the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 22 Jun 2026
Gravimetrically Measured Water Content of Filter Sampled Fine Particulate Matter at Low Relative Humidity: Insight from the Surface Particulate Matter Network (SPARTAN)
Abstract. Accurate measurements of the composition of fine particulate matter (PM2.5) are important for understanding its sources and health impacts. Water, a portion of PM2.5 mass, is difficult to measure. We describe developments to the Surface PARTiculate mAtter Network (SPARTAN) to better characterize the chemical composition of PM2.5 on polytetrafluoroethylene filters. A robotic weighing facility is used to estimate water content of PM2.5 sampled at three sites (Fajardo, Puerto Rico; Bujumbura, Burundi; Abu Dhabi, United Arab Emirates), and the chemical composition is used to attribute water content to three hygroscopic categories: high growth (sodium chloride), medium growth (ammonium-nitrate-sulfate-potassium), and low growth (organics). The growth rates are tested using two additional sites (Beijing, China; Halifax, Canada). The water content at 35 % relative humidity (RH) is estimated to be 16.5 % (11.6 %–26.5 %, 95 % confidence) for the high growth category, 3.9 % (3.0 %–4.2 %, 95 % confidence) for medium growth, and 1.0 % (0.6–1.3 %, 95 % confidence) for low growth. We calculate the average water content at 35 % RH for 2442 filters from 24 globally distributed sites from December 2019 to September 2024 to be 2.06 % (0.85 %–5.47 %, range), where 58 % of the aerosol water is associated with the medium growth category (22 % low growth, 19 % high growth).
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Measurement Techniques.
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 11 Aug 2026)
- EC1: 'Comment on egusphere-2026-3224', Mingjin Tang, 01 Jul 2026 reply
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RC1: 'Comment on egusphere-2026-3224', Anonymous Referee #1, 09 Jul 2026
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Aerosol water content plays an important role on atmospheric chemistry, especially for liquid reactions, but its mass concentration is difficult to measure directly. This study combined robotic weighing method and mass balance calculation (based on chemical compositions) to gravimetrically estimate water content of PM2.5 at different sites. The authors found water content can be attributed to three hygroscopic categories of chemical compositions following the Wilson equation. Overall, this study presents a remarkably detailed gravimetric analysis of aerosol water content, and the results are valuable for further studies in aerosol chemistry. I only have a few minor personal concerns for the authors' consideration.
- In Introduction, could you provide a short comparison on present methods of estimating aerosol water content in the atmosphere, for examples, thermodynamics model and/or hygroscopicity measurement?
- The results suggested that water content can be attributed to three hygroscopic categories of chemical compositions following the Wilson equation at 35%-55% RH conditions. How about higher RH conditions (eg. 70%-80%)?
- Its better to add a brief summary on the atmospheric implications of the gravimetric analysis results of water content in this study, such as in mass concentration determination, source apportionment, visibility effect, etc..
Citation: https://doi.org/10.5194/egusphere-2026-3224-RC1 -
RC2: 'Comment on egusphere-2026-3224', Anonymous Referee #2, 12 Jul 2026
reply
The manuscript by Oxford et al. presents a method for measuring water content of PM2.5 filter samples collected by the SPARTAN network. If successful, extending a global filter sampling network to quantify aerosol water content would provide valuable information for atmospheric aerosol research. However, in its current form, I believe that the manuscript has several fundamental technical issues that should be addressed before the methodology can be considered sufficiently validated for publication.
1) Lack of validation using standard chemical compounds
Validation using well-characterized reference compounds is a standard practice in hygroscopicity measurements because it demonstrates that the measured water uptake is physically meaningful and that the measurement system performs correctly. Measurements of atmospherically important compounds, such as ammonium sulfate, sodium chloride, and sucrose, are routinely conducted for this purpose, with ammonium sulfate being one of the most commonly used standards. I was unable to identify such validation measurements in the manuscript. Therefore, measurements of standard compounds should be conducted and compared with well-established literature data to demonstrate the validity of the measurement approach.
2) Limited RH range
According to the Introduction, the purpose of the study is to quantify the water content of PM2.5. However, the measurements were limited to a relatively narrow RH range (35–55%). This RH range does not cover the deliquescence relative humidity (DRH) of major atmospheric inorganic aerosol components such as ammonium sulfate and sodium chloride, where substantial water uptake occurs. Furthermore, multiphase chemical reactions that are important in the atmosphere are generally promoted under higher RH conditions. The reported mass growth factors (Figure 3) remain below approximately 1.05 throughout the investigated RH range, whereas ambient aerosol particles are frequently exposed to much higher RH in the atmosphere. Consequently, the atmospheric significance of the reported measurements remains unclear. Most hygroscopicity studies measure water uptake up to approximately 85–90% RH in order to characterize the full hygroscopic behavior of aerosol particles. If the authors believe that restricting the measurements to 35–55% RH is sufficient for the intended scientific objectives, a strong scientific justification should be provided. Otherwise, I suggest expanding the measurable RH range.
3) Insufficient technical description to demonstrate the validity of the methodology
The technical description is not sufficient for readers to evaluate the validity of the proposed measurement approach. For example, the manuscript does not adequately describe how RH was generated and controlled. In addition, information regarding RH stability, such as representative RH time series during equilibration and the standard deviation of RH, is not provided. Combined with the absence of validation using standard compounds, these omissions prevent the reader from assessing whether the proposed methodology accurately measures aerosol-associated water. Consequently, the technical validity of the methodology has not yet been demonstrated.
Because these issues require substantial additional experimental validation rather than clarification or minor revision of the existing manuscript, I do not believe that they can be adequately addressed through a normal revision process. Therefore, I cannot recommend publication of the manuscript in its current form. Resubmission should be considered after the authors have fully addressed these fundamental technical concerns.
Citation: https://doi.org/10.5194/egusphere-2026-3224-RC2
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Page 4, line 111-112: The authors stated that "Measurements of water mass, however, remain absent from filter-based PM2.5 measurements". This is not true, as the paragraph starting at line 125 provided an overview of previous studies which measured watter mass of filter samples.
Page 4-5, line 125-145: there are a few more studies which measured watter mass of filter samples. Please refer to Section 3.2 in a previous reveiw article (Tang et al., 2019) for more information.
References: Tang, M. J., Chan, C. K., Li, Y. J., Su, H., Ma, Q. X., Wu, Z. J., Zhang, G. H., Wang, Z., Ge, M. F., Hu, M., He, H., and Wang, X. M.: A review of experimental techniques for aerosol hygroscopicity studies, Atmos. Chem. Phys., 19, 12631-12686, 2019. [24]