the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 29 Apr 2026
Reducing uncertainties in elemental carbon quantification using solvent-extraction–based mass balance and temperature adjustment in thermal–optical protocols
Abstract. This study constrains protocol-dependent uncertainty in elemental carbon (EC) quantification by thermal–optical analysis (TOA) using a novel solvent-extraction-based mass balance framework. To eliminate organic particulate matter interference, PM2.5 samples underwent sequential water and organic solvent extraction. A backup filter was strategically employed to account for EC redistribution during the extraction process, which was found to involve 37 ± 6 % of the total EC. The resulting solvent-extracted EC, corrected for redistribution, served as an operational reference largely independent of thermal charring artifacts. Comparative analysis revealed that EC determined by the IMPROVE protocol was consistently higher, whereas the default NIOSH protocol yielded systematically lower values than the reference. By reducing the maximum OC analysis temperature (OC4) in the NIOSH protocol to 650 °C, the EC values showed improved agreement with the reference (ratio = 1.08 ± 0.13). Furthermore, a logarithmic regression of the solvent-extracted EC to bulk EC ratio as a function of OC4 temperature identified a unity condition at 615 °C, defined here as the “KRISS temperature”. This framework provides a robust, reproducible basis for OC4 temperature selection and enhances inter-protocol comparability by explicitly constraining protocol-dependent uncertainties.
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Status: open (until 05 Jul 2026)
- RC1: 'Comment on egusphere-2026-1213', Anonymous Referee #1, 03 Jun 2026 reply
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RC2: 'Comment on egusphere-2026-1213', Anonymous Referee #2, 07 Jun 2026
reply
This manuscript presents a solvent-extraction-based mass balance framework to constrain protocol-dependent uncertainties in elemental carbon (EC) quantification by thermal–optical analysis and proposes an optimized OC4 temperature (“KRISS temperature”) for EC determination. The topic is relevant to the scope of Atmospheric Measurement Techniques (AMT), as uncertainties associated with thermal–optical EC measurements remain an important methodological issue.
The proposed backup-filter correction and solvent-extraction framework are potentially innovative and may provide a useful perspective for improving EC quantification. However, the central conclusions of the manuscript rely heavily on the assumption that solvent-extracted EC represents an independent and more accurate reference for atmospheric EC. In its current form, this assumption has not been sufficiently validated, which weakens the scientific basis for defining the proposed “KRISS temperature”. In addition, the experimental dataset is limited to a small number of ambient samples collected at a single site, making it difficult to assess the broader applicability of the proposed framework.
Overall, although the manuscript presents a potentially valuable methodological approach, several critical issues remain unresolved, including the insufficient validation of the key underlying assumption, the limited dataset, the lack of independent method comparison, and the absence of direct experimental validation. I therefore recommend Major Revision. The manuscript requires substantial additional evidence and clarification before it can be considered suitable for publication.
The major concerns are summarized as follows:
1，The study is based on only 14 PM2.5 samples collected at a single site over a relatively short sampling period. Given that the thermal behavior of EC is strongly influenced by aerosol sources and chemical composition, the current dataset is insufficient to demonstrate the broader applicability and robustness of the proposed framework across different atmospheric environments.
2，The entire framework relies on the assumption that the solvent-extracted and redistribution-corrected EC represents a more accurate reference EC. However, the manuscript does not provide independent evidence demonstrating that the residual carbon after extraction truly corresponds to atmospheric EC rather than an operationally modified carbon fraction. Therefore, the validity of the proposed reference EC requires further verification.
3，The proposed reference EC is evaluated solely within the thermal–optical framework. Comparison with independent black carbon (BC) measurement techniques, such as SP2, MAAP, or Aethalometer, would substantially strengthen the validity of the proposed reference EC and help determine whether the solvent-extracted EC represents a physically meaningful carbon fraction rather than simply another operationally defined EC metric.
4，The observation that approximately 37% of EC is redistributed to the backup filter is one of the most important findings of this study. However, the physical and chemical characteristics of the redistributed carbon remain unclear, and no additional characterization is provided. Further evidence is needed to determine whether this material is truly EC or other carbonaceous species affected by the extraction process.
5，The proposed KRISS temperature (615 °C) is derived solely from regression analysis rather than direct experimental measurements. Given that this temperature constitutes one of the main conclusions of the study, direct validation at or near 615 °C is necessary to demonstrate its practical applicability and robustness.
6，The proposed optimal temperature (615 °C) is relatively close to the OC4 temperature employed in the EUSAAR2 protocol (~650 °C). The manuscript does not adequately demonstrate whether this difference is analytically significant or whether it provides a meaningful improvement over existing protocols. A more comprehensive comparison with EUSAAR2 and previous protocol optimization studies is therefore recommended.
