Increase in Carbon Monoxide (CO) and Aerosol Optical Depth (AOD) observed by satellite in the northern hemisphere over the summers of 2008&ndash;2023, linked to an increase in wildfires

Ehret, Antoine; Turquety, Solène; George, Maya; Hadji-Lazaro, Juliette; Clerbaux, Cathy

doi:https://doi.org/10.5194/egusphere-2024-3128

Preprints

https://doi.org/10.5194/egusphere-2024-3128

Preprints

14 Oct 2024

| 14 Oct 2024

Increase in Carbon Monoxide (CO) and Aerosol Optical Depth (AOD) observed by satellite in the northern hemisphere over the summers of 2008–2023, linked to an increase in wildfires

Antoine Ehret, Solène Turquety, Maya George, Juliette Hadji-Lazaro, and Cathy Clerbaux

Abstract. Biomass burning has a significant impact on the composition of the atmosphere due to large emissions of trace gases and aerosols. Previous studies have demonstrated the influence of biomass burning emissions on the spatial and temporal variability of carbon monoxide (CO) and aerosols concentration on hemispheric scales. This study aims to examine the correlation between fire variability and the mean and extreme values of CO and aerosol optical depth (AOD) observed by satellite (IASI/Metop for total column CO and MODIS/Terra and Aqua for AOD), focusing on the extratropical Northern Hemisphere (NH) from 2008 to 2023. While biomass burning due to agricultural practices is decreasing in many regions, boreal regions and the western United States have experienced a rise in burned area, up to 37 % in recent years (2017–2023) compared to the 2008–2023 period. This is consistent with an increase in meteorological fire risk in these regions. The increase in wildfires has led to a rise in the mean and extreme values of CO and AOD during the summer and early autumn across all NH, reaching 9.3 % and 33 % for extreme total CO and AOD in boreal regions and the western United States in recent years compared to 2008–2023. The number of days with extreme total CO and AOD has increased by over 50 % in recent years during summer in North America, the Atlantic and Europe, compared to the full period. A robust correlation (r=0.83) between the number of plumes and burned areas in the extratropical NH is obtained.

Received: 07 Oct 2024 – Discussion started: 14 Oct 2024

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Journal article(s) based on this preprint

26 Jun 2025

Increase in carbon monoxide (CO) and aerosol optical depth (AOD) observed by satellites in the Northern Hemisphere over the summers of 2008–2023, linked to an increase in wildfires

Antoine Ehret, Solène Turquety, Maya George, Juliette Hadji-Lazaro, and Cathy Clerbaux

Atmos. Chem. Phys., 25, 6365–6394, https://doi.org/10.5194/acp-25-6365-2025,https://doi.org/10.5194/acp-25-6365-2025, 2025

Short summary

Antoine Ehret, Solène Turquety, Maya George, Juliette Hadji-Lazaro, and Cathy Clerbaux

