the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 17 Apr 2025
Enhancing forest air sampling using a novel reusable ozone filter design
Abstract. Biogenic volatile organic compounds (BVOCs), such as monoterpenes, play essential roles in ecological and atmospheric processes, influencing air quality, climate and interspecies interactions. For accurate identification and quantification of these reactive compounds in the environment, active sampling on sorbent tubes followed by thermodesorption gas chromatography-mass spectrometry is commonly used. However, ozone present in the sampled air can degrade both the analytes and the sorbent material during the sampling process, leading to underestimation of target substances and overestimation of their degradation products. This study evaluates a novel reusable ozone filter designed for direct attachment to sorbent tubes and compatibility with multi-tube samplers. The filter utilizes potassium iodide (KI) or sodium thiosulfate (Na2S2O3) deposited on reusable glass filters and copper wool to improve the accuracy of BVOC measurements. Both types of ozone scrubbers were tested under varying ozone concentrations up to 50 ppb and relative humidity levels up to 90 %, utilizing a straightforward load-and-flush method as well as a permeation approach that simulates field sampling conditions. Furthermore, both methods were compared regarding their suitability for the systematic evaluation of ozone filters.
Results indicate, that both KI and Na2S2O3 effectively remove ozone, with KI showing a slightly higher performance and lower dependence on relative humidity, maintaining over 90 % removal efficiency even after 10 days of ambient air exposure. Recovery rates for four structurally different monoterpenes (α-Pinene, Myrcene, Limonene, Linalool) showed no significant differences between filtered and unfiltered samples at baseline ozone concentrations, demonstrating that the ozone filters did not negatively impact analyte recovery. When no filter was used, recovery rates for Myrcene, Limonene, and Linalool declined with increasing ozone concentration, while showing a method-dependent positive influence of increasing relative humidity. Both scrubber materials maintained high and comparable recovery rates across all tested conditions, except at very low relative humidity, thereby enhancing measurement accuracy and comparability under diverse environmental scenarios. Field tests confirmed the effectiveness of KI-loaded scrubbers in enhancing monoterpene detection in forest air while safeguarding the sorbent material. These results, combined with the easy reusability of the glass filters and the absence of additional equipment or power requirements, highlight that this scrubber design proves to be an optimal choice for the long-term environmental monitoring of volatile organic compounds.
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Notice on discussion status
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
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- Final revised paper
Journal article(s) based on this preprint
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2025-1387', Junfeng Liu, 16 May 2025
Biogenic volatile organic compounds (BVOCs) play essential roles in ecological and atmospheric processes, accurate identification and quantification of these reactive compounds in the environment is very important. The ozone present in the sampled air can degrade both the analytes and the sorbent material during the sampling process, leading to underestimation of target substances and overestimation of their degradation products. This study evaluates a reusable ozone filter designed for direct attachment to sorbent tubes and compatibility with multi-tube samplers. Results indicate, that both KI and Na₂S₂O₃ effectively remove ozone, with KI showing a slightly higher performance and lower dependence on relative humidity.
Overall, the results are meaningful for air pollution research. The manuscript is well organized and well written in accurate English using the correct synonyms to convey what the authors mean. Taking all factors above into consideration, I could favor of publishing this manuscript in the journal of “Atmospheric Measurement Techniques” after minor revision.
Minor comments:
- Figure 2 is not clear. Using different color, so in Figure 4.
- Figure 3. The time is so short for the ozone is raising quickly for the Na2S2O3-loaded filter.
- Figure 5. The result of the recovery rates of linalool at RH<7% is very difficult for understand, please repeat this experiment to ensure the correctness and repeatability of the experimental results.
Citation: https://doi.org/10.5194/egusphere-2025-1387-RC1 -
AC1: 'Reply on RC1', Robby Rynek, 17 Jun 2025
Dear Junfeng Liu,
Thank you for reviewing our manuscript and your positive evaluation. Please find our response to your comments below.
1. In the revised manuscript, we slightly adjusted the color scheme of Figures 2 and 4 to make the graphs more distinguishable.2. Indeed, the downstream ozone concentration starts to increase during the first two hours of flushing with air (60% rH, 50 ppb O3) at 80 mL/min. This is in contrast to the KI-loaded filters, which showed stable ozone concentrations at baseline level over the course of 10 hours. Nevertheless, the overall increase of the ozone concentration from around 5 ppb to around 9 ppb over the course of 10 hours is still relatively low, given the input concentration of 50 ppb.
