the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 29 Jul 2024
Accuracy and sensitivity of NH3 measurements using the Dräger Tube Method
Abstract. Regional estimates of ammonia (NH3) emissions are often missing data from heterogeneous or small fields. Areas with no experienced staff or in-field power supply also prevent the use of accurate and fully established micrometeorological measurement techniques. The Dräger Tube Method (DTM) is a calibrated open-dynamic chamber method, which requires little training to use and is comparatively inexpensive. It uses NH3 detector tubes (Dräger Tubes), an automatic pump, as well as a chamber system comprised of four stainless steel chambers connected with PTFE tubing. Even though the DTM is often used in countries such as Germany and China, the detection accuracy, precision and sensitivity have not been tested yet. In order to quantify those for the DTM, we simultaneously measured defined NH3 mixing ratios with the Dräger Tubes, with direct laser absorption spectroscopy (MGA7, MIRO Analytical AG, Switzerland) and with cavity ring-down spectroscopy (G2103, Picarro, Inc., USA). Second, we tested the exchange of the tubing material and heating of the tubing under laboratory conditions, as well as PTFE film attachments or wiping of the DTM chamber system with ethanol during outdoor measurements, on performance improvements. Results showed that the Dräger Tubes had a detection limit between 150 and 200 ppb, which is three to four times higher than originally assumed. Dräger Tube concentration measurements also underestimated NH3 concentrations by 43 up to 100 % for mixing ratios between 50 and 300 ppb, and by 28 up to 46 % for mixing ratios between 500 and 1500 ppb. The PTFE tubing material showed similar performances to the polyester-polyurethane tubing material regarding response time, which was further improved by heating of the tubing to 50 °C. The modifications of the chamber surface and cleaning in the outdoor experiment did not lead to any improvements of NH3 concentration measurements. The results suggest that the DTM should only be used where alternatives are unfeasible and high NH3 emissions are to be expected. A further assessment of calibrated DTM with reference methods is required for a comprehensive evaluation and alternative developments for a more appropriate method replacing the DTM in small plot applications is encouraged.
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Status: open (until 03 Sep 2024)
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RC1: 'Comment on egusphere-2024-1987', Anonymous Referee #1, 14 Aug 2024
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General comments
This manuscript critically assesses the accuracy and sensitivity of the Dräger Tube Method (DTM) for measuring ammonia (NH3) concentrations. The study finds that while DTM is easy to use and affordable, its applicability should be limited to cases when emissions are expected to be high. These findings are relevant for ATM because they highlight the practical limitations and considerations of using DTM measurements. Thus, it should be considered for publication after the concerns below have been addressed.
Specific comments
- L26: Refer to wet and dry deposition here.
- L35-37: In this sentence, it is put that it is difficult to estimate regional or national inventories due to large uncertainties in the NH3 field measurements, but the first two paper does not mention this, Wang et al. mention it specifically.
There are indeed uncertainties in measurements, but also very high variability in emissions due to variability in conditions. Newer studies have shown that the uncertainty of model estimation based on a large number of field measurements are on the same order of magnitude as between measurement methods. - Chamber methods
There must be a distinction between ‘normal’ chamber methods as dynamic flux chambers or wind tunnels that measure continuously over time and the open chamber DTM that measure at a discrete time.
g. L40-41: Pacholski et al. and Roelcke et al. use the DTM chamber, so the critique is justifiable here for DTM, but in my opinion that does not hold for the other chamber methods. Also, the underestimation mentioned here only applied for DTM in Kamp et al. and not for wind tunnels. Wind tunnels have often been used with acid traps where no interpolation is needed.
In general, for the manuscript there is too little nuance on the chamber methods and how they can be used, and it should be very clear that DTM is a special case of a chamber method thus general terms cannot be used for DTM as a chamber method. - It should also be clear that enclosure methods, including DTM, are not suitable for estimation of absolute emissions, but can be valuable when comparing e.g. treatments.
- DTM is mentioned as an alternative for remote areas without power, but other methods also exist (ALPHA samplers, absorbing sponges) that all integrate over time. Other alternatives should also be discussed to justify the use of DTM.
- DTM gets a snapshot in time, and this is a critical point for this method making it very labor intensive to obtain a good time resolution in the measurements. This needs more attention in the discussion.
Furthermore, the chambers are very small. There should be some discussion on how the chambers are placed in the field, e.g. after field application of slurry by band spreading or mineral fertilizers where there is a difference depending on the amount of granulate or slurry inside the ring.