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2024-3128', Anonymous Referee #1, 13 Nov 2024
In general, the manuscript discussed the change in wildfire and wildfire emissions in a wide range of the northern hemisphere using widely used variables to represent fire (fire activity, burned area, fire radiative power), land conditions (fire weather index), and emissions (CO and AOD). These selected variables permit a good comparison with the literature, and those variables and the distinct techniques used to analyse changes provide robust results in the manuscript. The regional base analysis for the northern region made it possible to discuss the change and trend of fire emission plume transport between them. This manuscript is suitable for publication in ACP but requires some improvements, mainly in consistency and structure.
General comments:
GC1: I noticed that the main objective of the study shifts throughout the manuscript. The objectives highlighted below were announced in order in the abstract, introduction, and methodology, but they are all different. I identified that they were actually addressed in the manuscript, so the suggestion is to coherently announce that the investigation has multiple objectives or announce them as part of a more general objective.
Abstract L3: “This study aims to examine the correlation between fire variability and the mean and extreme values of CO and aerosol optical depth (AOD) observed by satellite (IASI/Metop for total column CO and MODIS/Terra and Aqua for AOD), focusing on the extratropical Northern Hemisphere (NH) from 2008 to 2023.”
Introduction L73: “This study aims to quantify the impact of fires on the long-range transport of pollution in the Northern Hemisphere during the period 2008-2023, and how it has changed in recent years.”
L51: “The purpose of this article is to present an analysis of the extreme values of burned area, fire weather index, CO concentrations and AOD between 2008 and 2023 in the Northern Hemisphere.”
GC2: I would suggest rewording the start of Section 3 to “Results” or “Results: Variability and Trends in Fire Activity” or something along these lines. At present, it is unclear that Section 3 is actually the start of the results and Section 2 is the end of the Methods & Data section. The same is true for Section 2, currently “Observations and Indicators”. I would suggest “Data & Methods” or something similar.
Repetitive titles “3.2 Trends during 2008–2023”, “3.2 Trends during 2008–2023”
GC3. I've noticed that the authors often refer to the entire figure even when only a specific sub-panel is relevant to what is being described in the text. For example, in L279, the author refers to Fig. 5 but only describes Fig. 5a and Fig. 5b. Furthermore, since the figure has already been mentioned, the author didn’t mention when a specific subfigure should be highlighted. On L286, for instance, the reference to Figure 5c would enhance the understanding of the message.
Specific comments:
L83. In this paragraph, “Section 3.3” and “Section 4.4.” are summarised, but what happened with sections 3.1, 3.2, 4.1, 4.2, …? I appreciate the authors want to highlight some key sections; however, it would be nice to bring the general picture of the content of sections 3, 4 and 5 as well.
“TCO”? I believe this is the total column CO, but it was never referenced like that before in the manuscript.