3. This experiment was already repeated to ensure the correctness. In both cases, a decrease of the Linalool concentration with increasing ozone concentration was observed, both with and without the use of one of the ozone filters. The comparably high variations in the triplicates was also observed, showing that both ozone filters are not reliably depleting ozone under these conditions.
Citation: https://doi.org/10.5194/egusphere-2025-1387-AC1
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RC2: 'Comment on egusphere-2025-1387', Anonymous Referee #2, 26 May 2025
Biogenic volatile organic compounds such as monoterpenes are crucial for ecological and atmospheric processes, impacting air quality, climate, and interspecies interactions. This study introduces a novel reusable ozone filter for sorbent tubes to enhance the accuracy of BVOC measurements using a combination of potassium iodide (KI) or sodium thiosulfate (Na₂S₂O₃) and copper wool, tested across varying ozone concentrations and humidity levels. The results indicate both filter materials effectively remove ozone and improve measurement accuracy and comparability under diverse environmental conditions, with KI-based filters particularly enhancing monoterpene detection in forest air while protecting sorbent materials, demonstrating their suitability for long-term environmental monitoring of volatile organic compounds.
I find the scientific thesis under review to be exceptionally comprehensive. The experimental designs appear to have been carefully planned and executed with precision, while the data analysis is both lucidly presented and easily understandable. The readability of the document is notably high, enhancing its accessibility. Additionally, as a non-native English speaker, the English used throughout the document seems to me to be correct and well-formulated. The filter developed in this work is proposed as a viable, practical solution that has the potential to be adopted by future practitioners. Consequently, it is poised to become a pioneering approach for the measurement of terpenes.
I have just some minior comments:
Page 7 Line 191: Probably I haven’t read that. How do you know that both samplers perform in the same way?
Page 12 Line 320 ff: Four different humidities, three ozone concentrations each, and two scrubbers amount to 24 measurements, right. For each set, you measured three filtered and one unfiltered. ...I don’t get the 192.
Figure 3: Add in the caption “…over the course of 10 hours.”
Results in Figures 5 and 6: Do you have any idea why the recovery rate is that much above 100%?
Citation: https://doi.org/10.5194/egusphere-2025-1387-RC2 -
AC2: 'Reply on RC2', Robby Rynek, 17 Jun 2025
Dear Reviewer #2,
thank you for carefully reviewing our manuscript, the very positive evaluation and your constructive feedback. Please find below our response to your comments.
1. Both samplers are identical MTS-32 multi-tube sequential samplers (Markes International Ltd., UK), set up with the same parameters (sampling time, pump flow rate) and were operated in parallel next to each other. Additionally, the flow through all tube positions of both samplers was verified to be equal (±5%) before the experiment using a flow meter (7000 GC flowmeter, Ellutia Ltd., UK). This description was added to the revised manuscript.2. To evaluate the ozone filters, we used two scrubber materials (KI, Na2S2O3), two approaches (load-and-flush, permeation), each at four relative humidity levels (<7%, 30%, 60%, 90%) with three ozone concentrations (baseline, 25 ppb, 30 ppb). For each experiment, three samples and one control sample (without filter) were generated, resulting in a total number of 192 samples.
3. Thank you for pointing out the missing information. The figure capture was modified according to your suggestion.
4. Recovery rates exceeding 100% in the load-and-flush approach are likely attributable to inaccuracies in the preparation of the custom-made stock solution used for the experiments and the application of the diluted standard solution using a microliter syringe. In contrast, the calibration curves used for quantification were based on a commercially available, certified stock solution, offering greater accuracy.
In the evaluation of the permeation experiments, recovery rates were normalized to those of the unfiltered samples at baseline ozone concentrations at the corresponding relative humidity levels. As a result, values exceeding 100% are artifacts of the normalization procedure. Without normalization, the recovery rates were below 100%, except for α Pinene at <7% rH and <25 ppb O3, as detailed in lines 382f and illustrated Figure S5.Citation: https://doi.org/10.5194/egusphere-2025-1387-AC2
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AC2: 'Reply on RC2', Robby Rynek, 17 Jun 2025
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RC3: 'Comment on egusphere-2025-1387', Anonymous Referee #3, 14 Jun 2025
The article was written in a fairly good systematic manner by presenting a novel reusable ozone filter design for enhancing forest air sampling of biogenic volatile organic compounds (BVOCs), such as monoterpenes. The article evaluated reusable ozone filter design for the monitoring of biogenic volatile organic compounds using sorbent tubes and multi-tube samplers. The efficiency of ozone scrubber material in the analysis of terpenes compound were investigated both in laboratory and field experiments.
