Overall review: This paper presents a new instrument design, using nafion drying, denuding, and thermal dissociation with cavity-ringdown detection of NO2, in order to allow quantitative measurement of particulate nitrate (both inorganic and organic). The new instrument is compared to aerosol mass spectrometry (AMS) measurements of nitrate, tested with laboratory generated NH4NO3 and SOA from limonene + NO3 in order to benchmark performance.  The current instrument design overcomes previous challenges with humidity-dependent memory effects in the denuder. This step forward in denuder-based nitrate measurement techniques, and method validation by comparison to another technique, will be of interest to the scientific community making such measurements. The paper presentation is clear and well-written. I recommend publication after minor revisions.

Specific comments:

Title: might be a good idea to add your instrument acronym to the end of the title, for searchability?

Lines 11-13: first sentence of the abstract has 3 separate ideas in it. Suggest breaking up into 2 sentences & reword a bit: “Ambient inorganic and  organic particulate nitrate has been connected to cardiovascular and respiratory illness. Accurate measurement of nitrate in both gas- and aerosol-phases is essential for understanding its partitioning and thus its impact on human health.”

Line 14: define NOy at first instance

Line 28: define NOz

Line 30: “both NOx and OH radicals”

Line 32: add citation after “from the gas to the particle phase”

Line 35: don’t hyphenate sodium nitrate, and mention this is due to heterogeneous reactions of HNO3 on sea salt (and cite something for that). Maybe also add a sentence mentioning mineral dust inorganic nitrates (Ca, Mg) , with citation, before moving to organic nitrate literature.

Line 44-45: remove line break, continue paragraph.

Line 26: add recent Metrohm citation for MARGA, and also mention the AIM online ion chromatography system for completeness

Lines 48-50: Not sure I agree that mARGA type techniques can inform about organic nitrates. First, the absorbance solutions are not typically alkaline as you say (usually ultrapure H2O with a little H2O2), and hydrolysis timescale of even small, functionalized organic nitrates is likely to be too long for detection as HNO3. It certainly would not be something to trust quantitatively. So I would revise this piece to say that organic nitrate behavior in these IC systems is not well known, and omit the claim that you could use ion balance to infer org / inorg nitrates. Or, if there is literature supporting some modifications of IC systems to do this, please describe it in more detail, and cite it!

Line 52: MARGA resolution is 1 hr.

After line 53: Suggest to insert a line or two mentioning previous TD-based methods developed for nitrate aerosol determination: Garner et al Environ Sci Technol. 2020, Keehan et al AMT 2020. Then you can later also mention how your modifications improve on these previous methods.

Line 62: after citing Farmer 2010, also cite Day et al AMT 2022 (systematic re-evaluation of bulk org nitrate methods). Suggest to omit the parenthetical that follows.

Line 103: “nitrates, dissociating”

Line 105: “, 2020), a reduction in temperature is beneficial in avoiding”

Lin 138: Can you show this effect of detecting NH3 at higher temperature also on Fig. 1, by showing higher temperatures (and indicating in the figure that it is due to NH3 conversion and thus a reason to keep the TD oven at lower temperature)?

Line 145: Clarify that this means as opposed to introducing ZA downstream of the oven. Or would it be introduced even later?

Line 159: Explain the equation for competitive absorption in a bit more detail. What are the Klang’s for relevant NOy species?

Line 181: replacing every 45 hours is still pretty frequent … How low does the RH actually need to be? Earlier you mentioned effects above 20% RH. Good if you can make a recommendation for how often to regenerate in field applications

Line 190: End of section 2.3: One question that came up for me reading this: can you show the (lack of) effect on HNO3 measured of these inlet components?  Could pulling off HNO3 in the denuder / drying affect the partitioning of NH4NO3 in the instrument?

Line 193: “2020), which however”

Line 214: “loss is negligible.”

Line 236: Suggest expanding section header title, something like “Gas-phase breakthrough and reactivation of the denuder”

Lines 256-260: I think this error propagation could be better conveyed using a few inline equations, rather than just embedding in the paragraph text.

Line 264-272: same. Also nice to briefly show / mention the conversion factor for the ppb  <-> ug m-3 conversion . For particulate nitrate, do you assume the MW of NO3 group only? Or NO2?

Line 268: What does “Under laboratory conditions” mean in this context?

Line 305: explain what d(v) = 38.5 means. Diameter in nm?

Lines 325-326: Can you demonstrate this by including a timeseries panel of SMPS / RH?

Lines 330-332: how do you interpret this >1 slope? If there is an interpretation for it, could include the above small-particle hypothesis, if not, maybe omit both?

Related to figure 6: why is the D-TD-CRDS trace intermittent?

Line 347: Can reiterate that MARGA time resolution is 1 hr

Line 247: non-detection of organic nitrates

Data availability: cite public database and remove parenthetical

I did not find a link to the supplement.