L151, L166,L323. Here, and throughout the manuscript, the authors make reference to “CO concentration(s)”. The retrieved quantity from the satellite is a total column and NOT a concentration. Please update the manuscript accordingly.
L93. Reference?
L155. I suggest making X even clearer, with something such as “For each variable X (i.e., total CO, AOD, burned area or fire weather index)”.
L155. I imagine that the 97^th percentile is calculated over the fire seasons during the study period, right? All seasons from 2008 to 2023 were put together, and then the percentage 97^th was calculated, and the values below were excluded. Am I right? If so please add “during the study period”.
L157. The definitions of extreme events and plumes are a bit mixed in the paragraph. The paragraph should be rewritten to make it clearer to the reader.
What I understood is the following:
For AOD and total CO, the defined extreme events track extreme fire plumes. Besides the high CO and AOD load, the accounted plumes should be surrounded by pixels with extreme events. For that, in a box in size of 20°× 24° (i.e., ), the plumes are required to contain at least 5% of pixels categorised as extreme events for at least two days.
If correct, are the boxes already defined before the extreme event definition, or are they created based on the identified plumes?, And if so, how is this determined?
L185. I suggest being as specific as Eastern Central Asia (ECEAS), saying that Western boreal Asia (WBOAS) (i.e., not BOAS in general) has a large, burned area in spring. Additionally, more than “large BA are also observed during spring.” To be more transparent, I would say something like “large BA, frequently higher than in summer, are observed during spring.”
L192. The classified zone can be improved in consistency. For instance, for the type 1 region, fire radiative power wasn’t mentioned, and for type 1 and 2, the duration of the fires wasn’t mentioned. For types 1 and 2, the frequency is described, but the type 3 doesn’t have a description.
L202. I don’t completely understand the selection of the regions of primary interest, especially the exclusion of Western Central Asia.
Table A2. (As in Fig. 9) I find it beneficial to add the significance of the difference between Xrecent and Xfull to support the positive or negative value in this table. For instance, is extreme FWI significantly higher in Xfull for Europe than in Xrecent? With that calculated, stronger results can be presented, for instance, in L293: “The seasonal average FWI is lower during recent years across all periods.” Maybe significantly (?).
L316. I don’t see the high values mentioned there, but perhaps it is because of the colour bar. It might be worth using a more quantitative description, perhaps.
L387. I suggest keeping the labels for the regions selected at the beginning (i.e., use Eastern Boreal Asia and Eastern Central Asia, not only Central Asia). For example, in “Negative trends are observed in Eastern Asia and the Pacific Ocean for both the average and the extreme AOD,” I think the author refers to Eastern Central Asia only because I don’t see the negative trend described for Eastern Boreal Asia (it seems slightly positive).
L230 and L417. This particular way of calculating anomalies should be described in the methods. This would be beneficial after the dates for the recent and full periods were introduced.
Figure 11. I am confused by this figure, which is excellent at showing different features. Just one clarification: the burn area plot in x-axes excluded the Western Central and Boreal Asia regions, but the fractions presented by the pie charts include them as also include a category named “other”. Is that right? If so, what do others include? Are others considered in the x-axis burned area? As the different circle diameters diagram at the bottom left of the figure clarifies that the diameter is the total CO (it says ‘concentrations’), can you provide a legend which explains what the pie chart is describing? Maybe something like a segment of the circle and a small label would be enough.
L496. It was only until I read the complete label of Figure 11 that I was sure the number of plumes was added for all the regions. It might also be good to write that explicitly in the paragraph. Perhaps something like: “Figure 11 illustrates the cumulative number of days with total CO plumes (i.e., the sum of days with a detected plume in each region) and the total burned area for June-October of every year in the study period. Both cumulative days with plume and total burned area were calculated for the latitude range above 30°N, excluding the Western Central and Boreal Asia regions.” It is unclear if the second sentence is true; I don’t see it as explicit. Are both variables calculated with those conditions?
L510. The repetitive fragment “excluding the Western Central and Boreal Asia regions” is unclear. I am unsure if you are excluding Western Central Asia and Boreal Asia (as a whole) or Western Central Asia and Western Boreal Asia. This would not be the first time that Boreal Asia is named as a whole (e.g. L418), so it is still possible. I suggest consistently using the whole name introduced from the beginning (e.g. Western Central Asia and Western Boreal Asia).
L593. It is worth mentioning again the examples of agricultural regions.
L605. It might be worth discussing the role of land cover change in CO emissions and how this may change relations discussed here as AOD/CO or CO/ Burned area.
Technical corrections
L27 “The variation,” which variations? The sentence was written as a variation introduced before, but it was talking about stability. Maybe it would be better just to say, “The emissions.” This sentence needs a reference; I think it is the same source as the previous sentence (van Marle et al ., 2017), but this is not always true.
L33. I would suggest: “The extent of a wildfire after ignition depends on the availability of fuel and weather conditions (e.g., hot and dry conditions being favourable to propagation)”.
L35. I would suggest the verb phrasal verb “derived from” instead of “driven by” since the index is calculated based on those variables.
L58. The sentence “The plumes having CO concentrations well above background values, CO is considered a good proxy for biomass fire smoke plumes” needs to be rewritten. Maybe remove the second “CO”.
L183. It seems that the abbreviation BA was mentioned here for the first time without being introduced before. This is introduced for the first time in L210.
Caption on Fig. A1., Fig. 3 Please warn the reader about the different y-axis in the subplots.
Figure 6. In the colour bar of Total CO, the abbreviation TCO hasn’t been explained before.
Citation: https://doi.org/10.5194/egusphere-2024-3128-RC1
- AC1: 'Reply on RC1', Antoine Ehret, 13 Mar 2025
  
  You will find the answers (approved by all the authors of the manuscript) to the comments of the two reviewers in the attached document. Yours sincerely,
  Antoine Ehret
  
  Citation: https://doi.org/10.5194/egusphere-2024-3128-AC1
CC1:
'Comment on egusphere-2024-3128', Brice Barret, 26 Nov 2024