Remarks/additional comments were written in the pdf file. Please see the attachment for the reviewer’s comments for this article.
-
AC3: 'Reply on RC3', Robby Rynek, 17 Jun 2025
Dear Reviewer #3,
thank you for carefully reviewing our manuscript and your constructive comments, which helped to improve our work. Please find below our response to your comments.
1. We adjusted the schematic layout of the ozone scrubber design according to your suggestion and added markers for the individual parts. The figure caption was adjusted accordingly.2. The glass filters were placed in a filter holder and rinsed with the respective salt solution until complete wetting, as described in lines 126f. We expanded the experimental description for loading the ozone filters and added the preparation of the salt solutions.
3. The flow requirements of the ozone monitor (>600 mL/min) were added in the revised manuscript.
4. The desired flow rate through the single glass filters (80 mL/min) was added in the revised manuscript.
5. An explanation of the experimental conditions was added to the “Experiment” columns in Tables S3 and S4. Additionally, the experiments at low relative humidity were renamed to “<7 – 0”, “<7 – 25” and “<7 – 50” to be consistent with the conditions named throughout the manuscript.
6. For the purpose of the manuscript, a detailed description of the TD-GC-MS results, aside from the measured concentrations, is not the objective and would be beyond the scope of this paper. We therefore prefer to not include this detailed information in the manuscript. Nevertheless, the used TD-GC-MS system was calibrated using a terpenes standard solution (Terpenes MegaMix Standard #1, Restek GmbH, Germany), showing a sufficient chromatographic separation. The consistency of retention times and mass spectra for the mixture of α Pinene, Myrcene, Limonene and Linalool used in the experiments was also verified prior to conducting the experiments.
7. Looking at the reactions of the ozone depleting reagents with ozone (Eq. 1 and 2), both reactions require water to take place. While water is a reactant in the ozone depletion reaction using KI, the reaction of Na2S2O3 takes place in an aqueous medium. When water is present in the sampled air stream, both reactions can occur, with higher efficiency at increasing relative humidity levels. At very low relative humidity levels, both reactions are inhibited due to the lack of a reactant for KI and a suitable reaction medium for Na2S2O3. An explanation of the results was added to the revised manuscript.
8. Yes, the KI-loaded ozone filters still have the same ozone removal potential after 10 days of exposure to ambient air, as shown in Figure 4. Please note that exposure does not mean they are actively flushed with sampled air. Using the same ozone filter twice was not tested, but could be possible based on the breakthrough experiments (10 hours of flushing). Nevertheless, based on our experience from field experiments, we would advise against reusing the filters for a second sampling without applying new reagent, since visual changes (i.e. yellowing) of the filters are observed regularly.
9. We added a simplified reaction of copper with ozone in the revised manuscript. The detailed reaction mechanism would go beyond the scope of our work. More detailed information can be found in the cited publications by Kim et al. (2023), Lin and Frankel (2013), and Ma et al. (2024).
Citation: https://doi.org/10.5194/egusphere-2025-1387-AC3
-
AC3: 'Reply on RC3', Robby Rynek, 17 Jun 2025
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2025-1387', Junfeng Liu, 16 May 2025
Biogenic volatile organic compounds (BVOCs) play essential roles in ecological and atmospheric processes, accurate identification and quantification of these reactive compounds in the environment is very important. The ozone present in the sampled air can degrade both the analytes and the sorbent material during the sampling process, leading to underestimation of target substances and overestimation of their degradation products. This study evaluates a reusable ozone filter designed for direct attachment to sorbent tubes and compatibility with multi-tube samplers. Results indicate, that both KI and Na₂S₂O₃ effectively remove ozone, with KI showing a slightly higher performance and lower dependence on relative humidity.
Overall, the results are meaningful for air pollution research. The manuscript is well organized and well written in accurate English using the correct synonyms to convey what the authors mean. Taking all factors above into consideration, I could favor of publishing this manuscript in the journal of “Atmospheric Measurement Techniques” after minor revision.
Minor comments:
- Figure 2 is not clear. Using different color, so in Figure 4.
- Figure 3. The time is so short for the ozone is raising quickly for the Na2S2O3-loaded filter.
