Smoke Emissions and Buoyant Plumes above Prescribed Burns in the Pinelands National Reserve, New Jersey
, Michael R. Gallagher 1, Nicholas Skowronski 2, Warren E. Heilman 3, Joseph Charney 3, Matthew Patterson 2, Jason Cole 4, Eric Mueller 5 and Rory Hadden 6Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsOverall comment
This is an interesting and important study that assist in improving estimates of emissions from low and high intensity burns in pineland areas using pre-burn fuel loads and combustion completeness factors as well as improving our understanding into factors that affect smoke plume dispersion.
Detailed comments:
Materials and Methods
· Can you please reference/define what 1-hr, 10-hr and 100-hr fuels mean
· Based on equations (1) and (2), eq (3) should be: CC = Fuel consumed/FL(pre-burn).
Questioning whether CC calculations for the remainder of the analysis are correct, as these would use the postburn fuel load rather than the fuel consumed to derive CC.
Line 248: should this be 14 towers, based on data presented in Table 2?
Results
I suggest combining sections 3.1 and 3.3, starting with the results from the instrumental prescribed burns and then relating how they compare to the larger data set.
In Table 3, how is consumption defined? Is it Post fuel load – pre fuel load?
In Figure 2, estimated consumption is graphed against initial fuel load. Is estimated consumption the difference between post and pre fuel load? By combining eq (1) and eq (2), the slope of the graph would then represent the combustion completeness factor.
Why is the fine litter not shown in Figure 2?
How does the data from the instrumented burns compare to the larger data set in Figure 2? It would be interesting to add the instrumented data points to Figure 2.
In Section 3.2, is the combustion completeness factor derived using eq (3)? This wouldn’t be in agreement with combining eq(1) and eq (2).
I find sections 3.2 and 3.3 (2nd paragraph) very confusing given the difference in calculating CC.
What are biometric measurements?
In line 406, you refer to average weighted combustion completeness factors – how is the weighting defined? What equation is used to derive the CC factors?
Line 481: This should read Appendix F
Line 481: what do the authors mean by increased negative 10Hz wind velocities?
Figure 6: What’s the significance of the relationship between vertical wind velocity and air temperature?
Lines 522-525 refer to comparison of peak TKE values between fire front passage and control towers, however Figure 7b just shows the difference between low and high intensity burns? What Figure 7 is showing is that a significant difference between low and high intensity burns was observed for sensible heat flux but not for TKE.
Figure 8e – the relationship is very much skewed by 3 data points that showed high sensible heat flux. Similar for Figure 8b
Based on Figure 7 and lines 524/525, there does not seem to be a difference in TKE between low and high intensity burns. However, in Figure 9c and 9d you show a significant difference in TKE values between high and low intensity burns.
What’s the significance of the relationship between TKE and heat flux?
Section numbering needs to be updated.
Discussion
Line 629- the study showed that there was no relationship between sensible heat flux and TKE for low intensity burns (Fig 8)
Appendices
Appendix A - Please define PAR
Appendix B
Please reference equation to calculate EPP.
Equation to calculate ER, should that be total heat of combustion rather than mass, as stated in line 898?
Lines 905-906: Would Qc be the heat due to moisture rather than moisture?
Appendix D
Are CC factors calculated using eq (3)? Is it FLpost/FLpre or defined as Fuel consumed/FLpre?
If CC is defined as per eq (3), how can there be negative values? That would mean that you have negative fuel loads? Negative values for fuel consumed , would mean that you accumulated fuel?
Appendix E: Estimated consumption – is this the difference between post and pre fuel load?
What do negative values for fuel consumed mean? Fallen logs?
Author Response
In the text below, we address all of the comments made by Reviewer 1 point by point. Our answers and additional comments appear in italics.
Reviewer #1 comments
Overall comment
This is an interesting and important study that assist in improving estimates of emissions from low and high intensity burns in pineland areas using pre-burn fuel loads and combustion completeness factors as well as improving our understanding into factors that affect smoke plume dispersion.
Thank you for these positive comments. These are the main reasons we conducted this study to assist state and federal wildland fire managers estimate emissions from prescribed burns conducted in pine and oak-dominated forests on the mid-Atlantic coastal plain.
