Greenhouse gas emissions from laboratory-scale fires in wildland fuels depend on fire spread mode and phase of combustion

Surawski, N. C.; Sullivan, A. L.; Meyer, C. P.; Roxburgh, S. H.; Polglase, P. J.

doi:https://doi.org/10.5194/acp-15-5259-2015

Articles | Volume 15, issue 9

https://doi.org/10.5194/acp-15-5259-2015

© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/acp-15-5259-2015

© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 15, issue 9

Research article

|

13 May 2015

Research article |

| 13 May 2015

Greenhouse gas emissions from laboratory-scale fires in wildland fuels depend on fire spread mode and phase of combustion

N. C. Surawski, A. L. Sullivan, C. P. Meyer, S. H. Roxburgh, and P. J. Polglase

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Final revised paper (published on 13 May 2015)
Preprint (discussion started on 08 Sep 2014)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC C7446: 'Referee Comment', Anonymous Referee #1, 26 Sep 2014
- AC C8754: 'Author responses to reviewer 1's comments.', Nicholas Surawski, 03 Nov 2014
RC C7784: 'Referee 2 Comments', Anonymous Referee #2, 07 Oct 2014
- AC C8755: 'Author responses to reviewer 2's comments.', Nicholas Surawski, 03 Nov 2014

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Nicholas Surawski on behalf of the Authors (19 Jan 2015) Author's response Manuscript

ED: Referee Nomination & Report Request started (21 Jan 2015) by Rupert Holzinger

RR by Anonymous Referee #1 (24 Jan 2015)

Suggestions for revision or reasons for rejection

Reasons for rejecting paper a second time:

The authors claim to address the problem of poor mixing, but were completely off base as they tried to do this by discussing “chemical reactions” as in this quote from their response: “In this new section, we calculate the reaction Damköhler number (Da) which is the ratio of the flow time scale to the chemical reaction time scale (Law, 2006). We calculate Da at two flame heights and axial positions within the flow with Da exceeding 106 14 in all cases. Therefore, for the species we measure in this experimental effort, the timescale required for chemical reaction is very short relative to the flow timescale in our combustion wind tunnel. Therefore, the chemical reactions are at equilibrium (or are “frozen”) by the time our sampling manifold is reached and furthermore do not depend on sampling height.”

This response is completely irrelevant since the concern is not chemical reactions, but mixing and the appropriate “number,” if there is one, is the Reynolds number. Thus lack of demonstrated representative sampling still stands as sufficient reason to reject the paper without further reading.

However, I read more and found additional problems:

On response 2a, the authors say: “One factor that the reviewer has neglected to consider in their comment is that burnt fuel carbon does not necessarily have to be emitted to the atmosphere, even though most of it is. As we detail in later in this set of responses (i.e. major comment 2), burnt carbon could be present in the post-fire combustion residues as black carbon, ash or partially charred/combusted fuel.”

Once again, the authors are off topic. The reviewer does understand that not all carbon in the vicinity of a fire (or touched by flames) is emitted to the atmosphere. What the authors fail to understand is that carbon that is not emitted is not counted as fuel consumption. In lab experiments it remains on the scale so as to not affect the weight change. In field experiments, most often, the residual is collected and weighed in paired plots and fuel consumption is calculated as prefire-postfire mass. In an alternative method, logs are wrapped with wires before a fire. Post-fire, the wires are tightened and amount of tightening is used to calculate a reduction in fuel volume that is combined with the density of the wood to estimate mass change. Partial burning and charred surfaces are therefore correctly accounted for in the fuel consumption estimate.
Another relevant point is that the 100% fuel consumption for fine fuels in the Ward et al 1992 study cited in my first review, and a result obtained by many others, was not for extreme wildfires, but for small prescribed fires. In summary, the authors can increase the relevance of any future fire research they may do by burning under more representative conditions and they should not confuse residue and combustion factors.

On the topic of greenhouse gas mitigation. The authors fail to understand that once a fire is lit it creates its own micrometeorology, which also is impacted by variable terrain. But above all, even for any potential future experiments with redesigned representative sampling, by testing wind direction effects in the lab at only one windspeed, fuel type, and moisture, etc, they would not be able to predict real-world emissions, which are impacted by numerous environmental variables.

I would encourage the authors to take a step back. (1) Do the simple experiment of measuring the temperature profile in the wind tunnel. (2) Experiment with ways to induce and confirm good mixing. (3) Re-examine their research goals after visiting some real fires and reading papers to determine what the major unknowns are.