This paper provides very valuable information about the impact of fires on the evolution of atmospheric CO and particles. Concerning IASI CO used to infer the trend of this trace gaz, it is mentioned “The CO columns are obtained from the CO partial column profiles retrieved from the IASI spectra (Hurtmans et al., 2012), reprocessed backward in time by Eumetsat (CO CDR [...]) to provide a consistant climate record from 2008 to 2023”.
The reference to (Hurtmans et al., 2012) is rather old. Together with colleagues co-author to the present study, we performed recently a global validation study of two IASI-A-CO products with in-situ IAGOS airborne data for the 2008-2019 (Barret et al., Egusphere 2024, accepted for publication in AMT). Among these products is a version of FORLI-CO (v20151001) older than CO CDR used here. The validation with the longest and densest time serie of IAGOS CO recorded at Frankfurt has shown that FORLI-CO was impacted by discontinuities with larger negative biases between 2011 and 2015 than for the other periods probably related to discontinuities in the EUMETSAT temperature L2 products used for the FORLI radiative transfer calculations. The use of updated EUMETSAT L2 data has probably reduced these discontinuities and improve the evaluation of CO trends with CO CDR. Nevertheless this should be discussed and demonstrated (by comparisons of the two products for instance) to strengthen the the CO trend evaluation.

Citation: https://doi.org/10.5194/egusphere-2024-3128-CC1
- AC3: 'Reply on CC1', Antoine Ehret, 13 Mar 2025
  
  You will find the answers (approved by all the authors of the manuscript) to the community comment in the attached document. Yours sincerely,
  Antoine Ehret
  
  Citation: https://doi.org/10.5194/egusphere-2024-3128-AC3
RC2:
'Comment on egusphere-2024-3128', Anonymous Referee #2, 30 Jan 2025

The authors investigate mean and extreme carbon monoxide (CO) and aerosol optical depth (AOD) measured from space between 2008 and 2023. They focus on regions of the Northern Hemisphere (NH) and evaluate trends and variability for the fire season June to October. They find larger values for both mean and extreme values of CO and AOD for summer and autumn in recent years since 2017 for most regions, than for the full record. The study uses satellite measured burned area to link composition trends over extratropical NH with biomass burning signatures. The authors postulate through investigating fire weather index, where changing weather has impacted BB and where influences are from potential direct human activity.
Long term measurements, like the ones presented in this manuscript, are extremely important in understanding atmospheric chemistry changes. Evaluation of atmospheric chemistry long-term behavior, as is achieved in this manuscript, is imperative for understanding how pollution sources such as biomass burning might be changing over time. The manuscript is very well written and the methodology is rigorous. I have some minor comments below.
------------------------------------------------------
Comments:
1. The manuscript would benefit from a description of why the reference period for comparison was chosen to be the whole record (2008–2023) instead of an earlier period (such as 2008 – 2016). I suggest to add this justification in around Line 178.
2. Line 161: I am a little confused about the definition of spatial anomalies and the usage of the larger boxes. How was the box size 20 degrees × 24 degrees chosen as the spatial extent for plume analysis, and how exactly is it used on plumes? For example, is it centered on each anomaly point in turn?
3. Figure 1: I suggest adding a box to highlight the region “above the Atlantic”.
4. On line 246 to 247, the definitions are slightly different to the definitions on line 179. It would be wise to be consistent with the definitions. I suggest using the latter definitions, which are a little more general: full season (June–October), the early season (June–July), the middle season (July–August) or the late season (September– October).
5. On page 7, it might be valuable to give the three main regimes heading names. For example, 1: Human-managed fire 2: Wildfire, 3. Remote fire
6. In the caption of Figure 4, consider mentioning the blue diamonds are the difference values presented in Table 1.
7. Line 310, 312, and elsewhere. My opinion is “total CO” would be better described as “total column CO”. Because this work only evaluates total column CO, I would suggest to define/clarify that in section 2.2 as well as at the beginning of section 4.1, and then continue to talk about “CO” (without the “total” classifier).
8. Line 316: Approximately what values are considered “particularly high values of CO”. It is difficult for me to determine the high values from the color scale in Fig 2 c).
9. In central Asia, I am wondering if the authors have considered clouds or high aerosols affecting retrievals being a contributing factor to low CO measured there?
10. Line 344: Add more details about how the correlation coefficient is calculated between CO and AOD, e.g. how is it aggregated over time and space?
------------------------------------------------------
Technical Corrections:
Line 42 to 44: This sentence is a little convoluted and difficult to understand. Consider re-wording.

