the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 09 Jan 2025
Development and validation of a NOx+ ratio method for the quantitative separation of inorganic and organic nitrate aerosol using CV-UMR-ToF-ACSM
Abstract. Particulate nitrate is a major component of ambient aerosol around the world, present in inorganic form mainly as ammonium nitrate, and also as organic nitrate. It is of increasing importance to monitor ambient particulate nitrate, a reservoir of urban nitrogen oxides that can be transported downwind and harm ecosystems. The unit-mass-resolution time-of-flight aerosol chemical speciation monitoring equipped with capture vaporizer (CV-UMR-ToF-ACSM) is designed to quantitatively monitor ambient PM2.5 composition. In this paper, we describe a method for separating the organic and ammonium nitrate components measured by CV-UMR-ToF-ACSM based on evaluating the NO2+/NO+ ratio (NOx+ ratio). This method includes modifying the ACSM fragmentation table, time averaging, and data filtering. By using the measured NOx+ ratio of NH4NO3 and a plausible range of NOx+ ratio for organic nitrate aerosol, the measured particulate nitrate can be split into inorganic and organic fractions. Time averaging and data filtering results in a concentration limit of 0.6 μg m-3 total particulate nitrate, above which this method could be used. We show that this method is able to distinguish periods with inorganic or organic nitrate as major components at a rural site in the Netherlands. A comparison to a high-resolution time-of-flight aerosol mass spectrometer equipped with a standard vaporizer (SV-HR-ToF-AMS) shows a good correlation of particulate organic nitrate fraction between the instruments (CV/SV = 1.59; r2 = 0.92). We propose that researchers use this NOx+ ratio method for CV-UMR-ToF-ACSM to quantify the particulate organic nitrate fraction at existing monitoring sites in order to improve understanding of nitrate formation and speciation.
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RC1: 'Comment on amt-2024-191', Anonymous Referee #1, 31 Jan 2025
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Nursanto et al. present methodological development to separate ammonium nitrate and organic nitrate signal in the time-of-flight aerosol chemical speciation monitor (TOF-ACSM) with capture vaporizer (CV). This has been a challenge due to the unit mass resolution (UMR) of the instrument, limiting the ability to separate different ions at the same nominal m/z as the NO (30) and NO2 (46) signal. A further challenge is the inclusion of a CV, which is used to improve quantification of aerosol concentration but also induces more thermal fragmentation of the ions, leading to most of the nitrate signal occurring at m/z 30 (NO), limiting the ability to use the methods previously published about using the calibrated NO to NO2 ratio from ammonium nitrate and the derived average ratio of pure organic nitrate aerosol, a.k.a. the "ratio-of-ratio" (RoR) method .
Using data previously collected from different aerosol mass spectrometers (AMS) and ACSMs with CV, the authors first investigated improving the fragmentation table, a tool used to separate ions at the same nominal m/z to differentiate the signal. As discussed in prior publications, a revised fragmentation table was necessary for the CV TOF-ACSM that they apply for the paper and recommend for future users. Next, they investigate the limits of quantification of the CV TOF-ACSM due to the low signal of NO2, and what nominal RoR to utilize for the TOF-ACSM (which is different than what is used for an AMS with standard vaporizer). After determining the limits of quantification and error propagation, the authors provide initial results from measurements conducted at an long-term monitoring site and from a chamber experiment.
This paper is of use for the TOF-ACSM community, as there are many TOF-ACSM with CV collecting long-term measurements. As emissions change (and thus aerosol chemistry), being able to differentiate ammonium nitrate from organic nitrates is of great value, as these two different NOx reservoirs have different properties for the aerosol, and provide insight into the chemistry controlling the pollution. After the authors address the following comments, the paper fits into AMT.
1) There is concern about frag_org[46] vs frag_org[45], as the R^2 is very weak. What is the general fractional contribution of frag_org[46] and frag_org[45] to the total signal (e.g., does it need to be corrected if this signal is low, especially in regards to NO2)? Further, the correction of frag_org[46], as the authors conduct throughout the paper, is dependent on the aerosol being observed. As ambient aerosol is difficult to a priori know what is the origin, how much further uncertainty is introduced into this correction. E.g., looking at Fig. 1d, for less oxidized organic aerosol (LO-OOA) and aerosol influenced by isoprene and a-pinene, which would all be scenarios expected to generally have high contribution of signal towards organic nitrate aerosol instead of ammonium nitrate, it appears the correction over corrects the signal at m/z 46. Wouldn't this then lead to a too low contribution of signal to NO2 and thus under reporting organic nitrates?
2) Section 4.2: It is not clear why geometric mean was used to derive the ratio of pure organic nitrate (RpON). Not being a statistician, I do not understand the full reasoning behind using geometric mean, and why it makes more sense than arithmetic mean. If the authors could provide more details and references why geometric mean between two extreme values was used would strengthen the selection and section.
3) Section 6.1: Co-located measurements of pON is extremely challenging and rarely possible, which is understood. If there was anyway to have a co-located measurement, from a chamber study or somewhere else where there was another ACSM with CV and another pON, would strengthen this section. Currently, the results shown in Fig. 5 and 6 are hard to judge if the trends and mass concentrations make sense.
4) Section 6.2: This section is not very convincing in that the TOF-ACSM CV is sensitive towards pON. Combination that the fraction of pON reported by AMS and ACSM diverge, indicating that a single correction value for the fragmentation table may not be applicable, and that the scatter plot (Fig. 8) is really driven by two points (e.g., the values before limonene was injected, which is ~0, and the values after limonene was injected, which could be averaged into one point). Thus, the analysis from this one chamber experiment is suggestive that the CV TOF-ACSM may not be able to quantify pON and would potentially over attribute nitrate signal to pON instead of ammonium nitrate (by ~50-60%). Further analysis of this one experiment, or analysis of another chamber experiment with different chemistry, if possible, is needed to better understand the uncertainty and whether is is a precursor dependency and/or uncertainty with a constant fragmentation table correction.
Citation: https://doi.org/10.5194/amt-2024-191-RC1 -
RC2: 'Comment on amt-2024-191', Anonymous Referee #3, 17 Feb 2025
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Please find my comments in the attached pdf file.
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