Detailed comments:
Materials and Methods
- Can you please reference/define what 1-hr, 10-hr and 100-hr fuels mean
We apologize for this omission. We used the standard definition of 1-hour, 10-hour and 100-hour fuels: Woody fuels < 0.64 cm diameter are 1-hour fuels, ≥ 0.64 to < 2.54 cm diameter are 10-hour fuels, and ≥ 2.54 to 7.60 cm diameter fuels are 100-hour fuels. We have inserted these definitions into the revised manuscript text.
- Based on equations (1) and (2), eq (3) should be: CC = Fuel consumed/FL(pre-burn).
Thank you for pointing out our omission of one of the terms in the parentheses in Equation 3. We apologize for this omission, and the correct equation is:
CC(x) = (FL preburn – FL postburn)/ FL preburn (3)
We have included the correct form of Equation 3 in the revised manuscript.
Questioning whether CC calculations for the remainder of the analysis are correct, as these would use the postburn fuel load rather than the fuel consumed to derive CC.
Given our unintended omission of one of the terms in Equation 3, this is understandable. Throughout the remainder of the manuscript, we used the correct formulation of Equation 3. Thus, all of the combustion completeness factors throughout the original manuscript are correct.
Line 248: should this be 14 towers, based on data presented in Table 2?
Yes, this is correct, and we have corrected “13” to “14” towers. Thank you for pointing this out. We had accidentally omitted the burn area tower from the last burn conducted.
Results
I suggest combining sections 3.1 and 3.3, starting with the results from the instrumental prescribed burns and then relating how they compare to the larger data set.
We considered reorganizing Sections 3.1 and 3.3 in this manner but chose not to do so because we wanted to first calculate values of the combustion completeness factors using the larger dataset (n = 48 pre- and post-burn measurements) and then compare emission estimates using these CC values to those calculated using the pre- and post-burn measurements made during the 11 instrumented prescribed burns, as shown in Figures 3a-c. Our intent here is to provide State and Federal wildland fire managers with the most straightforward and accurate method to calculate emissions during operational prescribed burns in pine and oak-dominated forests in the Pinelands National Reserve. We feel that this approach and the order of presentation provides the best evaluation of emission estimates calculated using our locally-derived combustion completeness factors.
In Table 3, how is consumption defined? Is it Post fuel load – pre fuel load?
Consistent with the calculations throughout the manuscript, we calculated estimated consumption as Pre-burn fuel loading – post-burn fuel loading = fuel consumption.
In Figure 2, estimated consumption is graphed against initial fuel load. Is estimated consumption the difference between post and pre fuel load? By combining eq (1) and eq (2), the slope of the graph would then represent the combustion completeness factor.
Yes, estimated consumption was calculated as the difference between pre-burn fuel loading and post-burn fuel loading. The slopes of the relationships between initial (pre-burn) fuel loading and estimated consumption shown in Figure 2 do provide an estimate of the combustion completeness factors for each component, although the zero intercept would also need to be included. We instead used the average combustion completeness factors, as shown in Appendix D, for all emission calculations. We used the appropriate subsets of these averaged values to calculate emissions from prescribed burns conducted in each forest type (e.g., pine-scrub oak, pine-oak, oak-pine).
Why is the fine litter not shown in Figure 2?
We opted to show the loading and consumption of forest floor material, which was the combination of fine litter and 1-hour + 10-hour fuels, in Figure 2, because this seemed to make more sense to State and Federal wildland fire managers, who typically would not separate these components when assessing fuel loading in the field.
We do present the calculations for mean combustion completeness factors for fine litter, 1-hour + 10-hour fuels and fine litter and 1-hour + 10 hour fuel together as forest floor material in Appendix D. We also provide additional information regarding the consumption of fine litter during low and high intensity prescribed burns in lines 386 - 390 in the Results section. Similar results were presented for prescirbed burns and wildfires in Gallagher (2017).
How does the data from the instrumented burns compare to the larger data set in Figure 2? It would be interesting to add the instrumented data points to Figure 2.
Thank you for this suggestion. However, we have already included all of the data collected from the instrumented burns in the original Figure 2.