Citation: https://doi.org/10.5194/egusphere-2024-3128-RC2
- AC2: 'Reply on RC2', Antoine Ehret, 13 Mar 2025
  
  You will find the answers (approved by all the authors of the manuscript) to the comments of the two reviewers in the attached document. Yours sincerely,
  Antoine Ehret
  
  Citation: https://doi.org/10.5194/egusphere-2024-3128-AC2

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2024-3128', Anonymous Referee #1, 13 Nov 2024
In general, the manuscript discussed the change in wildfire and wildfire emissions in a wide range of the northern hemisphere using widely used variables to represent fire (fire activity, burned area, fire radiative power), land conditions (fire weather index), and emissions (CO and AOD). These selected variables permit a good comparison with the literature, and those variables and the distinct techniques used to analyse changes provide robust results in the manuscript. The regional base analysis for the northern region made it possible to discuss the change and trend of fire emission plume transport between them. This manuscript is suitable for publication in ACP but requires some improvements, mainly in consistency and structure.
General comments:
GC1: I noticed that the main objective of the study shifts throughout the manuscript. The objectives highlighted below were announced in order in the abstract, introduction, and methodology, but they are all different. I identified that they were actually addressed in the manuscript, so the suggestion is to coherently announce that the investigation has multiple objectives or announce them as part of a more general objective.
Abstract L3: “This study aims to examine the correlation between fire variability and the mean and extreme values of CO and aerosol optical depth (AOD) observed by satellite (IASI/Metop for total column CO and MODIS/Terra and Aqua for AOD), focusing on the extratropical Northern Hemisphere (NH) from 2008 to 2023.”
Introduction L73: “This study aims to quantify the impact of fires on the long-range transport of pollution in the Northern Hemisphere during the period 2008-2023, and how it has changed in recent years.”
L51: “The purpose of this article is to present an analysis of the extreme values of burned area, fire weather index, CO concentrations and AOD between 2008 and 2023 in the Northern Hemisphere.”
GC2: I would suggest rewording the start of Section 3 to “Results” or “Results: Variability and Trends in Fire Activity” or something along these lines. At present, it is unclear that Section 3 is actually the start of the results and Section 2 is the end of the Methods & Data section. The same is true for Section 2, currently “Observations and Indicators”. I would suggest “Data & Methods” or something similar.
Repetitive titles “3.2 Trends during 2008–2023”, “3.2 Trends during 2008–2023”
GC3. I've noticed that the authors often refer to the entire figure even when only a specific sub-panel is relevant to what is being described in the text. For example, in L279, the author refers to Fig. 5 but only describes Fig. 5a and Fig. 5b. Furthermore, since the figure has already been mentioned, the author didn’t mention when a specific subfigure should be highlighted. On L286, for instance, the reference to Figure 5c would enhance the understanding of the message.
Specific comments:
L83. In this paragraph, “Section 3.3” and “Section 4.4.” are summarised, but what happened with sections 3.1, 3.2, 4.1, 4.2, …? I appreciate the authors want to highlight some key sections; however, it would be nice to bring the general picture of the content of sections 3, 4 and 5 as well.
“TCO”? I believe this is the total column CO, but it was never referenced like that before in the manuscript.

