Mixing state of Carbonaceous Aerosol Emissions from an Ecodesign Woodstove

Cheng, Zixuan; Kılıç, Doğuşhan; Wilson, Daniel; Lea-Langton, Amanda; Flynn, Michael; Kirago, Leonard; Shaw, Marvin; Rickard, Andrew; Hopkins, Jim R.; Bryant, Daniel; McFiggans, Gordon; Hamilton, Jacqueline F.; Coe, Hugh; Allan, James

doi:10.5194/ar-2026-24

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https://doi.org/10.5194/ar-2026-24

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30 Jun 2026

| 30 Jun 2026

Status: this preprint is currently under review for the journal AR.

Mixing state of Carbonaceous Aerosol Emissions from an Ecodesign Woodstove

Zixuan Cheng, Doğuşhan Kılıç, Daniel Wilson, Amanda Lea-Langton, Michael Flynn, Leonard Kirago, Marvin Shaw, Andrew Rickard, Jim R. Hopkins, Daniel Bryant, Gordon McFiggans, Jacqueline F. Hamilton, Hugh Coe, and James Allan

Abstract. Residential wood burning (RWB) has become an increasingly significant source of carbonaceous aerosols (CAs) in the UK and worldwide. Black Carbon (BC) and Organic Aerosol (OA) fractions of CAs are of particular concern due to their impacts on climate and human health. Measuring the mixing state of CA is important as the mixing state can influence key aerosol properties including light absorption, hygroscopicity, cloud scavenging, atmospheric lifetime, and toxicity. While emissions and mixing states from traditional stoves have been previously reported and characterized, key uncertainties remain in the influence of user behaviour on emissions and the emission characteristics from the modern ‘Ecodesign’ appliances. Recent emissions tests imply that these can emit more pollutants such as polycyclic aromatic hydrocarbons (PAHs) and BC under certain circumstances and here we utilise online instrumentation to probe the mechanisms behind this and whether user behaviour has a role in impacting emissions. Hardwood logs were burned in a controlled test system using a UK Ecodesign-compliant woodstove under five operating protocols: standard, overload, underload, hot-reload and open-door protocols. Instantaneous particle emissions were quantified using a Single Particle Soot Photometer (SP2), a Differential Mobility Sizer (DMS500) and an Aerosols Mass Spectrometer (AMS). Modified combustion efficiency (MCE) was derived from CO and CO₂ concentrations measured by a Fourier-Transform Infrared (FTIR) spectrometer. Three typical combustion phases were observed: a pre-ignition phase, a flaming phase (with rich and non-rich flaming distinguished by an MCE of 0.95), and a smouldering phase. Stove operation can affect emissions by altering particle size, leading to ultrafine particle (UFP) formation (e.g., open-door, underload and overload conditions) and also prolong the rich flaming phase (e.g., hot-reload and overload), increasing PAH emissions. The findings here demonstrate how user behaviour may increase emissions of certain pollutants from modern stoves, partially offsetting the benefits compared to older designs.

Received: 15 Jun 2026 – Discussion started: 30 Jun 2026

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RC1:
'Comment on ar-2026-24', Anonymous Referee #1, 15 Jul 2026 reply
This study investigates how different operating conditions of an Ecodesign residential wood stove influence the emissions and mixing state of carbonaceous aerosols with couples of online measurements. The authors investigate the evolution of combustion phases, BC mixing state, particle size distributions, and organic aerosol composition, with the aim of understanding how user behaviour influences carbonaceous aerosol emissions from modern residential stoves. While the scope of this study is well suited to AR and the experimental dataset is comprehensive, the manuscript in its current form lacks sufficient mechanistic interpretation and discussions. In the Introduction (Lines 89–93), the authors define three scientific objectives for this study. However, these objectives are not fully addressed in the current results and discussions. Therefore, I recommend that the manuscript be reconsidered after major revision.
Major comments:
Section 3.2 primarily provides a descriptive comparison of AMS mass spectra across different combustion phases. While these observations are useful, the chemical information is not sufficiently integrated into the subsequent analyses to address the mechanistic questions posed in the Introduction (Lines 89–93). Since the authors have simultaneous measurements of BC mixing state from the SP2 and organic aerosol composition from the AMS, the manuscript would benefit from examining the relationships between BC mixing state and key AMS markers, such as m/z 43, m/z 44, m/z 60 and/or m/z 202, should also be explored. Such analyses would provide further support for the proposed BC mixing state mechanisms. For example, in Lines 510–511, the authors state that “Under low MCE, a large proportion of OM, especially PAHs, may prevent the BC particles from forming a core-shell morphology.” However, this interpretation is not supported by the current presented results. Why “especially PAH” will prevent the BC being internally mixed? Could it be the oxidization state of OM influence the BC mixing state?

In the Introduction (Lines 62–63), the authors state that coagulation rates increase significantly under high particle concentrations, implying enhanced particle mixing. However, in the Conclusions (Lines 674–675), the authors state that the formation of thickly coated BC under high-OM conditions is “consistent with” increased external mixing. This conclusion is not sufficiently justified. Although the results are interesting and suggest that a greater BC coating thickness does not necessarily correspond to a higher degree of internal mixing, the underlying mechanism is not adequately explained. Additional analyses and discussion are needed to provide mechanistic support for this conclusion.