We believe it is more effective to visually compare pre-burn and post-burn loading by component (understory vegetation, 1-hr and 10-hr fuels, and fine litter) in the larger prescribed burn data set to the pre-burn and post-burn fuel loading in the sub-set of instrumented burns, as we have done in Figure 1a and 1b. There were no statistically significant differences among the data presented in these two datasets when compared, although sample sizes for instrumented burns conducted in pine-oak and oak-pine forest types were low (n = 2 for each of these forest types).
In Section 3.2, is the combustion completeness factor derived using eq (3)? This wouldn’t be in agreement with combining eq(1) and eq (2).
Combustion completeness factors in Section 3.2 were derived using the CORRECTED version of Equation 3. Again, we apologize for the omission of terms in the version of Equation 3 presented in the first version of the manuscript, and the confusion it caused.
I find sections 3.2 and 3.3 (2nd paragraph) very confusing given the difference in calculating CC.
We agree, and we apologize for any confusion, given the original incorrect version of Equation 3. However, we believe that these sections are important to include in the manuscript. In the second paragraph of Section 3.3, we are calculating PM 2.5, CO2 and CO emissions two different ways; 1) using the estimated consumption multiplied by published emission factors (Ex) for PM 2.5, CO2 and CO as shown in Equation 2, and 2) using pre-burn fuel loading and the correct CC values, as shown in Equation 1. We believe that it is important to calculate and present these two estimates, because it is a way to evaluate the use of pre-burn loading and CC values against pre- and post-fuel loading sampled in the field. The use of CC values will be much easier for wildland fire managers who may not have the time or ability to sample post-burn fuel loading, but do have reasonable estimates of initial fuel loading. We discuss this approach further in the Discussion section.
What are biometric measurements?
These are the pre- and post-burn measurements made in the field, as well as the tree plot sampling we used to characterize forest types. We have retained this standard term for field measurements throughout the manuscript.
In line 406, you refer to average weighted combustion completeness factors – how is the weighting defined? What equation is used to derive the CC factors?
Thank you for pointing this out. Lines 407-409 are correct but confusing. We have reworded this sentence in the revised manuscript to read:
“Combustion completeness factors for fine litter, 1-hr + 10-hr wood, understory vegetation, and average weighted values for the sum of all available fuels are shown in Table 4 and Appendix D.”
The average weighted combustion completeness factors for each forest type (pine-scrub oak, pine-oak, and oak-pine) were calculated using the correct Equation 3 by weighting CC factors by the mass of each fuel type (fine litter, 1-hour and 10-hour wood, and understory vegetation) consumed. We present the weighted values for each forest type on the last line in Table 4, labeled as “All fuels”. We realize that this is only an approximation of the appropriate CC factors, but will provide wildland fire managers with values that can be used for rapid estimation of emissions during prescribed burns.
Line 481: This should read Appendix F
We have corrected the mislabeled “Appendix E” to “Appendix F”. We apologize for this error.
Line 481: what do the authors mean by increased negative 10Hz wind velocities?
These are the enhanced downdrafts associated with enhanced high-frequency turbulence during flame front passage, and typically occur to a greater extent during high intensity prescribed burns. As shown in the yellow trace line in Figure 4b, fire front passage during the high intensity burn at Warren Grove in 2013 was associated with enhanced positive (updrafts) and negative (downdrafts) values of vertical wind velocities measured at 10 Hz. We have substituted the term “enhanced” for “increased” for greater clarity.
Figure 6: What’s the significance of the relationship between vertical wind velocity and air temperature?
We believe that the instantaneous (10 Hz) relationship between air temperature and vertical wind velocity at the top of the canopy was important to explore because the upward movement of warm air in buoyant plumes is the primary mechanism transferring PM2.5, CO2, CO and other combustion products above the canopy to the free atmosphere.
We have attempted to show that not only are air temperatures and vertical wind velocities enhanced to a greater extent during high intensity prescribed burns compared to low intensity burns, but that their relationship is more tightly coupled during high intensity burns. We evaluated the statistical significance of this relationship for selected towers using Spearman’s nonparametric correlation coefficients in Figure 6, and present average values for Spearman’s nonparametric correlation coefficients for all low vs. high intensity prescribed burns in Table 6. Mean Spearman’s correlation coefficients were nearly significantly greater for high intensity prescribed burns when compared to low-intensity burns when evaluated using a Mann Whitney U test.
