the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 17 Jun 2021
Detection of Sulfur Dioxide by Broadband Cavity Enhanced Absorption Spectroscopy (BBCEAS)
Abstract. Sulfur dioxide (SO2) is an important precursor for formation of atmospheric sulfate aerosol and acid rain. We present an instrument using Broad Band Cavity Enhanced Absorption Spectroscopy (BBCEAS) for the measurement of SO2 with a minimum limit of detection of 0.6 ppbv using the spectral range 305.5–312 nm and an averaging time of 60 seconds. The instrument consists of high reflectivity mirrors (0.9984 at 310 nm) and a deep UV light source. The effective absorption path length of the instrument is 610 m in a 0.957 m base length. Published reference absorption cross-sections were used to fit and retrieve the SO2 concentrations and were compared to a diluted standard for SO2. The comparison was well correlated, R2 = 0.9985 with a correlation slope of 1.01.
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Status: closed
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RC1: 'Comment on amt-2021-172', Anonymous Referee #1, 06 Jul 2021
The authors describe a new instrument for the detection of SO2 using BBCEAS. Though the application of the BBCEAS method in the wavelength region around 310nm for SO2 measurements is new, there is no major technical progress compared to previous reports about BBCEAS instruments other than the change of the LED to provide the UV light at the required wavelength. In the paper there is no hint that the application of BBCEAS has a major advantage compared to other methods that are used for atmospheric detection of SO2. In fact, a discussion of the performance of the instrument in the context of other techniques is missing. Overall, the description of the instrument and its properties and the discussion are rather sparse and do not show that this instrument provides a major progress. The authors only show investigations of the linearity and limit of detection and a comparison with an established instrument during calibration measurements proving that absolute concentrations are correctly measured for these conditions. The investigation of the applicability of the instrument in ambient air is missing. For these reasons, I do not think that the content of the paper is sufficient to be published in AMT.
Citation: https://doi.org/10.5194/amt-2021-172-RC1 -
AC1: 'Reply on RC1', Jaron Hansen, 20 Oct 2021
We appreciate Reviewer 1’s diligence in responding to our article and the feedback given. We would like to respond to the following points outlined in the reviewer’s comments:
The instrument presented represents no significant progress compared to previous reports about BBCEAS instruments.
The presented instrument moves BBCEAS detection further into the Ultra-violet region than previous applications. This is important for several reasons, which we have added to our discussion of the motivation of the paper: A wider variety of species have structured absorption cross-sections in the deeper UV regions and thus the BBCEAS technique can be utilized to measure these, specifically the 305-310 nm region includes absorption lines for hydroxyl radical (OH) an important short-lived reactant in nearly all reactions of organic molecules in the atmosphere, and finally the instrument extensively utilizes 3D printing technology to reduce cost and increase availability of cavity enhanced instruments for atmospheric monitoring and research. These points are expanded upon in the updated draft of the paper.
“In the paper there is no hint that the application of BBCEAS has a major advantage compared to other methods that are used for atmospheric detection of SO2.”
We discussed other techniques in detail in the Introduction including interfering species specifically for fluorescence-based detection instruments. We have updated this section to add more details to this effect and elaborated the advantages of direct absorption measurements with DOAS analysis in the results and conclusion sections.
“The investigation of the applicability of the instrument in ambient air is missing”
Originally, ambient measurements were not completed due to the lack of variability of SO2 in the vicinity of the locations of our testing (Richfield and Provo, UT, historical data for the last 6 months shows almost no measured concentrations in excess of 1 ppbv over that period). In response to the reviewers, ambient data was acquired to assess fitting of interfering species (such as NO2) as well as performance in an ambient matrix. SO2 was injected into the inlet line to provide varying SO2 levels to measure. The description of these experiments was added to the Experimental, Results and Conclusion sections.
Citation: https://doi.org/10.5194/amt-2021-172-AC1
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AC1: 'Reply on RC1', Jaron Hansen, 20 Oct 2021
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RC2: 'Comment on amt-2021-172', Anonymous Referee #2, 14 Jul 2021
Thalman et al. present an investigation of the feasibility of the measurement of SO2 by the IBBCEAS technique. I can get the novelty that this technique has never been reported in SO2 measurement in the deep UV window, and the work firstly tested it. But I cannot recommend this version of the manuscript published in AMT. This paper, to some extent, looks like an experiment report with limited discussion about their results. The detection limit (0.6 ppbv) mentioned in the abstract is not obtained based on experiments but inferences, which is not acceptable. The measurement uncertainty of the technique is not mentioned and assessed. I encourage the authors to provide a more detailed characterization of this instrument and provide at least one field measurement test to show the advantages of IBBCEAS or its potential compared with the UV fluorescence method.
