the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 30 May 2024
A Portable Nitrogen Dioxide Instrument Using Cavity-Enhanced Absorption Spectroscopy
Abstract. The Portable (2.7 kg) Cavity-enhanced Absorption of Nitrogen Dioxide (PCAND) instrument for measuring in situ nitrogen dioxide (NO2) was developed using incoherent, broadband cavity-enhanced absorption spectroscopy (IBBCEAS). An LED light source centered at 408 nm was coupled to a cavity 15 cm in length, achieving an effective optical pathlength of ~520 m. Our precision was measured as 94 pptv (1 s). To date, we have flown this instrument on 3 balloon and 1 UAV test flights. This instrument records data to an SD card and outputs data (via an RS232 port) to external devices including a commercial radiosonde (iMet) for real-time data downlink.
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Status: final response (author comments only)
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RC1: 'Comment on amt-2024-61', Anonymous Referee #2, 03 Jun 2024
LED spectrum stability: LED emission spectrum can be very sensitive to its temperature and as you described, the LED here is temperature controlled at 25 degrees. Can you show that this is also the case at high altitudes with environment temperatures well below zero? presumably, that would make life easier for the TEC unless the LED is not generating enough heat.
optical alignment: I can see how fine alignment of the cell is not required. However, the device can experience a significant temperature gradient from ground level to a height of several km. In the altitude profile you presented this gradient can easily reach 40-50 degrees. With thermal expansion, especially when part of the device is kept at 25 C, how confident are you that the device is not too misaligned? can you provide data similar to Figure 4 from an actual flight? with temperature profile and LED temperature if you can.
section 3.2 is confusing to me. presumably, you are referring to the molecular absorption cross-section which should have cm2 or cm2 / molecule units. you used molecules / cm2. For water vapour, this should be orders of magnitude lower.
in the caption of Figure 8, you refer to the device as a "Sonde" although it was not flown here.
alongside a comparison to a state-of-the-art device at ground level I would like to see a comparison while performing under the intended instrument usage - i.e. during flight, under severe temperature and pressure gradients, and atmosphere composition changes.
under Acknowledgements, you refer to drone flights. did I miss that in the main text? I didn't see a reference to any drone flights.
I would like to see a discussion about the possible photolysis of NO2 at 405 nm considering its high quantum yield, or at least an explanation of why it is perhaps insignificant in this case.
Citation: https://doi.org/10.5194/amt-2024-61-RC1 - AC1: 'Reply on RC1', S.A. Bailey, 12 Jun 2024
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RC2: 'Comment on amt-2024-61', Anonymous Referee #3, 12 Jun 2024
Review of “A Portable Nitrogen Dioxide Instrument Using Cavity-Enhanced Absorption Spectroscopy” by Bailey et al.
Bailey et al. present a description of a novel instrument for measuring NO2 in a small footprint for airborne or portable detection at high sensitivity. I recommend publication following the correction of the minor corrections listed below.
Line 16: 3 balloon and 1 UAV flight are listed in the abstract. At the end of the Introduction (line 56) it is stated that one flight is used. Please standardize across the paper.
Line 26: Check reference style for Cersosima et al. as shown in paper text with first initial.
Line 34: remove comma between optical and absorption
Lines 33-35: Check formatting of parentheticals with references. Also, list is missing any reference to electrochemical sensors.
Line 154: aerosol can also attenuate the light in the cavity, not just make the mirrors dirty.
Line 178: I0 font consistency (should be italicized).
Line 221: Avoid personal pronouns. Replace “our pump” with “the pump”, same on line 220.
Citation: https://doi.org/10.5194/amt-2024-61-RC2 -
AC2: 'Reply on RC2', S.A. Bailey, 17 Jun 2024
- Comment from Referee #3: Line 16: 3 balloon and 1 UAV flight are listed in the abstract. At the end of the Introduction (line 56) it is stated that one flight is used. Please standardize across the paper.
Response: Thank you for your comment. I have removed all reference to the UAV flight in the paper. Although we did have 3 balloon flights, only one gave us useful NO2 data as we were working out radio interference problems on the other two flights which contaminated the NO2 data. I changed text in the Summary and conclusions section, sentence 4 (line 274), from “It was successfully tested on 3 balloon flights producing NO2 vertical profiles for each.” to “It was successfully tested on 3 balloon flights”.
- Comment from Referee #3: Line 26: Check reference style for Cersosima et al. as shown in paper text with first initial.