An assessment of the calibration to obtain absolute emissions should be added. - L44-46: Very much has happened and ammonia emissions have been measured in many field plots over the last 22 years (Bouwman et al., 2022). The present study should put into context literature published in recent years.
- L57-59: A calibration was applied, but as literature shows, there is ‘inherent underestimation of NH3’. Please rephrase to make it clear that the calibration itself is uncertain and a subject that needs to be addressed more in the future.
- L90: A reference for the 10-15% should be added including how this is determined. In my experience there is some uncertainty related to the manual read of and how long time you wait after exposure.
- MGA7 is mentioned in the introduction, but CRDS is used in laboratory setup. Please adjust introduction. L127-128 + 133-134: CRDS is the reference, which is an ok choice, but it should be explained why and why it was not the MGA.
Also, please consider not to use MGA7 and G2103 as abbreviation as the names point to the manufacturer. - L209: Is the precision range for MGA correct? (0.04–1.80 %)
- Section 2.3: Add the actual temperatures. Also add to section 3.2.
- For the outdoor measurements. The air exchange rate is the same for DTM and MGA, but as I understand it from the description, air is pulled through the chamber for 60 minutes whereas the DTM was only placed there for 1 min. This should be justified as the release over time can change. A constant emission over 60 min must be the same as for the 1 min with the DTM.
- Figure 2 and Figure 3: Please consider using a different plot type. The individual points can be added for each concentration with a horizontal line for average with error bar or boxplot.
- Section 3.1: The SD’s for DTM are very different between measurements. Please elaborate more on this.
I’m missing the plots of linear regression, please consider including them as minimum to a document with supporting material. - Section 3.2: I’m missing the plots of the response time for the different materials, please consider including them as minimum to a document with supporting material.
- Sections 3.1-3.3: Please consider collected relevant statistical information in a table and remove it from the text to increase readability.
- L259: change ‘This is correct, as shown by’. Svensson & Ferm did not test this setup and cannot conclude the reason for the underestimation of this system.
- L268-269: Not clear what ‘0.25/a’ means.
- L287-311: The effort here to estimate the over- and underestimation is good, but there are some major flaws in my opinion.
Emissions vary over time and as mentioned previously, DTM only provide a spot sample in time. How often should the measurements be conducted to interpolate between measurement points.
The assumption of zero emission during nights must be justified by literature.
Why use ‘…’ specifying the ranges? - L312-314: I my opinion it can never be recommended to use DTM for absolute emissions, which is also state in some of the literature already cited in this paper.
- L320: Kamp et al. found that wind tunnel estimated depended on the air exchange rate of the tunnel as the most important parameter.
- Conclusion:
- It must be clear that there are other alternatives.
- It should be added that DTM should only be used for comparisons, not determination of absolute emissions.
- Conclusions on wind tunnels should be removed. Wind tunnels were not used in this study, and it is difficult to see on what basis it’s justifiable to conclude that ‘wind tunnels are prone to errors’.
- The reference in the conclusions seems to misplaced as the conclusion should present the most important findings of this work.
- Include the shortcomings of the DTM method with spot samples in time giving long interpolation periods (unless many manual samples are taken) and the low surface area the chambers. - L376: Add README file to the data repository explaining the scripts and data.
- Add references to the supporting material (Table A1-4) in the manuscript.
- Language can be more accurate and clearer. See a few specific suggestions below.
Technical corrections
- General: Please check if ‘l’ should be capitalized or not.
- L4: ‘comparatively inexpensive’ change to ‘relatively inexpensive’
- L13: Remove ‘up’ (both places)
- L14: Performance, singular.
- L15: Remove ‘of’ before ‘the tubing’.
- L17-18: ‘Further assessment of calibrated DTM using reference methods’
- L114: 50 ppm NH3 in N2.
- Fig 1: Arrows indication flow direction is missing. The caption must make the figure self-explanatory, include explanation for the dotted box and the circles after NH3 and H2
- L140: Add ‘mm’ after 4.2.
- L141: Add ‘Section’ before 2.2.
- L141: Use other word than ‘section’.
- L162-164: Add details for: ClimaVUE50 and TRIME PICO64
- Equation 1: replace DT,MGA with i and describe i as either DTM or MGA.
- L208: Remove ‘up’ in ‘up to 0%’.
- L219: Add ‘test’ after ‘ANOVA’.
Citation: https://doi.org/10.5194/egusphere-2024-1987-RC1
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