Citation: https://doi.org/10.5194/egusphere-2026-1213-RC2
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- 1
General comments
The manuscript presents a solvent extraction method to constrain protocol-dependent uncertainties in EC measurements by thermal–optical analysis and to derive an operational “KRISS temperature” for OC4 in NIOSH-like protocols. The topic is suitable for AMT, since differences between OC/EC protocols remain a major source of uncertainty. The approach is interesting, but the current version of the manuscript has several serious weaknesses in methodology, documentation, and interpretation. In its present form, it does not yet support the strength of the claimed conclusions.
The dataset is small and narrow in scope. Fourteen PM2.5 samples collected at a single suburban site over a two-week period are used to derive an “optimized” OC4 temperature. With such limited temporal and spatial coverage, a relatively uniform aerosol composition is likely. This strongly restricts the applicability of the proposed KRISS temperature and the method to other environments. At the same time, key information on the instrument type, optical correction mode, temperature protocol (including durations), filter homogeneity, and uncertainty treatment is missing or is inconsistent. These are major issues that need to be addressed.
Overall, the work has potential, but substantial revision is required. In particular, the instrumentation and protocol description must be clarified, the uncertainty analysis strengthened, and the claims on universality and inter-protocol comparability significantly toned down and aligned with the actual evidence.
Specific comments
The study uses PM2.5 samples from one suburban site in Daejeon, Korea, collected over two weeks (31 March–13 April 2014). With such limited coverage, a rather homogeneous aerosol composition can be expected. This limitation is not discussed adequately. The current text suggests a broad relevance of the derived OC4 temperature and the KRISS temperature. This is not justified.
The authors should clearly state that the proposed temperatures are operational and specific to the aerosol type and conditions covered here. Claims that the framework provides a “robust basis” or “enhances inter-protocol comparability” must be softened and strictly linked to this dataset only. Without additional evidence from other environments and laboratories, the manuscript cannot argue for a universal or widely applicable OC4 setting.
Line 71 describes a “thermal–optical carbon analyzer (Sunset Laboratory Inc., Model RT-3140)”. “RT” typically refers to a real-time instrument. The method described in the manuscript, however, is clearly an off-line analysis of 1.5 cm² punches with sequential extractions. This inconsistency is critical, especially as most cited studies use laboratory analyzers.
The authors must clearly identify the exact instrument model and configuration (real-time vs laboratory unit). If a laboratory analyzer was used, the “RT-3140” wording is misleading and should be corrected. Any non-standard configuration should be described in sufficient detail to allow reproduction and fair comparison with the literature.
The manuscript states that thermal–optical transmittance (TOT) was used for charring correction with both IMPROVEA and NIOSH protocols. Most of the key IMPROVE-related studies cited here use reflectance (TOR) as the default optical correction. Transmittance and reflectance can yield significantly different OC/EC splits, even with the same temperature protocol. This issue is central and is not treated properly in the current version.
The authors need to:
- Explicitly confirm that IMPROVEA was implemented with TOT and not TOR.
- State clearly that, as a result, the measurements are not strictly comparable to earlier IMPROVEA/TOR results cited in the manuscript.
- Revise the discussion and conclusions wherever direct comparability with IMPROVEA/TOR literature is implied.
- Add a concise discussion on how the use of TOT, rather than TOR, may have influenced the observed protocol differences.
Table 1 lists only the temperature steps. For IMPROVEA, the OC4 step duration is modular and controlled by the optical signal returning to baseline, whereas NIOSH and EUSAAR2 apply fixed step durations. The manuscript focuses on changes in OC4 temperature, but completely omits the corresponding durations. This is a major omission because incomplete OC4 evolution due to short durations can strongly bias the results.
The authors should:
- Report the OC4 step duration for each protocol and each modified OC4 temperature.
- Clarify whether the duration was kept constant when the OC4 temperature was adjusted.
- Discuss explicitly how the chosen durations may have affected OC4 peak development and the differences observed between NIOSH variants.
Laser stability, optical signal behavior, and split point determination are not adequately documented. These aspects are crucial for OC/EC separation and for the KRISS-temperature concept. The manuscript must include:
- A short description of laser stability checks and criteria for rejecting thermograms (e.g. signal drift, noise, shifts).
- Information on oven cleanliness and any temperature-offset calibration performed. Earlier studies have shown that soiled ovens can lead to pre-oxidation and bias in split point determination. Given the focus on an “optimized” OC4, this information is essential.