L151, L166,L323. Here, and throughout the manuscript, the authors make reference to “CO concentration(s)”. The retrieved quantity from the satellite is a total column and NOT a concentration. Please update the manuscript accordingly.
L93. Reference?
L155. I suggest making X even clearer, with something such as “For each variable X (i.e., total CO, AOD, burned area or fire weather index)”.
L155. I imagine that the 97^th percentile is calculated over the fire seasons during the study period, right? All seasons from 2008 to 2023 were put together, and then the percentage 97^th was calculated, and the values below were excluded. Am I right? If so please add “during the study period”.
L157. The definitions of extreme events and plumes are a bit mixed in the paragraph. The paragraph should be rewritten to make it clearer to the reader.
What I understood is the following:
For AOD and total CO, the defined extreme events track extreme fire plumes. Besides the high CO and AOD load, the accounted plumes should be surrounded by pixels with extreme events. For that, in a box in size of 20°× 24° (i.e., ), the plumes are required to contain at least 5% of pixels categorised as extreme events for at least two days.
If correct, are the boxes already defined before the extreme event definition, or are they created based on the identified plumes?, And if so, how is this determined?
L185. I suggest being as specific as Eastern Central Asia (ECEAS), saying that Western boreal Asia (WBOAS) (i.e., not BOAS in general) has a large, burned area in spring. Additionally, more than “large BA are also observed during spring.” To be more transparent, I would say something like “large BA, frequently higher than in summer, are observed during spring.”
L192. The classified zone can be improved in consistency. For instance, for the type 1 region, fire radiative power wasn’t mentioned, and for type 1 and 2, the duration of the fires wasn’t mentioned. For types 1 and 2, the frequency is described, but the type 3 doesn’t have a description.
L202. I don’t completely understand the selection of the regions of primary interest, especially the exclusion of Western Central Asia.
Table A2. (As in Fig. 9) I find it beneficial to add the significance of the difference between Xrecent and Xfull to support the positive or negative value in this table. For instance, is extreme FWI significantly higher in Xfull for Europe than in Xrecent? With that calculated, stronger results can be presented, for instance, in L293: “The seasonal average FWI is lower during recent years across all periods.” Maybe significantly (?).
L316. I don’t see the high values mentioned there, but perhaps it is because of the colour bar. It might be worth using a more quantitative description, perhaps.
L387. I suggest keeping the labels for the regions selected at the beginning (i.e., use Eastern Boreal Asia and Eastern Central Asia, not only Central Asia). For example, in “Negative trends are observed in Eastern Asia and the Pacific Ocean for both the average and the extreme AOD,” I think the author refers to Eastern Central Asia only because I don’t see the negative trend described for Eastern Boreal Asia (it seems slightly positive).
L230 and L417. This particular way of calculating anomalies should be described in the methods. This would be beneficial after the dates for the recent and full periods were introduced.
Figure 11. I am confused by this figure, which is excellent at showing different features. Just one clarification: the burn area plot in x-axes excluded the Western Central and Boreal Asia regions, but the fractions presented by the pie charts include them as also include a category named “other”. Is that right? If so, what do others include? Are others considered in the x-axis burned area? As the different circle diameters diagram at the bottom left of the figure clarifies that the diameter is the total CO (it says ‘concentrations’), can you provide a legend which explains what the pie chart is describing? Maybe something like a segment of the circle and a small label would be enough.
L496. It was only until I read the complete label of Figure 11 that I was sure the number of plumes was added for all the regions. It might also be good to write that explicitly in the paragraph. Perhaps something like: “Figure 11 illustrates the cumulative number of days with total CO plumes (i.e., the sum of days with a detected plume in each region) and the total burned area for June-October of every year in the study period. Both cumulative days with plume and total burned area were calculated for the latitude range above 30°N, excluding the Western Central and Boreal Asia regions.” It is unclear if the second sentence is true; I don’t see it as explicit. Are both variables calculated with those conditions?
L510. The repetitive fragment “excluding the Western Central and Boreal Asia regions” is unclear. I am unsure if you are excluding Western Central Asia and Boreal Asia (as a whole) or Western Central Asia and Western Boreal Asia. This would not be the first time that Boreal Asia is named as a whole (e.g. L418), so it is still possible. I suggest consistently using the whole name introduced from the beginning (e.g. Western Central Asia and Western Boreal Asia).
L593. It is worth mentioning again the examples of agricultural regions.
L605. It might be worth discussing the role of land cover change in CO emissions and how this may change relations discussed here as AOD/CO or CO/ Burned area.
Technical corrections
L27 “The variation,” which variations? The sentence was written as a variation introduced before, but it was talking about stability. Maybe it would be better just to say, “The emissions.” This sentence needs a reference; I think it is the same source as the previous sentence (van Marle et al ., 2017), but this is not always true.
L33. I would suggest: “The extent of a wildfire after ignition depends on the availability of fuel and weather conditions (e.g., hot and dry conditions being favourable to propagation)”.
L35. I would suggest the verb phrasal verb “derived from” instead of “driven by” since the index is calculated based on those variables.
L58. The sentence “The plumes having CO concentrations well above background values, CO is considered a good proxy for biomass fire smoke plumes” needs to be rewritten. Maybe remove the second “CO”.
L183. It seems that the abbreviation BA was mentioned here for the first time without being introduced before. This is introduced for the first time in L210.
Caption on Fig. A1., Fig. 3 Please warn the reader about the different y-axis in the subplots.
Figure 6. In the colour bar of Total CO, the abbreviation TCO hasn’t been explained before.
Citation: https://doi.org/10.5194/egusphere-2024-3128-RC1
- AC1: 'Reply on RC1', Antoine Ehret, 13 Mar 2025
  
