| 29 Jan 2026
Particle nitrate measurement using a thermal-dissociation, cavity-ringdown-spectrometer with gas-phase denuder
Abstract. Ambient inorganic and organic particulate nitrate has been connected to cardiovascular and respiratory illness and accurate measurement of its concentration is essential for our understanding of its impact on human health and the partitioning of reactive nitrogen between the gas- and particle-phases. We report modifications to an existing Denuded-Thermal-Dissociation-Cavity-Ring-Down Spectrometer system (D-TD-CRDS) system that reliably measured gas-phase NOX and NOy but suffered from a positive bias in particle nitrate measurement owing to denuder breakthrough and memory effects associated with changes in relative humidity. We describe an air drying system with low particle transmission losses that reduces the relative humidity at the inlet of the denuder to < 5 % so that no measurable denuder breakthrough of NOY (even of highly volatile species such as NO) was observed after continuous use over the course of month-long campaigns. The D-TD-CRDS measurement of particulate nitrate has a limit of detection (1 minute) of ~ 0.035 mg m-3 under laboratory conditions and ~ 0.085 µg m-3 during field deployment. The associated uncertainty is estimated to be < 15 %. Laboratory experiments in which either inorganic nitrate aerosol (ammonium nitrate) or organic nitrate aerosol (generated from the NO3-induced oxidation of limonene) were sampled simultaneously from an environmental chamber by the D-TD-CRDS and with an Aerosol Mass Spectrometer (AMS) showed excellent agreement.
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