Multiple dilution stages were applied prior to the SP2 measurements, yet the possible effects of dilution on the gas–particle partitioning of semi-volatile organics and the resulting impacts BC coating thickness are not considered. Since the proposed coating formation mechanisms rely on SP2-derived coating metrics analysis, the authors should discuss whether the observed mixing state represents the primary emissions at the stove outlet or has been modified during dilution and sampling.

The current Abstract requires substantial revision. Although the manuscript is entitled “Mixing State of Carbonaceous Aerosol Emissions”, the current abstract provides almost no information regarding the aerosol mixing state or the key findings related to it. Instead, a large proportion of the abstract (Lines 16–30) is devoted to background information and a description of the experimental setup and instrumentation. In contrast, the main findings of the study are only briefly summarized in a few lines (Lines 30–34). The abstract should place much greater emphasis on the principal scientific results and their significance, rather than on methodological details.

Minor comments:
Line 44: provide the reference.

Line 51 – 53: cite Riemer et al. (2019).

Please unify the term “BC” and “rBC” throughout the manuscript.

To what extent was BC considered to be thickly coated? The authors classify BC particles based on Esca (Line 416). Including the associated Dp/Dc range or representative values would help readers better understand the degree of BC coating.

Line 429 - 430: Could BC originate from secondary sources?

Line 430, The statement “only a fraction …” is unclear. Please specify.

Line 505: Should this be "OM coagulates with BC"? Please also check similar descriptions throughout the manuscript. I think OM generally refers to particulate organic matter in the manuscript, and the term "condense" may not be appropriate.

Line 515, in Figure 7 caption, Pearson correlation coefficient should be r?

Figure 6: Are standard error bars available for all particle size distributions?

The axis labels and tick labels in several figures (e.g., Figures 5 and 10) are too small. Please increase the font size to improve readability.

There is no clear determination of UFP size range. For example, in Line 476 - 477: When the author stated “A large UFP peak was observed during the pre-ignition phase of single big log protocols”. What is the size range of the UFP peak?

Reference:
Riemer, N., Ault, A. P., West, M., Craig, R. L., & Curtis, J. H. (2019). Aerosol mixing state: Measurements, modeling, and impacts. Review of Geophysics, 57, 187–249. https://doi.org/10.1029/2018RG000615

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Citation: https://doi.org/10.5194/ar-2026-24-RC1
RC2: 'Comment on ar-2026-24', Anonymous Referee #2, 20 Jul 2026 reply

This manuscript presents a study investigating the carbonaceous aerosol emissions from residential wood burning using a wood stove under non-ideal operating conditions. The topic of this study falls within the scope of the journal Aerosol Research. This manuscript is generally laid out well and shows its academic value. The manuscript is recommended to be published after addressing the following comments.
Title: If this study focuses on non-ideal operations of a wood stove, it should be specified in the title.
Line 60: Please double check if the term "Aiken" is a typographical error of "Aitken" and make revisions if necessary.
Line 118: Please double check if the word "quantiying" is a typographical error of "quantify" and make revisions if necessary.
Lines 131 - 133 and in general: To characterize the emissions under real-world conditions, it seems that the catalyst should be used. The removal of catalyst during the experiments of this study seems to simulate the emissions of the stove under abnormal operating conditions where the catalyst is not functioning. Please provide more detailed descriptions to clarify this step of experimental design, otherwise the emissions estimated in this study would be misleading and give the overestimated results compared with the real-world situation. If the results are used to compare with those from traditional wood stoves, the emissions from non-ideal operations of those stoves should also be used for a meaningful comparison. These should be considered and specified in the manuscript.
Lines 157 - 159: Is the catalyst removed in all these protocols? The removal of catalyst is also a deviation from the standard operating condition in the real world.
Line 159: Please double check if the word "protocols" is a typographical error of "protocol" and make revisions if necessary.
Line 204: Please double check if the word "particles" is a typographical error of "particle" and make revisions if necessary.
Lines 215 - 216: How are the data points with E_sca < 1 treated in this study?
Line 374: Please double check if the word "has" is a typographical error of "have" and make revisions if necessary.
Line 423: Please double check if the word "important" is a typographical error of "importance" and make revisions if necessary.
Figure 5: Please provide the color bar for this figure.
Line 492: Why would a big log have a higher surface area to volume ratio? This statement seems counter intuitive if multiple smaller logs with a total volume equal to the volume of the big log are considered and compared.
Figure 8: Please provide the color bar for this figure and cite the reference for the classification criteria (Liu et al. 2019) in the caption to improve readability.
Line 543: Please double check if the word "absents" is a typographical error of "absent" and make revisions if necessary.
Figure 9: Please provide the color bar for this figure.
Line 562: Please double check if the word "particel" is a typographical error of "particle" and make revisions if necessary.
Figure 10: Panels (a) and (b) are suggested to be added to the caption.
Lines 607 - 608: Does it mean that 20 nm is larger than 190 nm?
Table 3: Where is the second standard load cycle?

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Citation: https://doi.org/10.5194/ar-2026-24-RC2

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Short summary

Wood burning is seen as cleaner in modern stoves, but user behaviour can change this when burning dry wood. We burned dry ash wood in a UK modern stove with six common operations and measured the wood smoke. Reloading while hot, using too much or too little wood, or opening the stove door increased small particels and harmful smoke emissions. We found a burning stage that helps explain these emissions. Using big pieces of wood and following guidance can reduce pollution and health risks.


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