  You will find the answers (approved by all the authors of the manuscript) to the comments of the two reviewers in the attached document. Yours sincerely,
  Antoine Ehret
  
  Citation: https://doi.org/10.5194/egusphere-2024-3128-AC1
CC1:
'Comment on egusphere-2024-3128', Brice Barret, 26 Nov 2024

This paper provides very valuable information about the impact of fires on the evolution of atmospheric CO and particles. Concerning IASI CO used to infer the trend of this trace gaz, it is mentioned “The CO columns are obtained from the CO partial column profiles retrieved from the IASI spectra (Hurtmans et al., 2012), reprocessed backward in time by Eumetsat (CO CDR [...]) to provide a consistant climate record from 2008 to 2023”.
The reference to (Hurtmans et al., 2012) is rather old. Together with colleagues co-author to the present study, we performed recently a global validation study of two IASI-A-CO products with in-situ IAGOS airborne data for the 2008-2019 (Barret et al., Egusphere 2024, accepted for publication in AMT). Among these products is a version of FORLI-CO (v20151001) older than CO CDR used here. The validation with the longest and densest time serie of IAGOS CO recorded at Frankfurt has shown that FORLI-CO was impacted by discontinuities with larger negative biases between 2011 and 2015 than for the other periods probably related to discontinuities in the EUMETSAT temperature L2 products used for the FORLI radiative transfer calculations. The use of updated EUMETSAT L2 data has probably reduced these discontinuities and improve the evaluation of CO trends with CO CDR. Nevertheless this should be discussed and demonstrated (by comparisons of the two products for instance) to strengthen the the CO trend evaluation.

Citation: https://doi.org/10.5194/egusphere-2024-3128-CC1
- AC3: 'Reply on CC1', Antoine Ehret, 13 Mar 2025
  
  You will find the answers (approved by all the authors of the manuscript) to the community comment in the attached document. Yours sincerely,
  Antoine Ehret
  
  Citation: https://doi.org/10.5194/egusphere-2024-3128-AC3
RC2:
'Comment on egusphere-2024-3128', Anonymous Referee #2, 30 Jan 2025

