere-2025-2243

This paper describes the design and testing of a novel online mass spectrometer for studying real time organic aerosol composition using atmospheric pressure chemical ionisation coupled to high resolution orbitrap mass spectrometry. The authors provide a compelling case for the effectiveness of the new methodology and outline its advantages compared to other techniques. I recommend publication after the following comments are addressed.

Overall comments

• I would change the structure of the results section. The field data is interesting but it would be much better if this followed on from the technical descriptions of the importance of high resolution, sensitivity and the use of fragmentation. For instance, there is a discussion in the fragmentation section about the assignment of MBTCA that I would have appreciated to understand before looking at the field observed data. Then I could take into account the discrepancies between the lab generated spectrum and the reference spectrum. The importance of having high resolution and the number of compounds that can be resolved at each nominal mass is one of the most important features of the new method and I think it would be more appropriate to have this as the first part of the results section.
• There is very little presented on the limitations of the technique. I would suggest adding a paragraph outlining the issues and how/if they could be overcome in future studies. The isobaric interferences is mentioned but there are other limitations around calibration and unassigned peaks.
• The field data is interesting but I feel that the power of the high resolution isn’t really drawn out. For instance, could you show two compounds that have very similar mass that you could not separate at lower resolutions? Or discuss how the observed trends for the target compounds might be incorrect using a lower resolution instrument.

Minor comments

• Figure 1 – this is blurry and hard to read. Its also not clear – are the blue and red clouds supposed to represent gas phase species?
• Table 1 seems unnecessary as most information is given in the text.
• Section 2.2: The concentration used here are very high and this is likely to lead to different chemistry than in the real atmosphere. The impact of this should be acknowledged.
• Line 253: what does “intensity” mean here? Do you mean the number of compounds?
• Line 257: (and further refer to) – seems like the rest of this is missing?
• Line 259: Could reduction in pinonic acid during the day also be related to secondary chemistry during the day?
• Line 266: Levoglucosan behaved similarly to MBTCA. This seems unusual given there very different sources. In the fragmentation figure 8, you compare MBTCA to the PAM-OFR data. What does the comparison with the ambient data look like? Are you sure this is the correct species?
• Figure 3: The colours are quite hard to differentiate – two greens and two blues. The should be changed to make it easier to read.
• Figure 3: Can factor analysis be done on the ACSM data or is the resolution not good enough? It would have been nice here to show that the points that doent correlate as well were related to a higher f43 or other hydrocarbon fragment ion.
• Line 304: You have a lot of unassigned m/z values. Do you have any suggestions for why these are not assigned?
• Figure 4: Is there pinonic acid data for this site? It would be helpful to compare this between the two sites. Also, I would suggest that C8H13O8N could have multiple monoterpene sources rather than just a-pinene. Additionally, I would like to see a zoomed in version of 4c to see the correlation when the f44 is high.
• Figure S8: I think the legend is incorrect. The “other” category is black not cyan.
• Line 375: Please give % of peaks that could be resolved at a resolution of 10k.
• Line 416: I would remind the readers here that the standards were introduced as a nebulised methanol/water solution.
• Section 3.5: This section needs to have a more critical evaluation of the spectra obtained. Some of the comparisons are not great – what are the similarity or reverse fit values? For MBTCA, why is the ambient data not used. Also, why have you chosen these three compounds? Are they fairly unique or dominant masses or simply because they are well known tracer compounds? At present, the identification of the pinene derived SOA components is not very convincing.
• Line 450: What is the m/z isolation window used for MS2 and are there any other peaks found within this window?