Lines 522-525 refer to comparison of peak TKE values between fire front passage and control towers, however Figure 7b just shows the difference between low and high intensity burns? What Figure 7 is showing is that a significant difference between low and high intensity burns was observed for sensible heat flux but not for TKE.
Lines 522-525 are worded correctly because 1-minute TKE values were greater for burn area towers than control towers during these prescribed burns. However, we agree that this is somewhat confusing, and have inserted “(data not shown)” following this first phrase. The second phrase is also correct, and we have not altered the wording in the revised manuscript. Reviewer #1 is correct in the overall interpretation of Figure 7.
Figure 8e – the relationship is very much skewed by 3 data points that showed high sensible heat flux. Similar for Figure 8b
This is true. The prescribed burn shown in Figure 8e is correctly characterized as a low intensity mixed-behavior burn, but was associated with a few relatively high 1-minute sensible heat flux values when the fire front was close to the tower. It should be noted that the scale on the x-axis for sensible heat flux is half of that for the paired high intensity burn shown in Figure 8f. A similar phenomenon occurred during the mixed-behavior fire at a pine-oak stand in 2011. Overall, the higher intensity prescribed burns, and especially head fires, were associated with very high 1-minute sensible heat fluxes as fire fronts passed rapidly by the instrumented towers.
Based on Figure 7 and lines 524/525, there does not seem to be a difference in TKE between low and high intensity burns. However, in Figure 9c and 9d you show a significant difference in TKE values between high and low intensity burns.
Reviewer #1 is correct in the interpretation of Figures 7 and 9. We used Figure 7 to illustrate 1-minute values of sensible heat and TKE between two of the 11 instrumented prescribed burns; one low intensity prescribed burn (shown by the blue line) and the other a high intensity burn (shown by the yellow line). However, values in Figure 9 were calculated for mean (Figure 9c) and maximum (Figure 9d) 1-minute values for all 11 instrumented prescribed burns together, and significant differences among low and high intensity burns were detected using the non-parametric Mann Whitney U test.
What’s the significance of the relationship between TKE and heat flux?
Similar to the relationship between air temperature and vertical wind velocity during fire front passage, we believe that exploring the relationship between 1-minute values of sensible heat flux and TKE has led to a better understanding convective heating and turbulence in buoyant plumes during low- vs. high intensity prescribed burns. Higher intensity head and flanking fires are producing a greater enhancement of sensible heat flux in buoyant plumes, creating greater turbulence at and above the top of the canopy and resulting in greater lofting and dispersion of smoke.
We present the statistical significance of the relationship between sensible heat flux and TKE for selected pairs of towers in low and high intensity burns in Figure 8, and here report significance values for each burn. We then pooled data for multi-tower experiments, and present data for all low and high intensity burns in Table 7, including Spearman’s rank correlation coefficients, values of Student’s T, and statistical significance levels.
Section numbering needs to be updated.
Yes, we agree. We apologize, because somehow section numbering was incorrect. We now have Sections 3.4, 3.5 and 3.6 in the correct locations.
Discussion
Line 629- the study showed that there was no relationship between sensible heat flux and TKE for low intensity burns (Fig 8)
For most of the low intensity burns, there was no relationship between sensible heat flux and TKE. However, in Figure 8e, sensible heat flux and TKE are positively associated during fire front passage at the prescribed burn conducted in a pine-scrub oak stand at Cedar Bridge in 2020, and in summary data presented in Table 7, sensible heat flux and TKE are positively associated during two of the low intensity burns. Therefore, we have retained the term “weaker” rather than “no” relationship between sensible heat flux and TKE during fire front passage in low intensity prescribed burns, as this seems to be the correct term.
Appendices
Appendix A - Please define PAR
PAR is an abbreviation for “photosynthetically active radiation”, in units of μmol m-2 s-1. We have omitted the reference to PAR in Appendix A because although we measured these data, we do not use PAR is any of the analyses in this study.
Appendix B
Please reference equation to calculate EPP.
We now provide an appropriate reference for the equation to calculate Epp, the energy consumed in preheating and pyrolysis. Kremens et al. 2012 (reference [26] provide an excellent overview and description of energy fluxes from fuels during combustion. processes.
Equation to calculate ER, should that be total heat of combustion rather than mass, as stated in line 898?
