the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Portable, low-cost samplers for distributed sampling of atmospheric gases
Alejandra Caceres
Deborah F. McGlynn
Mary Tovillo
Suzanne Pinar
Roger Schürch
Ksenia Onufrieva
Abstract. Volatile organic compounds (VOCs) contribute to air pollution both directly, as hazardous gases, and through their reactions with common atmospheric oxidants to produce ozone, particulate matter, and other hazardous air pollutants. There are enormous ranges of structures and reaction rates among VOCs, and consequently a need to accurately characterize the spatial and temporal distribution of individual identified compounds. Current VOC measurements are often made with complex, expensive instrumentation that provides high chemical detail, but is limited in its portability and requires high expense (e.g., mobile labs) for spatially resolved measurements. Alternatively, periodic collection of samples on cartridges is inexpensive but demands significant operator interaction that can limit possibilities for time-resolved measurements or distributed measurements across a spatial area. Thus, there is a need for simple, portable devices that can sample with limited operator presence to enable temporally and/or spatially resolved measurements. In this work, we describe new portable and programmable VOC samplers that enable simultaneous collection of samples across a spatially distributed network, validate their reproducibility, and demonstrate their utility. Validation experiments confirmed high precision between samplers as well as the ability of miniature ozone scrubbers to preserve reactive analytes collected on commercially available adsorbent gas sampling cartridges, supporting simultaneous field deployment across multiple locations. In indoor environments, 24-hour integrated samples demonstrate observable day-to-day variability, as well as variability across very short spatial scales (meters). The utility of the samplers was further demonstrated by locating outdoor point sources of analytes through the development of a new mapping approach that employs a group of the portable samplers and back projection techniques to assess a sampling area with higher resolution than stationary sampling. As with all gas sampling, the limits of detection depend on sampling times and the properties of sorbent and analyte. Limit of detection of the analytical system used in this work is on the order of nanograms, corresponding to mixing ratios of 1–10 pptv after one hour of sampling at the programmable flow rate of 50–250 sccm enabled by the developed system. The portable VOC samplers described and validated here provide a simple, low-cost sampling solution for spatially and/or temporally variable measurements of any organic gases that are collectable on currently available sampling media.
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James F. Hurley et al.
Status: open (until 15 Jun 2023)
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RC1: 'Comment on egusphere-2023-825', Anonymous Referee #1, 18 May 2023
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This manuscript describes the development of a portable, low-cost adsorbent tube sampler for field deployment. The authors describe their testing of sampler reproducibility and ozone scrubbing, as well as their deployments to investigate long-term temporal variability and spatial variability measurement capabilities. The programmable nature of this sampler along with its small size make it an attractive tool for researchers or industry professionals to monitor air quality. I particularly enjoyed reading about the transect mapping approach the authors discussed. I do think the sampler’s applications are somewhat limited – e.g., can only sample on one cartridge at a time before needing to be replenished, still has complex data analysis and thus requires operator expertise – but the study represents useful developments in lower cost sampling strategies that preserve a high level of chemical detail. The manuscript was very well written and the figures were clearly presented. I recommend this paper for publication in AMT after minor revisions.
Introduction: very nicely written. A few typographical comments:
Line 40: suggest revising to “does damage” instead of “doing damage”
Line 68: “reduce” instead of “reduces”
Line 83: Typically these adsorbent cartridges do need to be cleaned (e.g., via thermal conditioning)
Line 89: I’d also consider re-emphasizing the need for relatively complex data analysis with adsorbent sampling
Line 93: Would you say that you can capture temporal variability adequately with the hours-long sampling here? Certainly you can to some extent, but I’d consider qualifying what you mean by temporal variability somewhere. When I think of temporal variability, I really think of online VOC measurements like PTR-MS. For clarity, it would help to mention “over hours-long periods” or similar.
Materials and methods:
I appreciate the low operator involvement associated with your design (i.e., pre-programmable, very few controls to navigate), but since it can only sample one cartridge at a time, the operator involvement actually seems fairly high if cartridges need to be continually replaced. Would you consider building a multi-cartridge system in the future? While that might come with its own challenges, it would enhance your instruments capabilities significantly. I’d suggest mentioning this somewhere as a limitation of your proposed system.