Major comments.
- Line 122, the LOD is obtained based on the measurement by Avanties? The plot of the time series of the baseline measurement should be presented rather than a value.
- The measurement uncertainties are not discussed in the manuscript.
- Field application should be added to prove the feasibility of this instrument.
- I believe the data used in Figure 6 was based on the time series from Figure 4, if the intercomparison measurement is conducted by 43c simultaneously, this plot can be revised as a scatter plot rather than used six average values. Even if the six points were used to do the intercomparison, the measurement error should be given.
- Why this instrument do not need the purge flow, are there some new designs of the construction of the cavity to protect the high reflectivity mirror, or some other reasons? Is the filter used in the experiment tests?
Technique comments.
- Line 29, second, and sec should be unified throughout the manuscript.
- Figure 1, all the lines should be colored in black except BrO.
- Line 109, how many hours rather than “several”?
- Line 106 lpm, please give the full name when first appear in the text.
- Why other absorbers like HCHO, BrO are not considered in the spectrum fitting?
- Line 120, so which one of the cross-sections is used in the instrument characterization in the following sections?
- Figure 3, please label panel (A, B, C)
- Figure 4, please delete “:”.
- Line 127, the integration time of 2 seconds is not consistent with that mentioned in Section 2.2.
Citation: https://doi.org/10.5194/amt-2021-172-RC2 -
AC2: 'Reply on RC2', Jaron Hansen, 20 Oct 2021
Response to Reviewer #2
The authors thank referee #2 for the feedback given with regard to our paper “Detection of Sulfur Dioxide by Broadband Cavity Enhanced Absorption Spectroscopy (BBCEAS)". Below are specific responses to items raised in their general and specific comments.
“The detection limit (0.6ppbv) mentioned in the abstract is not obtained based on experiments but inferences, which is not acceptable. The measurement uncertainty is not mentioned and assessed. I encourage the authors to provide a more detailed characterization of this instrument and provide at least one measurement test to show the advantages of IBBCEAS or its potential compared to the UV fluorescence method.”
Further experiments have been carried out to assess the detection limit and measurement uncertainty as well as field applicability as described in the response to Reviewer #1. The results of these experiments are detailed in the revised version of the manuscript.
Major Comments:
Line 122, the LOD is obtained based on the measurement by Avantes? The plot of the time series of the baseline measurement should be presented rather than a value.
Because the Avantes proved so much worse in comparison with the Andor spectrometer/CCD combination, only the Andor data timeseries was shown. This is not unexpected as the Avantes spectrometer features a non-cooled detector, a high noise background, and an unoptimized slit resolution to wavelength range combination for this application. For the revision, all mention of the Avantes spectrometer has been removed to clarify the presentation of the results and focus on the optimal set up.
The measurement uncertainties are not discussed in the manuscript.
Discussion of measurement uncertainty is added to the section Noise Evaluation which includes discussion of absorption cross-section uncertainties and other measurement variables (uncertainty in R, etc). The measurement uncertainty is limited by the uncertainty of the reference cross-sections: 5% as given for the Rufus cross-section.
“In the paper there is no hint that the application of BBCEAS has a major advantage compared to other methods that are used for atmospheric detection of SO2.”
We discussed other techniques in detail in the Introduction including interfering species specifically for fluorescence-based detection instruments. We have updated this section to add more details to this effect and elaborated the advantages of direct absorption measurements with DOAS analysis in the results and conclusion sections. The advantages of BBCEAS come with simplified light sources (LED compared to lasers or flash lamps), calibration standard free calibration, absence of interfering species, and long-term stability.
