Schaum et al. describe the hyphenation of liquid chromatography (LC) with chemical ionization mass spectrometry (CIMS) for offline analysis of chamber-generated and ambient (secondary) organic aerosol. While the coupling between LC and mass spectrometry is well established since decades, to my knowledge, this is the first time with iodide CIMS as ionization technique after LC separation. Conventional LC-MS approaches use either electrospray ionization, atmospheric pressure chemical ionization (corona discharge) or atmospheric pressure photoionization. These are all ionization techniques with specific sensitivities toward certain compound classes, and approaches to compare these techniques have shown there is little compound overlap between e.g. offline ESI and semi-online FIGAERO using iodide as reagent ion (Caudilllo et al., Atmos. Chem. Phys., 23, 6613–6631, 2023). Hence, this is a substantial new concept, and the approach described here enables to compare OA chemical composition from filter samples with online VIA-I-CIMS, to systematically investigate possible artefacts through filter sampling and storage (e.g. Resch et al., Atmos. Chem. Phys., 23, 9161–9171, 2023). The experimental setup is well described, method characterizations (calibrations with organic acid standards, comparison of chromatographic peak shape against DAD) are clear, and the application on a-pinene SOA and ambient OA is presented. However, I miss the obvious: a comparison of mass spectra between the averaged spectrum of either chamber-generated or ambient OA using (1) offline HPLC-VIA-I-CIMS against (2) the corresponding online VIA-I-CIMS spectrum. Figure S4 and S6 are showing already the offline spectra – but how does this compare against online VIA-I-CIMS spectra? This is the central comparison that would enable the authors to make statements about the significance of filter sampling artefacts, and/or cluster artefacts in the online VIA-CIMS, to provide a more comprehensive organic aerosol characterization. I like to encourage the authors to add this comparison if the online data are available.

Overall, the paper addresses relevant scientific questions within the scope of AR, presents a novel tool, and is also giving proper credit to related work. The presentation is well structured and the language is very fluent and precise.

At the end I miss an outlook with a statement specifically on which other CIMS reagent ions would also work with this setup. E.g. can it be operated with nitrate-CIMS to eventually detect HOMs after LC separation? Or positive ion CIMS using ammonia / urea or even PTR?

The following points are minor and technical:

l. 15: IMO a company name should not be stated in the abstract, only in the methods part.

l. 21: Is this the amount injected on column? I find injected mass on column more intuitive.

l. 67: what is meant by “low transmission” specifically? From the GC column into the ion source? In modern GC-MS this is usually not an issue, and compounds which are passed over the column also make the way into the ion source in heated transfer lines.

l. 136: while it is true that a low pH improves chromatographic peak shapes, it also suppresses ionization of organic acids in ESI. Would this setup here work with post-column addition of ammonia to increase pH and potentially increase ionization efficiency of negative ions as in ESI?

l. 154: I am very surprised that this large void volume of 300 mL does not cause a chromatographic peak broadening. Why was such a large volume chosen?

l. 156: Is this large flow of dry N2 necessary to reduce the water dependency? Because it is diluting the eluting compounds from the LC.

Fig. 2: Can the increasing sensitivity with increasing retention time be caused by the higher organic content in the mobile phase? It would be interesting to see whether the formed aerosol size distribution after the nebulizer is different with different mobile phase composition. Did you try an isocratic HPLC run? This could show the effect of the different mobile phase composition on ionization efficiency.

l. 212: hydrogen should be written instead of H

Fig. 3: is the DAD signal blank corrected? Hence is the increased baseline from the sample?

l. 265: I cannot imagine that cannabis cultivation is dominant over emissions from forests?

l. 273 and l. 285 / Figure 4, Figure S6: I am surprised about all the organic nitrates detected on a filter considering their short lifetimes of a few hours (Lee et al., PNAS, 2016). Are you sure that all these CHNO compounds are organic nitrates? Are organic nitrates more stable than Lee et al. has published?

l. 279: “either ocimene or limonene” - this can be any other monoterpene

l. 304: Investigation on which compounds contribute to BrC with LC-DAD/PDA-HRMS is not new – has been done by several groups (e.g. Laskin, Nizkorodov, Moschos/Surratt, Huang, …). Mostly on nitroaromatics and BBOA.

The manuscript by Schaum et al. developed a new method to quantity organic aerosols by connecting HPLC with VIA-CIMS system. The technique presented in this manuscript is innovative and potentially have important application in quantifying ambient organic aerosols. The manuscript itself is well organized and clearly written. I recommend the manuscript for publication with minor revision, and ask the authors to consider the following suggestions during the revision process.

1. I believe that the advantage of HPLC-VIA-MS over traditional HPLC-MS should be elaborated further, as this point is not clearly to me. Line 80-81, the author compared the typical HPLC-MS with the HPLC-VIA-MS method. However, it looks to me that typical HPLC-MS can detect more compounds that HPLC-VIA-MS method, so it did not specify why HPLC-VIA-MS is going to be valuable in examining the chemical composition of organic aerosols.
2. Line 108, please include the weight of the filter before and after the collection in the SI.
3. Line 149, how was the flow rate of 3.5 lpm determined to be most efficient in limiting wall loss and thermal decomposition?
4. What is the residence time of aerosols in the nebulizer/flask interface before entering the VIA-CIMS? Did the author consider this residence time when comparing the timing of the DAD signal and the CIMS signal?
5. The time for the aerosols in the nebulizer/flask system should have a distribution as not all aerosols would exit the flask at the same time. Will this cause broadening of the peaks or overlapping of the adjacent peaks of different compounds in VIA-CIMS?
6. Sections 3.2 and 3.3 only showed a handful of compounds. How many compounds can the VIA-CIMS system see for the mass of the SOA collected in this study? Is it enough to perform non-target analysis, or the number of the detectable compounds are heavily limited by the mass of the SOA collected?