Yes, Reviewer #1 is correct and we have provided the correct term in the equation to calculate ER, the estimated energy released as radiative heat flux during combustion, following Kremens et al. 2012. Thank you for pointing out our error.
Lines 905-906: Would Qc be the heat due to moisture rather than moisture?
Yes, Reviewer #1 is correct and we have provided the correct definition for Qc, now defined as “…heat due to moisture released by combustion,…”. Thank you for pointing out our error.
Appendix D
Are CC factors calculated using eq (3)? Is it FLpost/FLpre or defined as Fuel consumed/FLpre?
Again, we apologize for our omission of one of the terms in parentheses in Equation 3. All of these CC factors were calculated using the correct equation:
CC(x) = (FL preburn – FL postburn)/ FL preburn (3)
If CC is defined as per eq (3), how can there be negative values? That would mean that you have negative fuel loads? Negative values for fuel consumed, would mean that you accumulated fuel?
We agree that a negative value for fuel consumption is counterintuitive, but did occur occasionally. Our pre- and post-burn fuel samples could not be collected in the identical location each burn area, and in some samples collected following low-intensity prescribed burns, 1-hour + 10-hour woody fuel mass was greater in post-burn samples compared to pre-burn samples. In addition, 1-hour and 10-hour fuels may have fallen on the forest floor during or immediately following prescribed burns, resulting in an apparent net accumulation of these fuels. We discuss some of these sampling limitations in the second paragraph of the section entitled “4.1 Fuel Loading and Consumption During Prescribed Burns”.
Appendix E: Estimated consumption – is this the difference between post and pre fuel load?
Yes, this is correct.
What do negative values for fuel consumed mean? Fallen logs?
Our pre- and post-burn fuel samples could not be collected in the identical location each burn area, and in some samples collected following low-intensity prescribed burns, 1-hour + 10-hour woody fuel mass was greater in post-burn samples compared to pre-burn samples. In addition, 1-hour and 10-hour fuels may have fallen on the forest floor during or immediately following prescribed burns, resulting in an apparent net accumulation of these fuels.
Reviewer 2 Report
Comments and Suggestions for AuthorsThe present paper titled " Smoke Emissions and Buoyant Plumes above Prescribed Burns in the Pinelands National Reserve, New Jersey" presents an interesting work and quite pertinent related to the fuel consumption and emission estimates of PM2.5, CO2, and CO with observations of above-canopy sensible heat flux and turbulence during operational prescribed burns of varying intensity in the Pinelands National Reserve in New Jersey.
The paper is well-written and structured but sometimes becomes too descriptive. The relevance of this research is underscored by the pressing need to protect the forest. At a global level, the world's forest is devastated annually by forest fires, causing serious and widely recognized consequences. It is true that the prescribed burning is a cost-effective method for reducing hazardous; however other solutions to eliminate forestry waste must first be evaluated, such as the energy use of forest residues. I think it would be important to address this topic in the introduction
Given the importance and relevance of the topic, I strongly believe that this article will significantly contribute to the advance scientific community, and I recommend accepting the article after addressing the following minor issues:
1) the article must be written impersonally, so the third person is used in the passive voice, line 27, 127, 620, 649, 747, 827, …
2) the websites relating to the bibliography consulted must only appear in the list of references and not in the text, lines 152, 220, 787
3) why were the samples dried at 70°C? What is the reason? This is not explained in the text. Drying at 70 C is not completely drying.
4) in the sentence “ … emissions and Δ air temperature, Δ wind velocity, Δ 1-minute sensible heat, and Δ TKE values.” the use of delta in conjunction with the variable written in extensive form becomes confusing for the reader.
5) The caption for figure 4 must be next to the images, and not on another sheet.
Since the standard deviation (air temp, RH and windspeed) is shown in table 5, these images may be unnecessary.
6) The caption for Table 7 must also be on the same page as table 7, on page 17.
Author Response
In the text below, we address all of the comments made by Reviewer 2 point by point. Our answers and additional comments appear in italics.
The present paper titled " Smoke Emissions and Buoyant Plumes above Prescribed Burns in the Pinelands National Reserve, New Jersey" presents an interesting work and quite pertinent related to the fuel consumption and emission estimates of PM2.5, CO2, and CO with observations of above-canopy sensible heat flux and turbulence during operational prescribed burns of varying intensity in the Pinelands National Reserve in New Jersey.