“The investigation of the applicability of the instrument in ambient air is missing”
Originally, ambient measurements were not completed due to the lack of variability of SO2 in the vicinity of the locations of our testing (Richfield and Provo, UT, historical data for the last 6 months shows almost no measured concentrations in excess of 1 ppbv over that period). In response to the reviewers, ambient data was acquired to assess fitting of interfering species (such as NO2) as well as performance in an ambient matrix. SO2 was injected into the inlet line to provide varying SO2 levels to measure. The description of these experiments was added to the Experimental, Results and Conclusion sections.
Field application should be added to prove the feasibility of this instrument.
A field application experiment has been added to the paper, with appropriate additions to the Experiment, Results, and Conclusions sections.
I believe the data used in Figure 6 was based on the time series from Figure 4, if the intercomparison measurement is conducted by 43c simultaneously, this plot can be revised as a scatter plot rather than used six average values. Even if the six points were used to do the intercomparison, the measurement error should be given.
The description of the calibration (intercomparison) experiment has been repeated and replaced with both data from a TECO 43i-TLE and the 43c instrument which were calibrated with the dilution calibrator. The real time data from these instruments is also incorporated and described for the ambient measurement comparison.
Why this instrument do not need the purge flow, are there some new designs of the construction of the cavity to protect the high reflectivity mirror, or some other reasons? Is the filter used in the experiment tests?
Several recent papers (Barbero et al., 2020, added to the instrument description) have described the lack of need for purge volumes in BBCEAS instruments. In this instance, a particle filter is always employed upstream of the cavity to remove deposition by particles on the mirror surfaces. In the current experiment no decay of the mirror reflectivity over time was observed to justify the added uncertainty (relative to sample length) and instrument complexity of purge volumes.
Technical Comments:
Line 29, second, and sec should be unified throughout the manuscript.
All instances of sec have been changed to second.
Figure 1, all the lines should be colored in black except BrO.
Figure 1 has been updated.
Line 109, how many hours rather than “several”?
The range was given because two separate experiments with different detectors were carried out. This has been clarified in the manuscript as the Avantes spectrometer data has been removed.
Line 106 lpm, please give the full name when first appear in the text.
The text has been updated.
Why other absorbers like HCHO, BrO are not considered in the spectrum fitting?
Other absorbers were not fit since only SO2 was supplied in this experiment. Other species (NO2) were included for the added ambient measurements. The relative strength of the differential absorption cross-sections of HCHO, BrO, and NO2 relative to SO2 in the wavelength range are: 0.69, 157, and 0.56 respectively. BrO is extremely short-lived and reactive, so it would not be detected in any of the configurations used in this work. While NO2 can be detected, there are better cavity configurations and wavelength ranges with better detection limits (360 nm, 405 nm, 455 nm, etc.)
Line 120, so which one of the cross-sections is used in the instrument characterization in the following sections?
The text has been updated to read: “The Rufus et al. absorption cross-section was used for fitting because it yielded a 20% lower residual than the Bogumil et al. (2003) cross-section.
Figure 3, please label panel (A, B, C)
Figure 3 has been updated with panel labels.
Figure 4, please delete “:”.
: has been deleted
Line 127, the integration time of 2 seconds is not consistent with that mentioned in Section 2.2.
The section has been updated to remove the discussion of the Avantes spectrometer.
Citation: https://doi.org/10.5194/amt-2021-172-AC2
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RC3: 'Comment on amt-2021-172', Anonymous Referee #3, 15 Jul 2021
Thalman and Hansen describe a promising new broadband cavity-enhanced absorption spectrometer for quantification of SO2 in air in the 305-312 nm region. Overall, I think this paper was perhaps submitted prematurely as some needed experimental work and a (critical) discussion of the results were absent. In my opinion, this paper should be rejected at this time, but I would like to see the authors submit a revised version once more data have been collected and the manuscript has been revised.
Major/general comments:
The authors present what looks like a promising instrument, but the manuscript is lacking a few key elements:
(1) Since the scope of this journal is on atmospheric measurement techniques, I would have expected to see some sample ambient air measurements with this new instrument, in parallel to a trusted reference method. Are there interferences in ambient air, for example? The authors state that other species were not included in the fit, which may be OK for a laboratory comparison, but does this approach hold up for ambient air measurements?
(2) What can the authors say about the stability of this instrument? The LED manufacturer shows a stability data over a 2,000-hour life span on their web site - does this imply that the LED will have to be replaced after 3 months of use?