The authors investigate mean and extreme carbon monoxide (CO) and aerosol optical depth (AOD) measured from space between 2008 and 2023. They focus on regions of the Northern Hemisphere (NH) and evaluate trends and variability for the fire season June to October. They find larger values for both mean and extreme values of CO and AOD for summer and autumn in recent years since 2017 for most regions, than for the full record. The study uses satellite measured burned area to link composition trends over extratropical NH with biomass burning signatures. The authors postulate through investigating fire weather index, where changing weather has impacted BB and where influences are from potential direct human activity.
Long term measurements, like the ones presented in this manuscript, are extremely important in understanding atmospheric chemistry changes. Evaluation of atmospheric chemistry long-term behavior, as is achieved in this manuscript, is imperative for understanding how pollution sources such as biomass burning might be changing over time. The manuscript is very well written and the methodology is rigorous. I have some minor comments below.
------------------------------------------------------
Comments:
1. The manuscript would benefit from a description of why the reference period for comparison was chosen to be the whole record (2008–2023) instead of an earlier period (such as 2008 – 2016). I suggest to add this justification in around Line 178.
2. Line 161: I am a little confused about the definition of spatial anomalies and the usage of the larger boxes. How was the box size 20 degrees × 24 degrees chosen as the spatial extent for plume analysis, and how exactly is it used on plumes? For example, is it centered on each anomaly point in turn?
3. Figure 1: I suggest adding a box to highlight the region “above the Atlantic”.
4. On line 246 to 247, the definitions are slightly different to the definitions on line 179. It would be wise to be consistent with the definitions. I suggest using the latter definitions, which are a little more general: full season (June–October), the early season (June–July), the middle season (July–August) or the late season (September– October).
5. On page 7, it might be valuable to give the three main regimes heading names. For example, 1: Human-managed fire 2: Wildfire, 3. Remote fire
6. In the caption of Figure 4, consider mentioning the blue diamonds are the difference values presented in Table 1.
7. Line 310, 312, and elsewhere. My opinion is “total CO” would be better described as “total column CO”. Because this work only evaluates total column CO, I would suggest to define/clarify that in section 2.2 as well as at the beginning of section 4.1, and then continue to talk about “CO” (without the “total” classifier).
8. Line 316: Approximately what values are considered “particularly high values of CO”. It is difficult for me to determine the high values from the color scale in Fig 2 c).
9. In central Asia, I am wondering if the authors have considered clouds or high aerosols affecting retrievals being a contributing factor to low CO measured there?
10. Line 344: Add more details about how the correlation coefficient is calculated between CO and AOD, e.g. how is it aggregated over time and space?
------------------------------------------------------
Technical Corrections:
Line 42 to 44: This sentence is a little convoluted and difficult to understand. Consider re-wording.

Citation: https://doi.org/10.5194/egusphere-2024-3128-RC2
- AC2: 'Reply on RC2', Antoine Ehret, 13 Mar 2025
  
  You will find the answers (approved by all the authors of the manuscript) to the comments of the two reviewers in the attached document. Yours sincerely,
  Antoine Ehret
  
  Citation: https://doi.org/10.5194/egusphere-2024-3128-AC2

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

AR by Antoine Ehret on behalf of the Authors (13 Mar 2025) Author's response Author's tracked changes Manuscript

ED: Publish as is (19 Mar 2025) by Chris Wilson

AR by Antoine Ehret on behalf of the Authors (24 Mar 2025) Manuscript

Journal article(s) based on this preprint

26 Jun 2025

Increase in carbon monoxide (CO) and aerosol optical depth (AOD) observed by satellites in the Northern Hemisphere over the summers of 2008–2023, linked to an increase in wildfires

Antoine Ehret, Solène Turquety, Maya George, Juliette Hadji-Lazaro, and Cathy Clerbaux

Atmos. Chem. Phys., 25, 6365–6394, https://doi.org/10.5194/acp-25-6365-2025,https://doi.org/10.5194/acp-25-6365-2025, 2025

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Antoine Ehret, Solène Turquety, Maya George, Juliette Hadji-Lazaro, and Cathy Clerbaux

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Biomass burning has a considerable effect on the chemical composition of the atmosphere and climate, due to the emission of trace gases and aerosols. We examine the relationship between fire variability and the values of carbon monoxide and aerosol optical depth observed by satellite. The observed increase in wildfires has led to a corresponding rise in the mean and extreme values of carbon monoxide and aerosol optical depth during the summer and early autumn across the northern hemisphere.


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