Response: Thank you for your comment. I have corrected the reference on line 26 from a Calibri font to a Calibri (Body) font. Thank you for catching that.
- Comment from Referee #3: Line 34: remove comma between optical and absorption.
Response: Thank you for your comment. I have removed the comma from line 34 between optical and absorption.
- Comment from Referee #3: Lines 33-35: Check formatting of parentheticals with references. Also, list is missing any reference to electrochemical sensors.
Response: Thank you for your comment. I cleaned up the sentences in lines 33-35 to make the parentheticals clearer and more concise. I also just rewrote the Introduction (paragraph 2, line 30) to explain why we think existing electrochemical NO2 sensors were not considered. The short answer is they do not have the precision and accuracy needed for determining a vertical profile of NO2 from a balloon flight.
- Comment from Referee #3: Line 154: aerosol can also attenuate the light in the cavity, not just make the mirrors dirty.
Response: Thank you for your comment. It is true an aerosol can attenuate light, but more important is if said aerosol sticks to one (or both) of the mirrors. That effect on mirror reflectivity is profound and permanent until the mirrors are removed and cleaned.
- Comment from Referee #3: Line 178: I0 font consistency (should be italicized).
Response: Thank you for your comment. I have gone through the paper and made all I0 symbols italicized. Thank you for catching that.
- Comment from Referee #3: Line 221: Avoid personal pronouns. Replace “our pump” with “the pump”, same on line 220.
Response: Thank you for your comment. I have removed the personal pronouns for line 221 and 220 along with a few others I found in the paper. Thank you for catching those.
Citation: https://doi.org/10.5194/amt-2024-61-AC2
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AC2: 'Reply on RC2', S.A. Bailey, 17 Jun 2024
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RC3: 'Comment on amt-2024-61', Anonymous Referee #1, 28 Jun 2024
The authors describe a new lightweight instrument for measuring NO2 from balloons or drones, using the IBBCEAS technique. They achieve 94 ppt precision in 1 second of averaging time, and demonstrate a test flight and comparison with an established NO2 instrument.
I think this paper is suitable for publication, but the authors should address the following concerns first:
- The authors compare PCAND to a recent lightweight LIF instrument, but should also compare its performance to other drone-based IBBCEAS NO2 instruments such as Zheng et al., 2024 and Womack et al., 2022.
- The sample cell is made of aluminum alloy, but the authors later say that FEP tubing was used in all plumbing, presumably to reduce NO2 losses. Have the authors tested losses of NO2 on the aluminum alloy surface? Similarly, did the authors test for NO2 losses on the Nafion dryer? And is the charcoal filter expected to completely scrub out the NO2 or will there be a small fraction remaining?
- There are inconsistencies in how the mirror reflectivity is reported. Line 68 says >99.9%, the Figure 1 says >99.98%, and Line 91 says 99.97%, which correspond to significantly different values when converted to effective pathlengths.
- The authors should discuss the 3 second flush time in the context of the speed of the drone or balloon. What kind of vertical resolution will be expected with this smearing? Is it sufficient for atmospheric chemistry studies?
- How frequently is the effective pathlength measured? Even if the cavity alignment is stable over months, is there any concern that mirror cleanliness will degrade faster than that?
- Section 4.1 is somewhat confusingly written. It’s not really clear how these two methods are derived from the equations. Has this method been used before?
- Additionally, I would recommend moving equation 8 to section 4.1, because it doesn’t follow from equation 7, but is rather derived in section 4.1
- Line 196: How are the “known” NO2 concentrations provided? More detail is needed here.
- Line 201: More details should be included about how leaks and contamination could affect the data. How would they affect the data? Are leaks independently checked for?
- Figure 7: There are quite a few data points in this vertical profile with values close to -1 ppb. However, the reported uncertainty in the laboratory is 0.1 ppb. Do the authors expect that the precision degrades at higher altitudes? If not, how do they explain these negative values?
- Additionally, the profile shows NO2 concentrations of >5 ppb at 7 km, which is unusually high. Did this occur in all the profiles? Was it possible the flight was affected by lofted biomass burning plumes? The authors should discuss this in detail, as accuracy at high altitudes will be critical if this instrument is to be used on balloon platforms.
References:
Zheng et al, 2024: https://doi.org/10.1016/j.atmosenv.2024.120361
Womack et al, 2022: https://doi.org/10.5194/amt-15-6643-2022
Citation: https://doi.org/10.5194/amt-2024-61-RC3 - AC3: 'Reply on RC3', S.A. Bailey, 17 Jul 2024
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