(3) A critical discussion of the results needs to be added. How does the performance of this instrument compare to other instruments, including commercially available ones? Is it better, worse, and why? What was the dynamic range, response time, power requirements, size compared to (many) other instruments out there? What has this paper contributed to the problem of SO2 measurements? Does this new instrument provide a faster, more sensitive, or more accurate data? What needs to be done to be improve matters further? Are any parts of the design advantageous or trouble (such as the 3D-printed cages, or the materials used for printing)? Since two spectrometers were evaluated, what are the advantages of the Avantes over the Andor (and vice versa), and are the observations consistent with what might have expected from the manufacturer specs?
Specific comments:
line 3 LOD is stated as 3.6 ppbv on line 122, but 0.6 ppbv on line 3.
lines 20 Consider consolidating all this information in a Table.
line 53 3D-printed cages are not very common. Please discuss (in the discussion section) pros and cons and the performance (do they last?) of the printed cages.
line 57 "The LED was temperature controlled with a Peltier cooler". More detail is needed here. How was the LED mounted/secured? How/where was the Peltier element attached? What model Peltier element was used? How/where was the temperature measured?
line 64 "260 - 820 nm" This huge range seems to be mismatch for this application - consider exchanging the grating in future.
line 67-69 "The Avantes spectrometer was not ideal" - consider moving this sentence to the discussion section.
pg 3 / Figure 2 - Panel A is of insufficient resolution.
line 72 "The material of choice was changed for various structural elements depending on the structural and design requirement." Please provide more details here. What are the structural and design requirements? Changed how?
line 94-95 This is not a grammatically correct sentence.
line 102 how were temperature and pressure measured?
line 104 What was the manufacturer-specified uncertainty of this standard?
line 120 Strike duplicate author names (twice); replace "rather that" with "rather than"
line 134 "Known interferences ... are avoided using the BBCEAS method". Please provide data to show that this is indeed the case.
line 136 "Improvements" - is this future work?
line 150 "Data for the experiments and figures are available in the Supplementary Material". There was no supplementary material uploaded.
Citation: https://doi.org/10.5194/amt-2021-172-RC3 -
AC3: 'Reply on RC3', Jaron Hansen, 20 Oct 2021
Response to Reviewer #3
Thalman and Hansen describe a promising new broadband cavity-enhanced absorption spectrometer for quantification of SO2 in air in the 305-312 nm region. Overall, I think this paper was perhaps submitted prematurely as some needed experimental work and a (critical) discussion of the results were absent. In my opinion, this paper should be rejected at this time, but I would like to see the authors submit a revised version once more data have been collected and the manuscript has been revised.
Major/general comments:
The authors present what looks like a promising instrument, but the manuscript is lacking a few key elements:
(1) Since the scope of this journal is on atmospheric measurement techniques, I would have expected to see some sample ambient air measurements with this new instrument, in parallel to a trusted reference method. Are there interferences in ambient air, for example? The authors state that other species were not included in the fit, which may be OK for a laboratory comparison, but does this approach hold up for ambient air measurements?
Ambient measurements have been added to the manuscript along with data evaluating interfering species. Additional species are fit for ambient measurements (NO2).
(2) What can the authors say about the stability of this instrument? The LED manufacturer shows a stability data over a 2,000-hour life span on their web site - does this imply that the LED will have to be replaced after 3 months of use?
This is a good point brought up by the reviewer. For the time period of this work (off and on for 6+ months), no appreciable decay in the LED intensity was observed.
(3) A critical discussion of the results needs to be added. How does the performance of this instrument compare to other instruments, including commercially available ones? Is it better, worse, and why?
Commercial viability for regulatory purposes is driven by standards for SO2 of 75 ppbv (EPA, 1-hour average) and 63 ppbv (WHO, 24 hour average). The reality is that current sensitive techniques and BBCEAS are much more than capable of measuring at these standard levels. Possible advantages in future developments for a BBCEAS include weight and power as well as cost, operation free of calibration standards, and lack of interfering species. These points have been added to the conclusions.
What was the dynamic range, response time, power requirements, size compared to (many) other instruments out there? What has this paper contributed to the problem of SO2 measurements? Does this new instrument provide a faster, more sensitive, or more accurate data? What needs to be done to be improve matters further? Are any parts of the design advantageous or trouble (such as the 3Dprinted cages, or the materials used for printing)? Since two spectrometers were evaluated, what are the advantages of the Avantes over the Andor (and vice versa), and are the observations consistent with what might have expected from the manufacturer specs?
