the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 24 Mar 2025
Continuous chemical characterization of ultrafine particulate matter (PM0.1)
Abstract. Ultrafine particles (diameter less than 100 nm) are primary suspects for enhanced negative health effects on humans. Measuring the chemical composition and physical properties of ultrafine particles on-line, continuously, and accurately is particularly challenging because of their typically low mass concentration (PM0.1) and susceptibility to interference from larger particles. The few past PM0.1 chemical composition measurement studies have used cascade impactors and at least daily temporal resolution. In this study we perform for the first time high temporal measurements of the composition and sources of PM0.1 using an aerodynamic aerosol classifier (AAC) to separate PM0.1 from larger particles. A high-resolution time of flight aerosol mass spectrometer (HR-ToF-AMS, for sulfate, nitrate, organics, chloride), a single particle soot photometer (SP2-XR, for black carbon) and an Xact625i (for elements) are also used.
Ambient PM0.1 composition measurements were conducted in a suburban area in Greece to test the system. The hourly PM0.1 levels varied from 0.4 to 1.5 μg m-3, with an average of 0.7 μg m-3. Most of the PM0.1 (45 %) was organic aerosol (OA). On average, sulfates contributed 14 %, ammonium 7 %, nitrate 3 %, and black carbon 4 % to PM0.1. Calcium (Ca) showed a surprising high average contribution to PM0.1 (18 %). The rest of the detected elements were Fe, K, Zn and Ti, contributing together 7 %. Source apportionment analysis showed that most of the PM0.1 OA, during this summertime period, was oxygenated OA (90 %), with 70 % being less oxidized and 20 % being more oxidized, while only 10 % was fresh hydrocarbon-like OA.
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Status: open (until 18 May 2025)
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RC1: 'Comment on egusphere-2025-1147', Anonymous Referee #1, 12 Apr 2025
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The work faces the challenge of measuring the chemical composition and physical properties of ultrafine particles on-line and continuously. The authors propose an original approach for selecting PM0.1 using an aerodynamic aerosol classifier (AAC) sending then the size-resolved sample to a HR-ToF-AMS, an SP2-XR and a Xact625i to achieve a full chemical composition characterisation. The topic is of broad interest for the scientific community and the research networks.
The paper is clear, the description of the methodologies is fine, and only a few comments are reported below to improve it.
The Reviewer suggests to accept the paper after minor revisions.
Suggestions:
- Line 257: please add a more detailed description of what do you mean with “positive/negative” artefacts (i.e., on which elements? Why?) and how much these artefacts affect X-ray fluorescence analysis. Indeed, the authors mention “These components included the tubing, the PM2.5 cyclone, the AAC, and one MFC.” (line 264-264) but it is not specified how each of these parts are impacting on positive/negative artefact and which are the elements gained/lost.
- As for the LOD: please add a table with the calculated values for all detected elements. From e.g., Figure S8 in most of the cases the concentrations were below or next to the LOD. The authors should discuss further this limitation of the Xact instrument when aiming at the on-line quantification of the elemental composition in the ultrafine PM fraction. Of course, some results could be also affected by the element’s size distribution so that – being the first application of the Xact to ultrafine particle samples – it is desirable to have information from parallel measurements of the elements size distribution (see e.g., https://doi.org/10.1016/j.aca.2021.339367) to better characterise its performance in the ultrafine size range.
- Line 299-301: the authors report that “When measured concentrations are close to the LODs the lighter elements Si, S, Cl, K, Ca are more susceptible to self-absorption effects” which should be better explained as it sounds quite strange in X-ray spectrometry. Indeed, the self-absorption effects do not strictly depend on the light element concentration closeness to LOD while they are known to depend on the matrix, on the energy of the fluorescence X-ray emitted, and potentially on the position of the light element inside the sample thickness if the it is not a thin sample.
- Line 314-315: while the S concentration comparison is fairly good, the assessment of Cl by Xact is almost half of the value given by AMS. This result should be underlined and better explained.
- Figure 4, panel (c): AMS data time-series with this y-scale are almost not visible for most elements, please modify it in order to see the patterns for all components. These AMS data appear very much “smoothed” due to the 4-h average representation? It would be useful to show the AMS data time-series on hogher time resolution to compare their pattern with also rBC. Please add a comment on the time-series variability and the possibility of identifying episodic emissions.
- Line 370-371: the authors summed up Ca, Fe, K, Zn, and Ti as elemental concentrations to the refractory components and to rBC. The Referee wonders about the chemical form of these elements in the ultrafine PM fraction : are they present as pure metals or oxides or others? Are there literature data to confirm that the pure elemental form is the right one in this size fraction? What are the sources?
- Line 438-441: the terminology used here is confounding. Indeed, the bilinear PMF model can be solved by different algorithms contained in both the PMF2 and ME-2 programs. The authors should specify that they used the algorithm of the PMF2 program for the unconstrained run and they should use the terminology “SoFi” when they are referring to the constrained run with fixed profiles using the algorithm contained in the ME-2 program. Indeed, also EPA-PMF v.5 uses ME-2 as a program to solve the PMF bilinear model but not necessarily the profiles must be constrained to obtain a reliable solution.
- Why the source apportionment was not performed including rBC and the elements?
- Conclusions: more comments on the limitations of the Xact performance in this setup should be added. What about possible improvements?
Citation: https://doi.org/10.5194/egusphere-2025-1147-RC1 -
EC1: 'Editor comment on egusphere-2025-1147: Review received', Hartmut Herrmann, 14 Apr 2025
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We have received a regular reviewer comment, however the reviewer was not able to upload it , so I am doing this
as the handling editor.
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