the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 09 Jul 2026
Opinion: From Conversion Factors to Diagnostic Signals: Interpreting Response Ratios among eBC, EC, and rBC
Abstract. Equivalent black carbon (eBC), elemental carbon (EC), and refractory black carbon (rBC) are widely recognized as distinct operationally defined metrics related to black carbon (BC). Building on the terminology framework of Petzold et al. (2013), this Opinion focuses on a practical question: how should empirical relationships among these metrics be represented and used? Conversion factors are often used for cross-metric harmonization, but their variability may also contain information on aerosol state and method response. We propose treating ratios such as eBC/rBC, eBC/EC, and EC/rBC as response ratios, namely derived diagnostic observables that can serve as empirical conversion factors when averaged for harmonization, but as indicators of changes in aerosol state and method response when retained as time-varying quantities. This dual role implies a practical equivalence trilemma: method specificity, state-independent numerical equivalence, and aerosol-state sensitivity cannot all be fully retained when cross-metric relationships are compressed into fixed conversion factors. The trilemma makes explicit a trade-off that the BC measurement community already navigates. It concerns cross-metric harmonization rather than within-method standardization or metrological traceability. We recommend transparent reporting of primary observables, conversion assumptions, and response-ratio time series in multi-method datasets. Retaining response-ratio variability alongside harmonized products would allow multi-method BC datasets to support both comparability and aerosol-state interpretation.
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Status: open (until 20 Aug 2026)
- RC1: 'Comment on egusphere-2026-3596', Anonymous Referee #1, 22 Jul 2026 reply
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RC2: 'Comment on egusphere-2026-3596', Anonymous Referee #2, 03 Aug 2026
reply
This comment constitutes an extension of the seminal paper by Petzold et al. (2013), regarding the definitions of black carbon. While this does not introduce any new concepts, this is a timely piece, as there is much discussion within the metrological community currently regarding the standardisation of BC measurement as part of measurement networks such as GAW and ACTRIS, the implementation of the 2021 WHO recommendations, and international frameworks such as the Paris agreement, Gothenburg Protocol and 2024 EU Air Quality Directive. In a number of forums, the disagreements between the different measurement techniques are effectively treated as measurement artefacts to be calibrated out, rather than measurements of different intrinsic properties of the substance.
This work argues that rather than striving for exemplar conversions between the different quantities such as EC, eBC and rBC, we should instead be striving to treat variations in these conversions as sources of information in their own right. Because this has implications for how "black carbon" is treated in models and inventories in addition to observations, I would say that while this conceptual paper is based around measurements, the implications extend beyond it, so it should be considered in-scope on that basis. The most common example of this currently in use is the MAC, that the author makes extensive reference to and while variations in this quantity are something that people modelling the fundamental physics are accustomed to, I have often observed a tendency to treat this more simply in the applied aspects of atmospheric science, so this bears discussing here.
Perhaps my biggest issue with the core thesis here is that for as much as I agree that these conversion factors do tell one about the fundamental properties of the black carbon, as stated in the article, there is no avoiding the fact that they also account for instrument issues as well. So for instance, when performing filter-based measurements of eBC, the Babs parameter derived can be subject to scattering artefacts that are dependent not just on the physical instrument but also other non-absorbing aerosol that are present. Another one is the widely-documented issues that can throw off the split point during thermal-optical analysis, which are again dependent on both the method and the aerosol under investigation. These are dealt with in the article, but as such, they detract from the notion of the differing conversion factors being useful in anything other than a technical context. If they are indeed mainly technical artefacts, then this would place the article outside of the scope of ACP and more the realms of AMT.In this spirit of this being a comment piece, I think the argument could be made far more strongly if better examples could be given for this approach demonstrating utility beyond simply diagnosing the technicalities of the measurement methods. One thing that may help to achieve this is presenting more quantitative data, as suggested by the other reviewer.
On a more minor note, I would say that much more attention needs to be paid to the definition of 'rBC' here because there are different ways of measuring this using incandescence. For the bulk Laser Induced Incandescence (LII) instruments, the calibration tends to relate to the mass-specific absorption cross section of the rBC, whereas with the Single Particle Soot Photometer (SP2), its is the boiling point. Currently in table 1, this is covered by the catch-all 'calibration' but this isn't particularly useful within the context of the paper.Citation: https://doi.org/10.5194/egusphere-2026-3596-RC2 -
RC3: 'Comment on egusphere-2026-3596', Anonymous Referee #3, 10 Aug 2026
reply
This manuscript presents an important and timely perspective on black carbon measurements. The key argument that eBC, EC, and rBC are method-dependent responses to an underlying aerosol state rather than interchangeable measurements of a single, directly measurable “BC” quantity is certainly valid. The observation-operator framework provides a way to further formalize this idea. In particular, the manuscript correctly highlights that simultaneous eBC, EC, and rBC observations provide more information than any single “BC-proxy” alone.
However, the conceptual framework would benefit from a clearer separation between the underlying state variables (S), which are not directly measured; the selected method parameters, such as wavelength, assumed MAC, thermal protocol, split point, and calibration factors/materials; and the reported or measured observables/responses (i.e. eBC, EC, and rBC). The manuscript could also add to each reported response their corresponding measurement uncertainties. Importantly, these uncertainties should refer to the actual measurement process—such as instrument calibration, sample flow, detection efficiency, operating conditions, data corrections, and repeatability—clearly separated from the selected method parameters or the underlying state variables and their unknown variability. This distinction should also be made more explicit in the equations and especially, in the discussion of Equation (9). As indicated, Equation (9) is an illustrative decomposition rather than a complete measurement model and both the effective MAC and the non-BC absorption contribution are state-dependent (and, in practice, wavelength-dependent) quantities, while the operational identification (BC mass equals rBC mass) neglects the uncertainty associated with the SP2 response.
Section 4 could also connect the described framework more directly to practical applications for different purposes, explain when different BC metrics are fit for purpose (currently briefly discussed in 4.3-4.4), discuss the complementary role of standardization (compare e.g. line 88 “Within-method standardization improves consistency, traceability, and comparability”), and provide true examples of available multi-method datasets (current section 4.2). The demand for fit-for-purpose measurements, within-method standardization, and multi-method supersites with additional measured quantities are only briefly discussed and could be elaborated further as they are in practice the drivers of several inconveniencies which are discussed in the paper. Overall, I find the Opinion manuscript a valuable and timely addition to the field, but the key messages and applicability remain somewhat vague and lack clarity to be truly adapted for the use of the scientific community.
Citation: https://doi.org/10.5194/egusphere-2026-3596-RC3
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This article highlights the inevitable uncertainties arising from the diversity of observational methods and the varying definitions of black carbon (BC) based on different physical parameters, while also offering preliminary recommendations. This perspective is constructive and valuable. However, the paper lacks a systematic, quantitative comparison of existing methods based on a comprehensive review of the literature. Moreover, it would benefit from more detailed suggestions on solving current problems rather than solely identifying them.
Specifically, the following aspects could be elaborated:
This article is generally constructive, but lack of quantitative comparison between different methodologies for BC measurements by really analysing on the existing literatures.
It has been separated too many subtitles.
It may benefit from giving more practical advice on how to deal with the exiting data, and what we can get from them.