Figure 1: “NOx and NOy” (not or). Also explain the filters (mention with symbol in caption, material); state temperature the TD inlets are heated to in caption.

Figure 2: Make left axis match right, “NO2 from HNO3”

Figure 4: all occurrences of “poly / mono dispersed” should be “polydisperse”

Figure 6: “1:1 line” not agreement

This paper presents a further development of a CRD instrument previously presented, modified such that it can detect particulate NOy through thermal desorption after the use of a stripper to remove gas phase NO_y. This is an interesting concept and offers an alternative method of integrated quantification of particulate NO_y compared to other techniques such as semicontinuous IC and AMS. Two specific conceptual advantages this has over the AMS is the lack of a size cutoff and the fact that this has a more direct quantification of atomic nitrogen, rather than electron ionisation fragments. The advantages over IC is the reduced complexity of the measurement and ability to measure total organic particulate reactive oxidised nitrogen.

The technique is described in detail and some initial validation work comparing against an AMS is presented resulting in very favourable correlations, so this is within scope for AMT. While I have a number of technical queries and corrections, this should not detract from the fact that this presents a novel approach to what has always been a tricky measurement in atmospheric science, so I am happy to recommend publication with minor corrections.

Major comments:

One major conceptual point not directly addressed is the propensity for ammonium nitrate to dissociate to ammonia and nitric acid at low relative humidities, which may happen to an extent during the drying stage, after which they would then be removed by the denuder. This process has previously caused artefacts with HTDMA systems (e.g. https://doi.org/10.5194/acp-7-6131-2007) and seems dependent on how long the particles were subjected to a low RH for. This potential issue needs addressing the and the typical time particles spend at low RH after being sampled should be specified. Note that the time spent in the chamber at low RH prior to sampling is a separate issue, but this will affect the AMS and D-TD-CRDS equally.

Minor comments:

Line 77: The other techniques summarised here only cover a specific subset of methods for the measurement of bulk oxides of nitrogen in aerosol, but other techniques are used, such as molecule-resolving techniques like CIMS, ESI and liquid/gas chromatography, or spectroscopic techniques such as NMR, FTIR and UV-Vis. While I would not expect a comprehensive review here, the authors should be more specific about the types of technology they are comparing their technique against.

Line 80: Are there problems with NO₂ photolysis at 405 nm, or extinction caused by the O₃? I’d like to assume not, but I couldn’t find references to either in the Fridrich et al. (2020) paper, so it would be good to have these confirmed.

Line 175: The technical details of the RH sensors must be specified, because the capacitance probes typically used for atmospheric work lose accuracy at lower humidities. Although it is noted that the RH thresholds referred to here are purely operational, so the absolute accuracy of the RH measurements probably isn’t important.

Line 283: What orientation was the t-piece in? If there was an asymmetric 10:1 split in flows (as implied) this could lead to an uneven transmission of particles over the two flows, as the centreline of a laminar flow could be exclusively following the major flow. If the minor flow was taken from the centreline using a subsampling tube (a common feature on AMS systems), this should be specified.

Line 298: A 10% error on the airbeam correction factor seems very high, especially for a discrete experiment such as this. How is this derived?

Line 307: Strictly, the LOD should be determined based on filtered air, not the ‘beam blocked’ signal, as there may be gas phase interferences. Although this is likely to be minor for m/z 30, but if the authors have this data they should use that instead.

Line 358: Not commented on is whether this instrument could be of use for more refractory nitrate species such as sodium or calcium nitrate, but given the desorption tube operates at a temperature higher than the boiling point of both, one might expect this to be the case. If it is able to, this would offer another potential benefit over the AMS, so should be mentioned.

Technical corrections:

Line 25: Technically the biggest source of reactive nitrogen to the atmosphere is NH₃. It would be truer to refer to combustion NO_x as the biggest source of “reactive oxides of nitrogen”. This terminology should also be used when referring to NO_z further down.

Line 35: The statement about sodium nitrate is overly simplistic. Technically the nitrate salt is formed with other marine cations such as potassium, calcium, magnesium, etc. Furthermore, calcium nitrate can also be formed from nitric acid reacting with calcium carbonate minerals in wind-blown dust.

Line 64: The AMS is capable of sub-second time resolution in 'fast' mode, which has been successfully used in eddy covariance and aircraft measurements, among other applications.

Line 282: Different models of aerodynamic lens are now available, so it should be specified that this is using the ‘Standard’ Aerodyne design. Recommend citing http://doi.org/10.1080/02786820701422278

Line 323: I wouldn’t refer to the shrinkage of ammonium nitrate as “thermal decomposition” because this implies an irreversible chemical transformation similar to the desorption tube. What’s more likely to happen is that the ammonia and nitric acid will be in dynamic equilibrium with the gas phase, but because the chamber walls are microscopically ‘flat’, the condensed phase will favour this surface over the particles in the chamber due to the Kelvin effect.