Instrument specifications including power consumption and interferences has been added to the paper, all relative to the Thermo UV flash fluorescence instrument. While the current iteration of the instrument is a longer cavity attached to a 19” instrument rack mount box (9”x 19”x 24”), there is still plenty of room for further optimization in size, power consumption, streamlined electronics, etc.
Specific comments: line 3 LOD is stated as 3.6 ppbv on line 122, but 0.6 ppbv on line 3.
lines 20 Consider consolidating all this information in a Table.
The information presented here covers only a few techniques and focuses on the fluorescence instruments produced by Thermo, so the text provides the smallest footprint in the paper.
line 53 3D-printed cages are not very common. Please discuss (in the discussion section) pros and cons and the performance (do they last?) of the printed cages.
This is a good point and some discussion has been added about cavity construction and stability has been added to the conclusions.
line 57 "The LED was temperature controlled with a Peltier cooler". More detail is needed here. How was the LED mounted/secured? How/where was the Peltier element attached? What model Peltier element was used? How/where was the temperature measured?
The instrument description has been updated for more detail and a figure (picture) has been added to the Supplementary Information.
line 64 "260 - 820 nm" This huge range seems to be mismatch for this application - consider exchanging the grating in future.
The Avantes spectrometer was one that was available for initial use and was not ideal, thus the paper will now only include results and discussion about the data taken with the Andor spectrometer.
line 67-69 "The Avantes spectrometer was not ideal" - consider moving this sentence to the discussion section.
Per comments from the other reviewers and further experiments, the data and discussion relative to the Avantes spectrometer has been removed from the paper as it is unnecessary for the results.
pg 3 / Figure 2 - Panel A is of insufficient resolution.
An updated figure has been provided and will be proofed to ensure proper resolution before final publication.
line 72 "The material of choice was changed for various structural elements depending on the structural and design requirement." Please provide more details here. What are the structural and design requirements? Changed how?
This section has been updated to have more detail relative to choices made in 3D printing material.
line 94-95 This is not a grammatically correct sentence.
The sentence has been updated.
line 102 how were temperature and pressure measured?
Details of the pressure and temperature measurement have been added to the instrument description.
line 104 What was the manufacturer-specified uncertainty of this standard?
The manufacturer specified uncertainty was 1.4% and has been added to the description of the standard.
line 120 Strike duplicate author names (twice); replace "rather that" with "rather than"
The duplicate reference has been fixed and the sentence updated to remove the “rather that”.
line 134 "Known interferences ... are avoided using the BBCEAS method". Please provide data to show that this is indeed the case.
NO was provided to the instruments as a comparison and the results of the interference have been added to the results and discussion.
line 136 "Improvements" - is this future work?
This section has been updated to reflect “Future Work” which is pertinent to the description of the instrument and obvious improvements that were not able to be tested.
line 150 "Data for the experiments and figures are available in the Supplementary Material". There was no supplementary material uploaded.
The text has been updated to reflect the data will be available through the Scholars archive.
Citation: https://doi.org/10.5194/amt-2021-172-AC3
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AC3: 'Reply on RC3', Jaron Hansen, 20 Oct 2021
Status: closed
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RC1: 'Comment on amt-2021-172', Anonymous Referee #1, 06 Jul 2021
The authors describe a new instrument for the detection of SO2 using BBCEAS. Though the application of the BBCEAS method in the wavelength region around 310nm for SO2 measurements is new, there is no major technical progress compared to previous reports about BBCEAS instruments other than the change of the LED to provide the UV light at the required wavelength. In the paper there is no hint that the application of BBCEAS has a major advantage compared to other methods that are used for atmospheric detection of SO2. In fact, a discussion of the performance of the instrument in the context of other techniques is missing. Overall, the description of the instrument and its properties and the discussion are rather sparse and do not show that this instrument provides a major progress. The authors only show investigations of the linearity and limit of detection and a comparison with an established instrument during calibration measurements proving that absolute concentrations are correctly measured for these conditions. The investigation of the applicability of the instrument in ambient air is missing. For these reasons, I do not think that the content of the paper is sufficient to be published in AMT.
