the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 27 Jun 2025
Laboratory characterization of furan, 2(3H)-furanone, 2-furaldehyde, 2,5-dimethyl furan, and maleic anhydride measured by PTR-ToF-MS
Abstract. Furanoids are significant contributors to volatile organic compound hydroxyl radical reactivity in biomass burning emissions, yet their accurate measurement using proton-transfer-reaction time-of-flight mass spectrometry (PTR-ToF-MS) remains challenging due to potential interferences and measurement uncertainties. In this study, we conduct detailed laboratory characterizations of furan (C4H4O, protonated m/z 69.033), 2(3H)-furanone (C4H4O2, m/z 85.028), 2-furaldehyde (C5H4O2, m/z 97.028), 2,5-dimethyl furan (C6H8O, m/z 97.065), and maleic anhydride (C4H2O3, m/z 99.008). Sensitivities for these compounds were found to have minimal dependence (less than 15 %) on both sample humidity and drift tube electric field strength (E/N). Fragmentation was observed for 2-furaldehyde (~8 %) at m/z 69.033, creating interference with furan measurements, while hydrolysis products corresponding to m/z+18 ions were detected for 2(3H)-furanone, 2-furaldehyde, and maleic anhydride. The hydrolysis of maleic anhydride to maleic acid was found to be most significant, accounting for 7–31 % of the parent ion signal across E/N conditions.
Gas standard recertification confirmed the long-term stability of furanoids, and 21 other VOCs, in compressed gas mixtures, with changes in mixing ratios of less than 5 % over seven years, although PTR-ToF-MS instrument sensitivities decreased by ~30 % during this time, likely due to aging of the microchannel plate (MCP). While the stability of gas standards and the minimal humidity and fragmentation effects support the accurate measurement of furanoids by PTR-ToF-MS, discrepancies with co-deployed gas chromatography-mass spectrometry highlight the need to further investigate potential isomeric and fragment interferences, particularly in aged BB smoke.
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Status: open (until 02 Aug 2025)
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RC1: 'Comment on egusphere-2025-2937', Anonymous Referee #1, 15 Jul 2025
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Comments:
This manuscript systematically validates the capabilities and limitations of PTR-ToF-MS in measuring furanoids, highlighting the need to account for fragmentation and isomeric interferences in biomass burning (BB) research. The findings provide essential data and guidance for improving the accuracy of VOC measurements. The results presented are insightful and are expected to be useful to the PTR-MS and BB communities. While the study presents a novel and valuable research angle, the manuscript's structure (e.g., Methods) requires significant refinement. I recommend its publication to AMT, after the authors have addressed the following comments.
1. For introduction, several issues in structure, clarity, and conciseness need to be addressed to improve readability and scientific communication. My detailed comments are as follows:
1.1 Organization and Logical Flow
The Introduction attempts to cover a broad range of information, but its current structure is overly dense and lacks a clear logical progression. Key ideas are often repeated or presented out of sequence.
1.2 Study Objective and Scope
The study objectives are mentioned twice, in two separate places, with some redundancy. It would be more effective to consolidate this into a single clear statement toward the end of the Introduction. Additionally, the last paragraph should more directly explain how the work addresses the identified gaps.
1.3 Redundancy and Repetition
Several concepts are repeated unnecessarily. For example, the dominant role of biomass burning as a source of furanoids is stated in multiple places with similar wording. Similarly, the discussion of specific furanoids and their atmospheric roles appears both before and after the study objective is introduced. These points should be consolidated for conciseness.
2. For the Methods section, It is recommended to divide the Methods section into several subsections for better clarity and readability. The details of standard gas should be shown here but not in results.
3. The results shown in Figure 2 demonstrate that the fragmentation of certain furanoid species can vary dynamically with changes in E/N. Therefore, readers may also expect to see a plot of fragmentation ratios as a function of E/N.
4. The y-axis in Figures 3 and 4 is labeled in Ncps, which may make it difficult for readers to interpret the relative changes in signal intensity. Using relative signal values—for example, normalizing the maximum to 1—would improve clarity.
5. Can you also provide the relationship of the Reagent ion signals and m39/m21 as a function of water vapor mixing ratios in the instrument? This can help clarify for readers the range of water vapor concentrations corresponding to your humidity experiments.
6. Lines 228-230: “For most species, the mixing ratios in the tank changed by less than 1 % in the seven years since the standard was made (Furanoids summarized in Table 2 with the remaining VOCs in Table A1).” Please clarify the details of how the absolute concentration in the tank was measured for 2017 and 2024, including the measurement methods used.
7. Lines 228-230: “Table A1 shows the same for the other 21 VOCs in our gas standards. For the furanoids reported here, we find that direct calibrations for all furanoids except 5-methylfurfural agree within less than 42 % of those calculated from their molecular properties.” Please provide the kPTR values used in your sensitivity estimations. It is also recommended to discuss the possible factors that may have contributed to the large discrepancy between the estimated and measured values for 5-methylfurfural.
8. Lines 264-266: “We also find that the correlation between kptr and the measured sensitivities for 25 directly calibrated VOCs decreased by 33 % in the seven years from 2017 to 2024, representative of the overall decrease in instrument sensitivity described above.” The correlation (R) between kptr and sensitivity should not change simply because the overall sensitivity decreases. Does 'correlation' in this context refer to the slope between the two variables or the correlation coefficient (R)? If the correlation (R) indeed decreases with a reduction in general sensitivity, please provide further explanation.
9. Lines 295-297: “Similarly, methyl furans at m/z 83.049 and furaldehydes at m/z 97.065 also have a higher unknown fraction in the field measurements. This may be due to the rapid change in smoke composition as it ages post emission, with unidentified isomers or fragments being formed.” Considering that the variability in fragmentation fractions may result from rapid changes in smoke composition, could the authors elaborate on how this might impact measurements in ambient air? Are there any ambient observations (e.g., urban environment but not wildfire) data available to support this? In light of the potential interferences, is furan—or other furanoid species—still a reliable tracer for biomass burning?
Other comments:
- The reference lists in several sentences are extensive and somewhat overwhelming. In some cases, citations could be streamlined or selected more selectively. Additionally, references should be consistently ordered chronologically, e.g., (Akagi et al., 2011; Stockwell et al., 2015; Koss et al., 2018; Andreae, 2019; etc.).
- Please unify 2,5-dimethylfuran or 2,5-dimethyl furan.
- Line 41 Typos for 2-furalehdye
- Lines 52 “photochemical chemical aging” should be photochemical aging.
- Table 3 What does 'unknown' refer to in your table? Please clarify its meaning in the text.
- Line 172 Typos for malic anhydride
- Lines 324-326 It is recommended to specify whether “higher” refers to concentration, sensitivity, or signal intensity to avoid ambiguity.
- Line 326 “The full extent of these interferences is currently difficult to quantify“ It can be helpful to state the specific reasons for this difficulty.
Citation: https://doi.org/10.5194/egusphere-2025-2937-RC1
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