The paper is well-written and structured but sometimes becomes too descriptive. The relevance of this research is underscored by the pressing need to protect the forest. At a global level, the world's forest is devastated annually by forest fires, causing serious and widely recognized consequences. It is true that the prescribed burning is a cost-effective method for reducing hazardous; however other solutions to eliminate forestry waste must first be evaluated, such as the energy use of forest residues. I think it would be important to address this topic in the introduction.
Thank you for these positive comments on our manuscript.
While we agree with Reviewer #2 that evaluating alternative fuel reduction treatments and the use of thinned and chipped forest products are important concepts, we believe that adding additional paragraphs to our manuscript would distract from the main concepts:
1)The development and evaluation of more accurate combustion completeness factors for fuels in pine- and oak-dominated forests of the Pinelands, and 2) an evaluation of how fire behavior affects above-canopy sensible heat flux and turbulence in buoyant plumes above prescribed burns.
Our research group has not evaluated any other types of fuel reduction treatments in the Pinelands, as they have been conducted less frequently and in only limited areas. In our case, thinned trees are typically masticated and then scattered onsite to decompose.
Given the importance and relevance of the topic, I strongly believe that this article will significantly contribute to the advance scientific community, and I recommend accepting the article after addressing the following minor issues:
Thank you again for these positive comments on our manuscript.
1) the article must be written impersonally, so the third person is used in the passive voice, line 27, 127, 620, 649, 747, 827, …
We have reworded most the 1st person (we) passive voice terms to third person in the passive voice. One exception is where we introduce our major objectives at the end of the Introduction Section, and a few other exceptions occur in the Discussion section where the use of "we" seemed more appropriate.
2) the websites relating to the bibliography consulted must only appear in the list of references and not in the text, lines 152, 220, 787
We apologize for formatting these references incorrectly. We have moved these to the Reference section, and renumbered the references in the text and reference list.
3) why were the samples dried at 70°C? What is the reason? This is not explained in the text. Drying at 70 C is not completely drying.
We believe that 70 °C is a standard drying temperature for tissue samples. We placed samples in paper bags and dried them at 70 °C in commercial laboratory drying ovens for at least 72 hours, and then weighed samples to 0.1 g accuracy when dry. We have evaluated this method by drying samples for at least 72 hours, weighing them, and then placing them in the driers for an additional 24 hour period to ensure no more mass loss occurred.
4) in the sentence “ … emissions and Δ air temperature, Δ wind velocity, Δ 1-minute sensible heat, and Δ TKE values.” the use of delta in conjunction with the variable written in extensive form becomes confusing for the reader.
We have attempted to clearly define the Δ symbol throughout the manuscript. For example, at the first use of Δ symbol on lines 291-293, we state:
“Maximum differences between burn area and control towers (Δ values) were calculated by subtracting the appropriate control tower data from burn area tower data for each value for the identical time period.”
We believe that the use of this symbol is the clearest way to denote the difference between data collected between burn area and control towers.
5) The caption for figure 4 must be next to the images, and not on another sheet.
We agree, and we will work with the editorial staff at the Journal Fire to format our manuscript correctly.
Since the standard deviation (air temp, RH and windspeed) is shown in table 5, these images may be unnecessary.
We believe that Figure 4 should remain in the manuscript, as it illustrates the differences in instantaneous (10 Hz) values of air temperature, and vertical and horizontal wind velocities that can occur in buoyant plumes during fire front passage in low- vs. high intensity burns. These are essentially the "raw" datasets that are used to calculate 1-minute sensible heat flux and TKE shown in Figure 7, and help illuminate how information from each tower was processed in the manuscript.
Table 5 is presenting ambient weather conditions during each prescribed burn collected from control towers, thus are independent datasets that do not include any information on air temperature or windspeed measured above prescribed burns on burn area towers.
6) The caption for Table 7 must also be on the same page as table 7, on page 17.
We agree, and again we will work with the editorial staff at the Journal Fire to format our manuscript correctly.
Round 2
Reviewer 1 Report
Comments and Suggestions for AuthorsI'm happy with the responses and the revised manuscript. I've nothing further to add.
