the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 06 Nov 2024
Developing a low-cost device for estimating air–water ΔpCO2 in coastal environments
Abstract. The ocean is one of the world’s largest anthropogenic carbon dioxide (CO2) sinks, but closing the carbon budget is logistically difficult and expensive, and uncertainties in carbon fluxes and reservoirs remain. Specifically, measuring the CO2 flux at the air–sea interface usually requires costly sensors or analyzers (>30,000 USD), which can limit what a group is able to monitor. Our group has developed and validated a low-cost ΔpCO2 system for ~1,400 USD with Internet of Things (IoT) capabilities to combat this limitation using a ~100 USD pCO2 K30 sensor at its core. Our Sensor for the Exchange of Atmospheric CO2 with Water (SEACOW) may be placed in an observational network with traditional pCO2 sensors to extend the spatial coverage and resolution of monitoring systems. After calibration, the SEACOW reports atmospheric pCO2 measurements within 2–3 % of measurements made by a calibrated LI-COR LI-850. We also demonstrate the SEACOW’s ability to capture diel pCO2 cycling in seagrass, provide recommendations for SEACOW field deployments, and provide additional technical specifications for the SEACOW and for the K30 itself (e.g., air and water-side 99.3 % response time; 5.7 and 29.6 minutes, respectively).
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RC1: 'Comment on egusphere-2024-3375', Anonymous Referee #1, 17 Dec 2024
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This manuscript details the design of a Do-It-Yourself (DIY) pCO2 sensor. The design cleverly uses two sample loops to successively measure steams air and in-water CO2, which are passively equilibrated across membranes. The CO2 detector used in this work is an inexpensive infrared gas analyzer used in previous designs of low-cost CO2 sensors. The authors do a nice job of providing the details of their design, including parts lists, 3D drawings, and circuit design. They describe some tests of sensor performance, and then present results from a two-week mesocosm-type evaluation of sensor response using tanks with and without seagrass. The fundamentals of a good technical note and here, but I was left with several questions which I think should be addressed.
MAJOR POINTS
-Use of the proposed design is framed around coastal or perhaps estuarine environments, but why? In theory this design should be applicable to freshwaters as well, where direct measurements of pCO2 are lacking. Why not use this design to study lakes, for example? Is there concern about the equilibration time for higher-CO2 waters? I think the authors are limiting themselves a bit. Similarly, the authors repeatedly describe how using this design as a “delta-pCO2” device helps compensate for potential CO2 detector drift, which is a good point, but I imagine many researchers would want to use this device to measure absolute pCO2 for studying quantities other than air-sea fluxes.
-I have concerns about the results in Figure 4B. If I am reading the Methods correctly (section 2.2.2), these results are from measurements of 2L of DI water which was bubbled with 1,000 ppm CO2 for 24 hours. It seems that the pCO2 of this water should therefore be right around 1,000 uatm, but SEACOW3 is reading a pCO2 of only about 550 uatm. Is there a potential accuracy issue shown here which is not discussed, or am I misunderstanding what these data are showing? I know that these data are mostly intended to show the response time, but the accuracy discrepancy caught my eye.
-The Discussion is very perfunctory. I think there is more that the authors should examine, including potential reasons for SEACOW 2 and 4 failures, other deployment use cases, potential improvements to the sensor design, and putting the data quality in perspective (perhaps in the climate/weather framework like that suggested by the IWG-OA).
MINOR POINTS
-These references are very relevant to this manuscript:
Lee, D.J.J., Kek, K.T., Wong, W.W., Mohd Nadzir, M.S., Yan, J., Zhan, L. and Poh, S.-C. (2022), Design and optimization of wireless in-situ sensor coupled with gas–water equilibrators for continuous pCO2 measurement in aquatic environments. Limnol Oceanogr Methods, 20: 500-513. https://doi.org/10.1002/lom3.10500
Robison, A. L., L. E. Koenig, J. D. Potter, L. E. Snyder, C. W. Hunt, W. H. McDowell, and W. M. Wollheim. 2024. Lotic-SIPCO2: Adaptation of an open-source CO sensor system and examination of associated emission uncertainties across a range of stream sizes and land uses. Limnology and Oceanography: Methods 22: 191–207. doi:10.1002/lom3.10600
-L17 what about water-side pCO2 accuracy? This is an important metric
-L13 the concept of IoT is mentioned a few times, but I’m not sure what IoT capabilities are added in the design. Some sort of communication upgrade?
