the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 24 Feb 2025
Characterization of filter photometer artefacts in soot and dust measurements – laboratory and ambient experiments using a traceably-calibrated aerosol absorption reference
Abstract. A novel reference absorption instrument, based on photothermal interferometry – the PTAAM-2λ, and scattering measurements are used to characterize filter photometer artefacts in measurements of absorption coefficients of soot and dust-dominated aerosol samples within laboratory and ambient campaigns.
The Aethalometer AE33 and the Continuous Light Absorption Photometer (CLAP) were characterized during a laboratory campaign where different soot-like and mineral dust samples were measured. Furthermore, ambient measurements during a campaign in Granada, Spain, were used to characterize the AE33 and MAAP (Multi Angle Absorption Photometer), a pseudo-reference absorption instrument.
The laboratory campaign showed significant wavelength dependence of the multiple-scattering parameter C. The C of AE33 at 450/808 was 4.08/3.95 and 6.25/5.27 for propane soot and diesel soot, respectively. For the CLAP the C was 5.10/4.26 and 6.79/5.80 for propane and diesel soot, respectively. For mineral dust, C at 450 nm ranged between 2.74 and 3.03 for the AE33 and between 2.50 and 2.80 for the CLAP. The ambient measurements showed an overall C of 4.72 at 450 nm and of 3.90 at 808 nm for the AE33. The results for both the AE33 and the CLAP show a dependence with the particle size, with fine particles having the highest C values, and as the aerosols become larger C levels off. Both the laboratory and the ambient measurements of the AE33 showed overlapping results.
The cross-sensitivity to scattering was smaller for the CLAP than for the AE33. The values of the cross-sensitivity parameter ms at 450/808 nm were 3.0 %/1.5 % for the AE33 and 2.4/0.9 % for the CLAP at 450/808 nm.
The intercomparison of the MAAP with the PTAAM-2λ during the ambient campaign in Granada showed that the MAAP-derived absorption coefficients feature a 47 % overestimation at 637 nm and a cross-sensitivity to scattering of 2.4 %.
Competing interests: Luka Drinovec and Griša Močnik are employed by Haze Instruments d.o.o., the manufacturer of PTAAM-2l. Other authors declare no potential conflict of interest
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this preprint. The responsibility to include appropriate place names lies with the authors.- Preprint
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Status: open (until 03 Apr 2025)
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RC1: 'Comment on egusphere-2024-3995', Anonymous Referee #1, 10 Mar 2025
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This study analyzes the multiple-scattering compensation parameter (C) for AE33 and CLAP photometers and characterizes the pseudo-reference MAAP. It emphasizes the importance of correcting scattering artifacts and knowing particle sizes to accurately compensate aerosol absorption coefficients. The accuracy of PTAM-2 λ, MAAP, and AE33 is decreasing, so it is necessary to correct the key parameters discussed in the article. But the readability of the article needs to be improved, especially the abstract and introduction, which need to be able to be quickly understood by the users of the instrument. Some details are as follows:
- Abstract: The abstract is unclear about the purpose, methods, and key results. You can add content similar to Lines 40-55 to the abstract to explain why you conducted this study.
- Introduction: The introduction is not clear for readers. How do you measure the absorption of soot and dust? What are the problems with current measurement methods? PTAAM-2λ provides reference data. I think these are your key points, but the information is spread across too many paragraphs and is not direct or clear for readers.
- For example, you can use clearer and simpler sentences to introduce that you will develop a novel method to measure dust using AE33, a measurement tool designed for black carbon (BC) (Lines 17-50).
- Lines 211: Provide more details about Equation (9) in Yus-Díez et al. (2021).
- Lines 181-182: Add sentences to emphasize that PTAAM-2λ provides reference data.
- Lines 272-274: This is a powerful conclusion, but how did you obtain it? Do you mean that the change is low, as shown in Table 3?
- Lines 317-319: This is a key point that should be included in the Introduction.