Does the temperature in the box increase at all with the operation of the pump and other electronics, and what does that do for the retention of volatiles in the cartridge? Do you have any sort of temperature monitoring or temperature tests to address this? If so I suggest including those results or commenting on what is expected. Without an on-board fan or anything, I am concerned that you are seeing significant temperature increases in your sampler box especially when deployed in the field in the sun.
Relatedly, did you perform any breakthrough testing at such long sampling times (e.g., the many hours described in the ozone scrubber testing, the roughly 1-day long samples described in the long-term temporal variability section)? I’d like to see some additional discussion of sorbent breakthrough as well as possible sorbent breakthrough under warmer temperature conditions in the box (as mentioned above).
Line 195-200: this description is challenging to understand on its own but much clearer once the reader looks at Figure 5, can you add a reference to figure 5 somewhere in this discussion to draw attention to it? The figure is great and helped clarify the text significantly.
Results:
Line 242: You describe possible analyte loss to the ozone scrubbing filter here, and mention that species with low ozone reactivity had minimal concentration difference with and without the scrubber, ruling out adsorption to the filter. How would you expect more functionalized gases to behave here? I would not anticipate that BTEX species would be especially susceptible to filter-related losses but I am concerned about losses of other oxygenated VOCs for example. Can you please comment on this possibility? Or provide some discussion of the limitations of this testing with high volatility non-functionalized species?
For field sampling, have you considered the impacts of water uptake on your sodium thiosulfate filter? How would that impact ozone scrubbing over long sampling times? Do you expect any reactivity of particle-phase species on the surface of your filter that might result in the volatilization of gases that you collect in the adsorbent tube? I suggest adding some comments about necessary considerations when deploying a glass fiber filter upstream of the adsorbent cartridge.
Figure 3: would be helpful to make BTEX and Monoterpenes different shapes.
Citation: https://doi.org/10.5194/egusphere-2023-825-RC1 -
RC2: 'Comment on egusphere-2023-825', Anonymous Referee #2, 23 May 2023
reply
This is a very nice piece of work demonstrating the development of a relatively inexpensive adsorbent cartage based portable VOC sampler for a field study. To ensure that multiple samplers can be used in a field study that examines temporal and spatial variability, the authors performed comprehensive sampler characterisation experiments that tested their stability, repeatability, and reproducibility. Furthermore, the authors showed a novel concept of transect mapping to pinpoint emission sources. Typically, a sampler needs to be located in each grid cell to pinpoint emission sources, but this makes such an exercise extremely costly. The authors demonstrated that their strategy allowed them to use much fewer samplers, for example, only 10 samplers to locate the emission sources from 25 grid cells. The manuscript is clear and concise and I recommend its publication once some minor technical issues are addressed.
Line 46: “hydroxyl” should be “hydroxyl radical”.
Line 83: A TD adsorbent tube requires a post-use ‘bake out’ step prior to reuse.
Line 101: Have the authors considered an umbrella or a similar attachment at the end of the Teflon tube for rain protection? If so, please describe it here.
Line 124: What are the authors’ criteria for selecting a specific adsorbing polymer for different experimental condition or target VOCs? An additional description about the choice of adsorbing polymer will be good here.
Section 2.4.2.: In my opinion, the authors described the complex sampling scheme well, but a simple strip drawing that shows the movement of samplers would make it easier for readers to understand.
Section 3.2.: Have the authors tested the ozone capacity (or longevity) of a self-made ozone scrubber? A typical holdup volume of a 25mm syringe filter is about 0.1 mL so it does not hold as much an ozone scrubbing reagent as some of commercially available ozone scrubbers for DNPH carbonyl sampling (e.g. 505285 Supelco LpDNPH Ozone Scrubber). A small discussion about the ozone capacity will be useful here.
Tables S2 and S3: Are these numbers the integrated area out of GC/MS? If so, this should be mentioned in the caption.
Citation: https://doi.org/10.5194/egusphere-2023-825-RC2
James F. Hurley et al.
James F. Hurley et al.
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