the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 07 Nov 2025
Reassessment of the glyoxal-to-formaldehyde ratio (RGF) as a proxy for VOC source identification
Abstract. The glyoxal-to-formaldehyde ratio (RGF) has been proposed as a proxy to distinguish sources of volatile organic compounds (VOCs) in the atmosphere. However, the interpretation of its variability remains uncertain because of the diverse processes that affect VOC emissions and chemistry. In this study, we revisit the applicability and limitations of RGF using multi-year ground-based MAX-DOAS measurements at four distinct sites: two biogenic (Orléans, France, and ATTO Tower, Brazil) and two anthropogenic (Athens, Greece, and Incheon, South Korea).
The results show higher RGF in anthropogenic environments and lower at biogenic sites. Seasonal RGF patterns are broadly similar across sites, with reduced values in summer and enhanced values in winter, driven by HCHO variability. Diurnal cycles, caused by CHOCHO variability, are more pronounced at urban sites, with weekend effects of 10 %. Correlations between RGF and NO2 vary, even among anthropogenic stations, indicating the importance of local emission contributions.
Additionally, our analysis shows that increasing temperatures leads to a decrease of RGF by up to 1.9 percentage points across all sites, due to the more rapid increase of HCHO levels with temperature than CHOCHO.
Moreover, we discuss four effects that reduce the comparability between different RGF values: measurement volume, vertical sensitivity, time dependence, and the impact of averaging-ratioing order.
Our findings suggest that ground-based remote sensing RGF contains valuable diagnostic information about VOC source environments. However, its use as a universal proxy remains challenging, as our incomplete understanding of the various effects currently limits the reliable use of RGF for VOC source attribution.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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Status: open (until 19 Dec 2025)
- RC1: 'Comment on egusphere-2025-5285', Anonymous Referee #1, 10 Nov 2025 reply
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RC2: 'Comment on egusphere-2025-5285', Anonymous Referee #2, 24 Nov 2025
reply
This study investigated the diurnal, seasonal, and weekly characteristics of glyoxal-to-formaldehyde ratio RGF over four ground-based MAX-DOAS measurement sites, with additional investigations about the dependence of elevation, temperature, and NO2 concentrations. The study also discussed the effects of different measurement techniques in the literature of ground-based versus column-based, vertical sensitivity, temporal sampling biases, and calculation order for RGF on the differences across various reports. The analyses are comprehensive and is helpful for the generalization on the usage of RGF on the understanding of VOC emission characteristics.
General Comments
- The study argued that there are various calculation methods adopted in the literature. When comparing the results in this study against prior studies, the differences are dominated by the calculation methods or the real variation of RGF temporally and spatially?
- How spatially representative are these ground-based measurement sites?
- As the reference was taken at the high elevation angles, then naturally the analyses about the elevation effects would be close to 1 with high elevation angles. Would the uncertainties increase with elevation angles as well?
- Why were the slant column sensitivities chosen instead of vertical column sensitivities?
Specific Comments
- Line 474-475: what would be the emission ratio of NOx and VOCs over these two measurement sites? Would different emitted VOCs from different sector contribute to the different values of RGF?
- Line 510-512: how would the RGF values compare between MAX-DOAS measurement sites and that coincidently sampled from TROPOMI observations?
Citation: https://doi.org/10.5194/egusphere-2025-5285-RC2 -
RC3: 'Comment on egusphere-2025-5285', Anonymous Referee #3, 28 Nov 2025
reply
General evaluation
The article presents an observational analysis of CHOCHO, HCHO, and their ratio (RGF) across contrasting environments using MAX-DOAS measurements. The study investigates temporal variability and relationships with meteorological parameters and NO₂, aiming to improve understanding of the applicability of RGF as an indicator for VOC source attribution. It also discusses the factors contributing to discrepancies among previous studies using different methodological approaches, thereby highlighting both the potential and limitations of employing RGF as a proxy for VOC source identification. The manuscript addresses a timely, underexplored topic and is generally well-structured and informative. Before publication, several issues should be clarified or strengthened as noted below.
Specific comments
- It would be recommended that the authors further elaborate on how the substantial differences in instrument placement (10–500 m above ground) and non-overlapping measurement periods (2017–2024, varying by site) may influence the retrieved diurnal, weekly, and seasonal patterns. MAX-DOAS sensitivity to boundary-layer gases depends strongly on viewing geometry and instrument height, and differences in measurement years imply variations in meteorology and emissions among the sites.
- MAX-DOAS data were aggregated into 30-minute bins, whereas accompanying reanalysis meteorology is hourly and subsequently interpolated. Please justify this choice. Using hourly averages for MAX-DOAS would reduce noise and ensure temporal consistency with the meteorology without interpolation.
- The diurnal cycles results show higher RGF-related values in ATTO than in Orléans, which seems counterintuitive given the expected dominance of HCHO in the Amazon. Figure 4 shows unscaled ATTO values as the lowest. Could the scaling applied introduce a bias in the RGF representation at ATTO?
- In Figure 11, it is difficult to visually confirm a systematic increase in RGF with viewing elevation. Since O₄-based light-path corrections have reduced influence at higher elevation angles, applying a uniform correction across sites with substantially different instrument heights (e.g., Athens at ~500 m a.g.l.) may introduce biases. Excluding the highest elevation angles appears to yield more consistent trends, and the median pattern remains relatively stable with only a slight U-shaped variation. Please clarify how these factors may influence the interpretation of elevation-dependent RGF behavior.
- Including relative humidity datasets in the analysis or discussion may be valuable, as it strongly influences glyoxal uptake and therefore RGF variability in different environments.
- The “seasonal” analysis currently reflects monthly averages. Because hemispheric seasons differ and the sites span both hemispheres, grouping by season or consistently referring to “monthly variability” would improve clarity.
- Differences in the correlation between RGF and NO₂ at the urban sites may reflect differing emission trends, such as the recent decline in NOₓ levels in Seoul (Incheon). A brief note on this possibility could enhance interpretation.
- Line 113 indicates 1–3° viewing angles, but figures include data beyond 30°. Please clarify the full range of elevation angles used in the analysis.
- Line 213: It would be helpful to include the latitude and longitude of each measurement site to clearly define their locations.
- Line 348: While temperature variability in ATTO is limited, a slight increase during September–October coincides with peak HCHO values; this may be worth noting.
- Line 496: Please clarify the spatial representativeness of the MAX-DOAS measurements. Over what approximate horizontal extension can these observations be considered representative, and how does this compare to a point measurement assumption?
- The description of how temperature dependence was evaluated is unclear. Were data grouped into temperature bins? How does this analysis add beyond what is shown in Fig. 7 where temperature influences are already visible?
- Figure 9 indicates a possible weekend effect in RGF. It would be helpful to assess whether the differences between weekdays and weekends are statistically significant.
Citation: https://doi.org/10.5194/egusphere-2025-5285-RC3
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The paper presents a thorough multi-site analysis of the glyoxal-to-formaldehyde ratio (RGF) derived from ground-based MAX-DOAS measurements across four environments (tropical forest, temperate forest, and two urban sites). The authors attempt to clarify why the RGF values reported in different studies often diverge, emphasizing observational geometry, aerosol and NOx regimes, and temperature dependence. The topic is timely and valuable for improving the interpretation of satellite and ground-based VOC proxies.
I should note that I am not a specialist in atmospheric chemistry or VOC oxidation modeling, so my comments focus mainly on the scientific reasoning, completeness of the analysis, and clarity of interpretation. Overall, the study is carefully executed and well documented, but it still lacks several essential elements that would make the findings more conclusive. Below I summarize major and minor scientific questions and suggestions.