-L25 be clear that “sources” are sources of CO2 from aquatic to atmospheric reservoirs
-L29: how high of a temporal or spatial resolution is needed to constrain CO2 flux budgets?
-L38: Is the CO2-Pro a “delta-pCO2” device as defined in this manuscript? I though the CO2-Pro only measured in-water pCO2. Furthermore, I think the authors should explicitly define what makes a “delta-pCO2” device earlier than L53, as this is a term I haven’t seen in common use. I suggest not using this term in the title either, perhaps substituting it for “air-water CO2 fluxes”.
-Figure 1: I like this figure a lot. It would be nice to see what the device looks like with the housing as well. Also, could there be some sort of air-water boundary line included, and maybe blue shading below this line to easily indicate the water side?
-L80: why was the ABC turned off?
-L81-87: can you talk more about the logging and communication systems more?
-Figure 2: some more detail would be useful: which lines are air in or out, and what are the various wires? The housing also has the BME280 inside I believe. I think this figure could be expanded a lot with pictures of the components described in Lines 90-105 too. This Figure can be a really useful resource to help readers understand how the various components work together.
-L112: 250 ppm steps are pretty large for examining atmospheric CO2
-L114: This confuses me. Eq. 2 requires inputs of m(dry) and b(dry), but Lines 113-114 says these parameters are produced by applying Eq 2-4. I’m not sure how these calculations were done.
-L138: I need more description of how this K30 reading vs vapor pressure curve was done, and maybe even an example in the Supplementary.
-L147-149: if the drying system is only removing some of the water vapor, is it really needed? What’s the advantage in keeping the drying system, especially if the 100% humidity response can be accounted for?
-L161: I think this is a typo and should say “control tank”. My understanding is that two “tanks” were used, and each tank had several “containers” of sediment. Is that right?
-L163: is a “power filter” some sort of pumped filter unit?
-L165: the tanks were “closed-loop” with respect to water flow, but not to air exchange I believe. Were they covered in any way?
L167: can you rephrase this line?
L168: I believe each “tank” had four “containers” inside. So each “container” had one light?
L186-189: I’m not sure that the discrete sample pCO2 calculations add much to this manuscript. The discussion of them is pretty minimal and the calculated pCO2 seems hard to interpret (perhaps unsurprisingly). I would be OK with them being omitted as analytical issues seem to keep them from being useful. However, if the discrete data remain, then there needs to be a lot more analytical detail regarding the TA and DIC measurements (accuracy/precision? CRM used? Propagated uncertainties in calculated pCO2? etc).
-Figure 3: It’s very interesting that plumber’s tape could be used as a cheap (although not very durable) membrane- I never considered that!
L197-201: Tie this paragraph more explicitly to the Methods. Was the same chamber used as described in 2.2.2?
-Section 3.2: It looks like there was some variability in the response time among the SEACOW units. Can you put some uncertainties around the 1T/5T estimates?
-Figure 4B: Why is only one SEACOW unit shown here?
-L236-237: can you explain “difference in sensor gain could contribute to inaccuracy as delta-pCO2 increases” more? I don’t follow that statement.
-Figure 6: can uncertainty bounds be put around the pCO2 lines in these plots? Also label each panel with the appropriate sensor unit 9SEACOW-1 on top, SEACOW-4 on bottom.
-L250: talk more about this air leak, either here or in the Discussion. Do you have advice on how to avoid this in the future? How could someone trying to replicate this design avoid this problem?
-L256-259: I don’t see the described pattern. To my eye, the data in Figure 6 show decreasing pCO2 during the day and rising pCO2 at night.
-L261: I don’t see a big change in air pCO2 after addition of the DI water (can this addition be marked with a vertical line or something in the Figures?)
-Table 2- Discuss these data more, perhaps in the Discussion section. What does the characterization indicate about potential uses for the device, comparison to other sensors, or use of the data, perhaps in the context of the climate vs. weather data quality criteria proposed by the IWG-OA? Also discuss why two of the units failed and how others might avoid these problems.
-L280-289: this paragraph reads more like a part of the Introduction.
-L303-304: if someone building this device still needs to characterize the humidity response, then is it still useful to have the Drierite system? Would removing this drying system bring down the cost or complexity or increase deployment times?
-Figure S1 (caption)- I see the DO as red instead of purple.
Citation: https://doi.org/10.5194/egusphere-2024-3375-RC1
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