The primary samples are large rectangular high-volume filters (20 × 25 cm). Only a single 1.5 cm² punch is analyzed per filter. With such a large area, non-uniform loading is a realistic and important concern, especially after sequential solvent extraction and handling. Yet, the manuscript does not state whether replicate punches were taken. This is a serious gap in the uncertainty analysis.
The authors should:
- State clearly whether duplicate or triplicate punches were analyzed for each filter.
- If replicates were analyzed, provide a brief summary of intra-filter variability and explain how it was propagated into the uncertainty budget.
- If no replicates were analyzed, discuss explicitly the likely magnitude of inhomogeneity and how this unaccounted variability affects the reliability of the results and conclusions.
For the backup filters, the procedure is not clearly described. It is not evident whether the whole backup filter was analyzed or only a portion of it. Since the backup filter plays a key role in the EC mass balance (with a large fraction of EC being recovered on it), this must be clarified in detail.
Please:
- Specify whether the entire backup filter was analyzed or a sub-area.
- If the entire filter was used, describe the analysis method and any checks for homogeneity.
- If only a portion was analyzed, explain the sampling strategy and the assumption of uniform loading, and discuss how this assumption influences the calculated “operational reference EC”.
The manuscript reports that, on average, 37.4 ± 6.4 % of EC is recovered on the backup filter after extraction. This is a substantial fraction, introducing additional uncertainty. The authors should assess whether this fraction is stable across samples or shows systematic variation with load, OC/EC ratio, or other indicators. This variability needs to be incorporated transparently into the uncertainty estimates.
Lines 114–119 refer to brown carbon and charred OC in the context of solvent extraction, drawing on studies that often use water-only extraction or different pretreatments. In this work, both water and organic solvents are used, and the filters are sourced from a specific suburban environment. The connection to the cited mechanisms is not always convincing, and the current wording is confusing.
This part should be rewritten to:
- Clarify whether significant brown carbon or charred OC is expected for the sampling period and site considered here.
- Clearly separate water-soluble brown carbon, solvent-extractable OC, and mechanically redistributed carbonaceous material collected on the backup filter.
- Avoid general statements that implicitly assume that mechanisms from water-only extraction studies apply unchanged to a more aggressive, multi-solvent protocol.
The thermograms in Fig. 3 show a clear shift in EC evolution to higher temperatures after extraction, for both the water-only and combined-solvent steps. The current explanation, which attributes changes mainly to the removal of thermally stable OC, does not fully capture this behavior. The data suggest that the extraction modifies the pool of EC-like material and its thermal stability, not only removes “stable OC”. The interpretation of these thermograms should be revised accordingly.
In Fig. 3, the OC peaks for C3 are very small. In contrast, Table 2 reports notable differences in OC4 concentration between NIOSH variants, especially at 650 °C. This is difficult to reconcile with the low OC4 signals seen in the example thermogram.
The authors need to clarify:
- Whether Fig. 3 describes a representative sample, and if all samples show similar thermograms.
- If not, why was this particular thermogram chosen, and how representative is it of the dataset?
- How can such small OC4 peaks produce the reported concentration differences in a statistically robust way?
The EC concentration range reported in Table 2 is relatively low, while the associated uncertainties are comparatively large. Several of the apparent differences between NIOSH variants are small relative to these uncertainties. It is not evident which differences, if any, are statistically significant. The current discussion tends to overinterpret trends that may be within the bounds of uncertainty.
The regression-based KRISS temperature of 615 °C is derived exclusively from this limited dataset. This must be made explicit. It is an operational parameter tuned to this specific aerosol type and to the particular implementation of the NIOSH protocol and optical correction used here. Without additional validation on other aerosol types and in other laboratories, the value cannot be promoted as generally applicable.
The manuscript states that lowering OC4 and introducing the KRISS temperature improves inter-protocol comparability and supports better interlaboratory reproducibility. In practice, changing OC4 and using a different optical correction mode defines a new protocol. This has serious implications for continuity with existing IMPROVEA and NIOSH datasets. Please clarify whether the KRISS temperature, in its current form, would effectively create a modified/new temperature protocol.
The recent interlaboratory work by Sipkens et al. (2024) is briefly mentioned but not used to place the uncertainties in context. That study reports reproducibility-related uncertainties on the order of 10–20 % for EC, OC, and TC. The manuscript needs to demonstrate that the additional method complexity yields a net benefit and does not merely shift or inflate uncertainties.
Recommendation
Given the issues outlined above, major revisions are necessary. The manuscript requires clearer, more complete documentation of the instrument and protocols, a more rigorous treatment of filter homogeneity and uncertainties, and refocusing of the conclusions to what the data can actually support.