- Figure 5. The result of the recovery rates of linalool at RH<7% is very difficult for understand, please repeat this experiment to ensure the correctness and repeatability of the experimental results.
Citation: https://doi.org/10.5194/egusphere-2025-1387-RC1 -
AC1: 'Reply on RC1', Robby Rynek, 17 Jun 2025
Dear Junfeng Liu,
Thank you for reviewing our manuscript and your positive evaluation. Please find our response to your comments below.
1. In the revised manuscript, we slightly adjusted the color scheme of Figures 2 and 4 to make the graphs more distinguishable.2. Indeed, the downstream ozone concentration starts to increase during the first two hours of flushing with air (60% rH, 50 ppb O3) at 80 mL/min. This is in contrast to the KI-loaded filters, which showed stable ozone concentrations at baseline level over the course of 10 hours. Nevertheless, the overall increase of the ozone concentration from around 5 ppb to around 9 ppb over the course of 10 hours is still relatively low, given the input concentration of 50 ppb.
3. This experiment was already repeated to ensure the correctness. In both cases, a decrease of the Linalool concentration with increasing ozone concentration was observed, both with and without the use of one of the ozone filters. The comparably high variations in the triplicates was also observed, showing that both ozone filters are not reliably depleting ozone under these conditions.
Citation: https://doi.org/10.5194/egusphere-2025-1387-AC1
-
RC2: 'Comment on egusphere-2025-1387', Anonymous Referee #2, 26 May 2025
Biogenic volatile organic compounds such as monoterpenes are crucial for ecological and atmospheric processes, impacting air quality, climate, and interspecies interactions. This study introduces a novel reusable ozone filter for sorbent tubes to enhance the accuracy of BVOC measurements using a combination of potassium iodide (KI) or sodium thiosulfate (Na₂S₂O₃) and copper wool, tested across varying ozone concentrations and humidity levels. The results indicate both filter materials effectively remove ozone and improve measurement accuracy and comparability under diverse environmental conditions, with KI-based filters particularly enhancing monoterpene detection in forest air while protecting sorbent materials, demonstrating their suitability for long-term environmental monitoring of volatile organic compounds.
I find the scientific thesis under review to be exceptionally comprehensive. The experimental designs appear to have been carefully planned and executed with precision, while the data analysis is both lucidly presented and easily understandable. The readability of the document is notably high, enhancing its accessibility. Additionally, as a non-native English speaker, the English used throughout the document seems to me to be correct and well-formulated. The filter developed in this work is proposed as a viable, practical solution that has the potential to be adopted by future practitioners. Consequently, it is poised to become a pioneering approach for the measurement of terpenes.
I have just some minior comments:
Page 7 Line 191: Probably I haven’t read that. How do you know that both samplers perform in the same way?
Page 12 Line 320 ff: Four different humidities, three ozone concentrations each, and two scrubbers amount to 24 measurements, right. For each set, you measured three filtered and one unfiltered. ...I don’t get the 192.
Figure 3: Add in the caption “…over the course of 10 hours.”
Results in Figures 5 and 6: Do you have any idea why the recovery rate is that much above 100%?
Citation: https://doi.org/10.5194/egusphere-2025-1387-RC2 -
AC2: 'Reply on RC2', Robby Rynek, 17 Jun 2025
Dear Reviewer #2,
thank you for carefully reviewing our manuscript, the very positive evaluation and your constructive feedback. Please find below our response to your comments.
1. Both samplers are identical MTS-32 multi-tube sequential samplers (Markes International Ltd., UK), set up with the same parameters (sampling time, pump flow rate) and were operated in parallel next to each other. Additionally, the flow through all tube positions of both samplers was verified to be equal (±5%) before the experiment using a flow meter (7000 GC flowmeter, Ellutia Ltd., UK). This description was added to the revised manuscript.2. To evaluate the ozone filters, we used two scrubber materials (KI, Na2S2O3), two approaches (load-and-flush, permeation), each at four relative humidity levels (<7%, 30%, 60%, 90%) with three ozone concentrations (baseline, 25 ppb, 30 ppb). For each experiment, three samples and one control sample (without filter) were generated, resulting in a total number of 192 samples.
3. Thank you for pointing out the missing information. The figure capture was modified according to your suggestion.
4. Recovery rates exceeding 100% in the load-and-flush approach are likely attributable to inaccuracies in the preparation of the custom-made stock solution used for the experiments and the application of the diluted standard solution using a microliter syringe. In contrast, the calibration curves used for quantification were based on a commercially available, certified stock solution, offering greater accuracy.
