the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 29 Jul 2025
Measurement Report: Collocated speciation and potential mechanisms of gaseous adsorption for integrated filter-based sampling and analysis of water-soluble organic molecular markers in the atmosphere
Abstract. To better understand the measurement uncertainties and sampling artifacts of particulate water-soluble organic molecular markers (WSOMMs), three quartz filters were stacked and installed in two collocated samplers (Sampler I and II) to simultaneously collect ambient WSOMMs. The paired top filters (Qf) loaded with PM2.5 were analyzed to determine the duplicate-derived uncertainty of particulate WSOMM concentrations. For several WSOMMs (e.g., levoglucosan) specifically associated with aerosol sources, the uncertainty was well below 20 %, which was commonly assumed in previous studies for the analysis of particulate WSOMMs. If the WSOMMs detected in the other two filters (Qb and Qbb) below Qf were caused by gaseous adsorption, the breakthrough value ([Qbb]/([Qb]+[Qbb])) can be used to estimate the sampling artifact of particulate WSOMMs due to gaseous adsorption on Qf. To understand the influence of acidic and alkaline conditions on the adsorption of gaseous WSOMMs or their precursors on quartz filters, the bottom filter (Qbb) of Sampler I was treated with (NH4)2SO4 or KOH on different sampling days. From the comparison of the measurement results between chemically treated and untreated Qbb samples, it was inferred that (NH4)2SO4 can increase the formation of isoprene secondary organic aerosol by reactive uptake of the oxidative intermediates; KOH can promote the adsorption of organic acids through neutralization reactions. Future studies are warranted to develop a suitable method for sampling gaseous WSOMMs using chemically treated adsorbents.
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RC1: 'Comment on egusphere-2025-2106', Anonymous Referee #1, 08 Aug 2025
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This study employed side-by-side filter samplers to characterize quartz filter sampling of organic molecular markers. The focus was on quantifying measurement uncertainty for primary and secondary water-soluble organic molecular markers. Although the topic of artifacts in the quartz filter sampling of organic compounds has been studied for decades, this study is novel because it focuses on many compounds that have not been characterized in such studies before. The manuscript is well organized and the writing is generally good. However, there are substantial technical problems with the manuscript, and I do not think most of the issues can be addressed through a major revision. These are detailed below.
- The authors confuse precision with uncertainty throughout the manuscript. These are not the same thing. Sources of uncertainty in a filter measurement of organics come from a combination of sampling and analytical factors (flow rate, filter recovery including extraction, positive and negative artifacts, contamination throughout the filter/sample handling process, and GC analysis). Most of these are neglected in the manuscript. There are many good resources that explain this concept, including:
- "Modeling Uncertainty in the Earth Sciences, First Edition. Jef Caers. 2011 John Wiley & Sons, Ltd ISBN: 978-1-119-99263-9"
- The authors quantitatively treat evaporation of semi-volatile WSOMMs from the front quartz filter (i.e., the negative artifact) but they are not able to assess positive artifacts that arise from vapor deposition to the front quartz filter.
- Overall, the sampling was limited. There were only 24 paired samples collected in total and the temperature range during sample collection was fairly narrow (Table S1). This is probably not enough samples to support the major conclusions that the study states. Ambient temperature will certainly affect filter sampling artifacts (positive and negative) so the translation of these results to other studies and/or locations is limited.
- From the results in Table 1, it appears there are systematic differences between Sampler I and Sampler II. The average concentration of the Qf sample for every compound is higher in Sampler II than for Sampler I. A statistical test can confirm if the difference is statistically significant but it certainly appears so. This presents significant complication for these analyses.
- As mentioned above, this general topic has been studied extensively. There are many nuances that are not addressed in this manuscript (effects of temperature, humidity, organic concentration, and filter face velocity, to name a few) – it is recommended that the authors delve into this huge body of literature to better understand their measurements and more accurately discuss the interpretations of their results.
- Section 3.1.1 is off topic from the rest of the manuscript.