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RR by Anonymous Referee #2 (13 Feb 2015)

RR by Anonymous Referee #3 (16 Feb 2015)

Suggestions for revision or reasons for rejection

Scientific Significance

This paper presents the results of experimental laboratory fires used to investigate the influence of fire spread mode and phase of combustion on gas emissions. According to van Leeuwen and van der Werf (2011), explaining the large within-biome variability of emission factors that is so pronounced in the findings of Akagi et al (2011; Table 2.2) remains one of the main challenges for biomass burning emissions science. By focussing on measuring fires burning with different fire spread modes, this paper addresses one of van Leeuwen and van der Werf’s main suggestions – that measurement campaigns need to pay more attention to ambient conditions. The significant differences found between emissions from different fire spread modes goes some way to explaining why we see such large variability within and between different studies of fire emissions from a particular biome (in this case, Australian temperate forests). More papers are needed like this one that attempt to describe AND explain emissions variability within a particular biome. The links made to carbon accounting and sequestration are important.

Scientific Quality

I am impressed by the attention to detail in the description of the experimental and analytical methods. A lot of effort has been made to explain and justify the experimental setup, and the particular equations used for calculating emission factors. These justifications are supported by referencing recent examples in the literature that use similar approaches. Few fire emissions papers attempt to explore statistical relationships between fuel/ambient/ignition conditions/characteristics and the resultant emissions (Meyer et al. 2012 is one exception). The MANCOVA and MANOVA tests proposed in this study are appropriate. This Section (2.4.3) lacks any reference to previous studies, so perhaps it is worth referring to Meyer et al. 2012 who also use statistical methods to explore relationships between fuel characteristics/ambient conditions and fire emission factors.

The discussion does a good job of comparing findings with the few previous studies of Australian temperate forest fire emissions. Whilst I appreciate that there has been little investigation into the influence of fire spread mode on emissions, and probably no studies of this for Australian temperate forests, it would be useful to see a comparison with any study that does look at this relationship. The only one that I can think of is that by Wooster et al. (2011) who report differences in emission factors between headfires, backing fires, and residual smouldering in field measurements of African savannahs (see Table 4 in their paper).

References:

Akagi, S.K., Yokelson, R.J., Wiedinmyer, C., Alvarado, M.J., Redi, J.S., Karl, T., Crounse, J.D., Wennberg, P.O. (2011) Emission factors for open and domestic biomass burning for use in atmospheric models. Atmospheric Chemistry and Physics 11: 4039–4072.

van Leeuwen, T.T., van der Werf, G.R. (2011) Spatial and temporal variability in the ratio of trace gases emitted from biomass burning. Atmospheric Chemistry and Physics 11: 3611–3629.

Meyer, C.P., Cook, G.D., Reisen, F., Smith, T.E.L., Tattaris, M., Russell-Smith, J., Maier, S.W., Yates, C.P., Wooster, M.J. (2012) Direct measurements of the seasonality of emission factors from savanna fires in northern Australia. Journal of Geophysical Research 117(D20305), doi: 10.1029/2012JD017671.

Wooster, M.J., Freeborn, P.H., Archibald, S., Oppenheimer, C., Roberts, G.J., Smith, T.E.L., Govender, N., Burton, M., Palumbo, I. (2011) Field determination of biomass burning emission ratios and factors via open-path FTIR spectroscopy and fire radiative power assessment: headfire, backfire and residual smouldering combustion in African savannahs. Atmospheric Chemistry and Physics 11: 11591–11615.

Referee Report: PDF

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ED: Reconsider after minor revisions (Editor review) (18 Feb 2015) by Rupert Holzinger

AR by Nicholas Surawski on behalf of the Authors (26 Feb 2015) Author's response Manuscript

ED: Reconsider after minor revisions (Editor review) (04 Mar 2015) by Rupert Holzinger

AR by Nicholas Surawski on behalf of the Authors (11 Mar 2015) Author's response Manuscript

ED: Reconsider after minor revisions (Editor review) (23 Mar 2015) by Rupert Holzinger

AR by Nicholas Surawski on behalf of the Authors (17 Apr 2015) Author's response Manuscript

ED: Publish as is (20 Apr 2015) by Rupert Holzinger

AR by Nicholas Surawski on behalf of the Authors (22 Apr 2015)

Short summary

By undertaking greenhouse gas emissions measurements (plus CO) in a combustion wind tunnel facility, we show that emissions from fire depend on how they spread relative to the wind. Statistically significant differences include fires spreading with the wind emitting twice as much CO as fires spreading perpendicular to or against the wind, and about 10-17% more carbon dioxide. Our results suggest that judicious use of ignition patterns could mitigate carbon emissions from forest fires.