In the evaluation of the permeation experiments, recovery rates were normalized to those of the unfiltered samples at baseline ozone concentrations at the corresponding relative humidity levels. As a result, values exceeding 100% are artifacts of the normalization procedure. Without normalization, the recovery rates were below 100%, except for α Pinene at <7% rH and <25 ppb O3, as detailed in lines 382f and illustrated Figure S5.Citation: https://doi.org/10.5194/egusphere-2025-1387-AC2
-
AC2: 'Reply on RC2', Robby Rynek, 17 Jun 2025
-
RC3: 'Comment on egusphere-2025-1387', Anonymous Referee #3, 14 Jun 2025
The article was written in a fairly good systematic manner by presenting a novel reusable ozone filter design for enhancing forest air sampling of biogenic volatile organic compounds (BVOCs), such as monoterpenes. The article evaluated reusable ozone filter design for the monitoring of biogenic volatile organic compounds using sorbent tubes and multi-tube samplers. The efficiency of ozone scrubber material in the analysis of terpenes compound were investigated both in laboratory and field experiments.
Remarks/additional comments were written in the pdf file. Please see the attachment for the reviewer’s comments for this article.
-
AC3: 'Reply on RC3', Robby Rynek, 17 Jun 2025
Dear Reviewer #3,
thank you for carefully reviewing our manuscript and your constructive comments, which helped to improve our work. Please find below our response to your comments.
1. We adjusted the schematic layout of the ozone scrubber design according to your suggestion and added markers for the individual parts. The figure caption was adjusted accordingly.2. The glass filters were placed in a filter holder and rinsed with the respective salt solution until complete wetting, as described in lines 126f. We expanded the experimental description for loading the ozone filters and added the preparation of the salt solutions.
3. The flow requirements of the ozone monitor (>600 mL/min) were added in the revised manuscript.
4. The desired flow rate through the single glass filters (80 mL/min) was added in the revised manuscript.
5. An explanation of the experimental conditions was added to the “Experiment” columns in Tables S3 and S4. Additionally, the experiments at low relative humidity were renamed to “<7 – 0”, “<7 – 25” and “<7 – 50” to be consistent with the conditions named throughout the manuscript.
6. For the purpose of the manuscript, a detailed description of the TD-GC-MS results, aside from the measured concentrations, is not the objective and would be beyond the scope of this paper. We therefore prefer to not include this detailed information in the manuscript. Nevertheless, the used TD-GC-MS system was calibrated using a terpenes standard solution (Terpenes MegaMix Standard #1, Restek GmbH, Germany), showing a sufficient chromatographic separation. The consistency of retention times and mass spectra for the mixture of α Pinene, Myrcene, Limonene and Linalool used in the experiments was also verified prior to conducting the experiments.
7. Looking at the reactions of the ozone depleting reagents with ozone (Eq. 1 and 2), both reactions require water to take place. While water is a reactant in the ozone depletion reaction using KI, the reaction of Na2S2O3 takes place in an aqueous medium. When water is present in the sampled air stream, both reactions can occur, with higher efficiency at increasing relative humidity levels. At very low relative humidity levels, both reactions are inhibited due to the lack of a reactant for KI and a suitable reaction medium for Na2S2O3. An explanation of the results was added to the revised manuscript.
8. Yes, the KI-loaded ozone filters still have the same ozone removal potential after 10 days of exposure to ambient air, as shown in Figure 4. Please note that exposure does not mean they are actively flushed with sampled air. Using the same ozone filter twice was not tested, but could be possible based on the breakthrough experiments (10 hours of flushing). Nevertheless, based on our experience from field experiments, we would advise against reusing the filters for a second sampling without applying new reagent, since visual changes (i.e. yellowing) of the filters are observed regularly.
9. We added a simplified reaction of copper with ozone in the revised manuscript. The detailed reaction mechanism would go beyond the scope of our work. More detailed information can be found in the cited publications by Kim et al. (2023), Lin and Frankel (2013), and Ma et al. (2024).
Citation: https://doi.org/10.5194/egusphere-2025-1387-AC3
-
AC3: 'Reply on RC3', Robby Rynek, 17 Jun 2025
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Thomas Mayer
Helko Borsdorf
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(1716 KB) - Metadata XML
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Supplement
(567 KB) - BibTeX
- EndNote
- Final revised paper