Citation: https://doi.org/10.5194/egusphere-2024-3995-RC1 -
RC2: 'Comment on egusphere-2024-3995', Anonymous Referee #2, 20 Mar 2025
reply
The present paper provides analysis of the artefacts in filter photometers widely used in field and laboratory studies. The paper refers to field observations at Granada and a laboratory setup, and uses different filter based instruments (AE33, CLAP, MAAP) and the PTAAM, based on photothermal interferometry, complemented by nephelometry and size distribution measurements. The analysis evidences a wavelength dependence of the multiple scattering correction factor (C) of the AE33 and the CLAP as well as a size dependence for C. The analysis also evidences a significant difference between the aerosol absorption coefficient measured with the PTAAM compared to the one of the MAAP, suggesting potential implications for the evaluation of the eBC (equivalent black carbon) from air quality measurements which are calibrated using MAAP as a reference technique. The paper is surely of relevance for AMT and provide an interesting investigation. Nonetheless some aspects deserve improvements and I suggest major revisions.
General comments
- The manuscript, in particular the introduction, is not providing all the necessary elements so that a non-fully expert reader can understand the problem and the chosen approach, as well the discussion of the results. I would encourage the authors to develop more in detail the different artefacts associated to filter-based measurements and the choice of the reference techniques.
- Tables of results and plots in the main manuscript are generally missing consideration of uncertainties or statistical variability and this would be required for supporting data presentation, discussion and results
- The discussion on the discrepancies between the PTAAM and the MAAP is interesting and of potential relevance for the community. The authors identify as the limitation of the MAAP the reduced angular measurements, and the authors mention other more angular resolved instruments – such as for instance the PPUniMI – developed to overcome these issues. Does any other studies have provided comparison against those other instruments and how these investigations, if they exist, provide support to your analysis?
- The text uses a lot of acronyms and the reading can result quite hard in some points also because many numbers (C for several instruments, aerosol types, wavelengths) are provided in the text. Please consider to introduce an acronym table and to simplify the text when possible to facilitate reading.
Specific comments:
- Line 1: define PTAAM at first use
- Lines 8-14: are the differences identified for C relevant within uncertainties
- Line 51: the C factor is mentioned but not introduced / explained
- Line 52: this is relevant for all absorbing species, not only dust
- Lines 67-68: it is not clear why the EMS method is more relevant for laboratory than field measurements, please explain
- Lines 74-75: the TAP and PPUNIMI should be spelled out
- Line 75: the MAAP was already introduced in line 39
- Line 82: the nephelometer is mentioned without any indication of which quantity is measured with this instrument
- Line 95: please recall how the soot are produced (combustion conditions) and their main properties (chemical, physical)
- Line 104: if a reference exists for the dust generation device, please cite it (manufacturer)
- Lines 110-123: as volumetric quantities, if the absorption coefficient measurements are all reported to the same temperature and pressure (i.e. Standard Temperature and Pressure) or not should be mentioned. If not, is the difference in T and p reference affecting the comparison and to what extent?
- Line 133: size instrumentation is introduced but some more details on the type of measured diameter and potential corrections applied to the data should be mentioned
- Line 137: if the Teri (2022) correction scheme requires information on the complex refractive index of the aerosols to be applied, the hypothesis used should be explained
- Line 158: define FLE
- Line 160: please be more precise in indicating the MAAP wavelengths
- Lines 167-169: not clear what is the “compensation scheme” used – lease give more details
- Lines 178-180: please provide more explanation as I am not sure this could be clear for non-fully expert readers
- Section 3.1 misses discussion about laboratory soot experiment
- Fig 2 caption: “random sample” to replace with more specific sample identification
- Fugure 3: the axes scales could be reduced to better visualize the data
- Line 254: as it is absorption, then scattering is also used for EMS calculation with CAPS, to be mentioned
- Lines 256-257: the fact that the C found in the present analysis using PTAAM data is lower than previous studies using MAAP observations as reference absorption is not in contradiction with the discussion in sect. 3.3?
- Lines 265-269: would add comparison with soot data from Weingartner et 2003
- Lines 279-281: is this because urban air masses are dominant?
- Sect 3.2: the comparison with the literature indicate both higher and lower values when referring to different aerosol types and photometer models. The conclusion of the comparison is not clear then, and would suggest to develop this point so that a clear message is delivered.
- Line 299: how much the differences in diameter definition can influence the results and their interpretation?
- Figure 6, panel b: units are missing
- Lines 364-369: would be good to have a reference for the results in relation to the stanBC campaign
Weingartner, E., Saathof, H., Schnaiter, M., Streit, N., Bitnar, B., and Baltensperger, U.: Absorption of light by soot particles: Determination of the absorption coefficient by means of Aethalometers, J. Aerosol Sci., 34, 1445–1463, 2003.
Citation: https://doi.org/10.5194/egusphere-2024-3995-RC2
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