Citation: https://doi.org/10.5194/egusphere-2025-2106-RC1 -
RC2: 'Comment on egusphere-2025-2106', Anonymous Referee #2, 18 Aug 2025
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This manuscript used stacked quartz filters to quantify the sampling uncertainties of WSOMMs. The paper provides useful information on many types of compounds and is valuable for the aerosol community. However, there are several issues that should be addressed before the manuscript can be considered for publication.
The authors compared their WSOMM concentrations with previous studies. While useful, these comparisons are limited as they were conducted in different years under potentially different meteorological conditions. If the authors could also compare their quartz filter results to simultaneous measurements using other techniques, e.g. Teflon filters or online instrument, if such data are available.
Besides temperature and relative humidities, particle loadings can be another factor affecting the gas-particle partitioning. The sampling campaign was conducted in summer. In winter, enhanced partitioning into the particle phase would be expected due to higher PM concentrations and lower temperatures. It would strengthen the manuscript if the authors can provide more discussions on how their findings may differ under such conditions, and to what extent the conclusions can be generalized.
For figure 2, can the authors describe the difference between black and red lines? Clear labeling in the figure caption would make it easier for readers to under the figure.
The authors only briefly mentioned that PM2.5 was collected. It is recommended to add more details of the collecting methods, including whether a size-selective inlet or cyclone was used, and what the sampling velocity or cut-off characteristics were.
Citation: https://doi.org/10.5194/egusphere-2025-2106-RC2 -
RC3: 'Comment on egusphere-2025-2106', Anonymous Referee #3, 19 Aug 2025
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This is an interesting study, with a focus on the measurement uncertainties of particulate water-soluble organic molecular markers (WSOMMs) during air sampling. It provides important technical details of the adsorption of many gaseous WSOMMs on filters. Such information offers guidance for future filed measurement of WSOMMs. The paper is generally well-written. That being said, a few major concerns need to be addressed.
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Major comments
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Many WSOMMs were investigated in this study, including isoprene SOA tracers, dicarboxylic acid, biomass burning tracers, etc. It is known that many factors including vapor pressure, humidity, and sampling flow rate would impact the adsorption of WSOMMs. However, a fundamental understanding of the adsorption mechanism is lacking.
1. Factor 1, vapor pressure. The author already have the concentrations of many WSOMMs in different filter. The WSOMMs can be classified into two groups: low-volatility and high-volatility compound. Then a comprehensive vapor pressure analysis can be done to illustrate the impact of vapor pressure on WSOMMs adsorption.
2. Factor 2, humidity. What is impact of RH on WSOMMs with different structures (polar vs. non-polar compounds)? As the authors have already mentioned in the paper, the coated (NH4)2SO4 on Qbb can absorb water vapor and act as an acid to promote the hydrolysis of IEPOX on filters to form 2-MTs. This suggests that RH can also impact surface heterogeneous reactions. For a given WSOMM, did you observe different behaviors at different RH?
3. Factor 3, sampling flow rate. It would be valuable to know the impact of sampling flow rate, this can provide guidance for future sampling activity. More studies are also needed to address this issue, i.e., comparing the adsorption behaviors at low-flow rate and high-flow rate conditions.
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Gas-filter interaction under different conditions (e.g., vapor pressure and humidity) is a complex process. An analysis of Koa (octanol/air partition coefficient) and Kow (octanol/water partition coefficient) of WSOMMs may be helpful when elucidating the measurement results.
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Minor comments
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4. Twenty-four pairs of samples were used for analysis. The sample number is relatively small.
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5. For the treated filters, what are remaining concentrations of (NH4)2SO4 and KOH on different filters? The amount of (NH4)2SO4 and KOH on filters will significantly impact the adsorption behavior of certain WSOMMs.
Citation: https://doi.org/10.5194/egusphere-2025-2106-RC3
Data sets
Replication Data for: Collocated speciation and potential mechanisms of gaseous adsorption for integrated filter-based sampling and analysis of water-soluble organic molecular markers in the atmosphere W. Feng et al. https://doi.org/10.7910/DVN/ZD0JQW
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