Citation: https://doi.org/10.5194/amt-2021-172-RC1 -
AC1: 'Reply on RC1', Jaron Hansen, 20 Oct 2021
We appreciate Reviewer 1’s diligence in responding to our article and the feedback given. We would like to respond to the following points outlined in the reviewer’s comments:
The instrument presented represents no significant progress compared to previous reports about BBCEAS instruments.
The presented instrument moves BBCEAS detection further into the Ultra-violet region than previous applications. This is important for several reasons, which we have added to our discussion of the motivation of the paper: A wider variety of species have structured absorption cross-sections in the deeper UV regions and thus the BBCEAS technique can be utilized to measure these, specifically the 305-310 nm region includes absorption lines for hydroxyl radical (OH) an important short-lived reactant in nearly all reactions of organic molecules in the atmosphere, and finally the instrument extensively utilizes 3D printing technology to reduce cost and increase availability of cavity enhanced instruments for atmospheric monitoring and research. These points are expanded upon in the updated draft of the paper.
“In the paper there is no hint that the application of BBCEAS has a major advantage compared to other methods that are used for atmospheric detection of SO2.”
We discussed other techniques in detail in the Introduction including interfering species specifically for fluorescence-based detection instruments. We have updated this section to add more details to this effect and elaborated the advantages of direct absorption measurements with DOAS analysis in the results and conclusion sections.
“The investigation of the applicability of the instrument in ambient air is missing”
Originally, ambient measurements were not completed due to the lack of variability of SO2 in the vicinity of the locations of our testing (Richfield and Provo, UT, historical data for the last 6 months shows almost no measured concentrations in excess of 1 ppbv over that period). In response to the reviewers, ambient data was acquired to assess fitting of interfering species (such as NO2) as well as performance in an ambient matrix. SO2 was injected into the inlet line to provide varying SO2 levels to measure. The description of these experiments was added to the Experimental, Results and Conclusion sections.
Citation: https://doi.org/10.5194/amt-2021-172-AC1
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AC1: 'Reply on RC1', Jaron Hansen, 20 Oct 2021
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RC2: 'Comment on amt-2021-172', Anonymous Referee #2, 14 Jul 2021
Thalman et al. present an investigation of the feasibility of the measurement of SO2 by the IBBCEAS technique. I can get the novelty that this technique has never been reported in SO2 measurement in the deep UV window, and the work firstly tested it. But I cannot recommend this version of the manuscript published in AMT. This paper, to some extent, looks like an experiment report with limited discussion about their results. The detection limit (0.6 ppbv) mentioned in the abstract is not obtained based on experiments but inferences, which is not acceptable. The measurement uncertainty of the technique is not mentioned and assessed. I encourage the authors to provide a more detailed characterization of this instrument and provide at least one field measurement test to show the advantages of IBBCEAS or its potential compared with the UV fluorescence method.
Major comments.
- Line 122, the LOD is obtained based on the measurement by Avanties? The plot of the time series of the baseline measurement should be presented rather than a value.
- The measurement uncertainties are not discussed in the manuscript.
- Field application should be added to prove the feasibility of this instrument.
- I believe the data used in Figure 6 was based on the time series from Figure 4, if the intercomparison measurement is conducted by 43c simultaneously, this plot can be revised as a scatter plot rather than used six average values. Even if the six points were used to do the intercomparison, the measurement error should be given.
- Why this instrument do not need the purge flow, are there some new designs of the construction of the cavity to protect the high reflectivity mirror, or some other reasons? Is the filter used in the experiment tests?
Technique comments.
- Line 29, second, and sec should be unified throughout the manuscript.
- Figure 1, all the lines should be colored in black except BrO.
- Line 109, how many hours rather than “several”?
- Line 106 lpm, please give the full name when first appear in the text.
- Why other absorbers like HCHO, BrO are not considered in the spectrum fitting?
- Line 120, so which one of the cross-sections is used in the instrument characterization in the following sections?
- Figure 3, please label panel (A, B, C)
- Figure 4, please delete “:”.
- Line 127, the integration time of 2 seconds is not consistent with that mentioned in Section 2.2.
Citation: https://doi.org/10.5194/amt-2021-172-RC2 -
AC2: 'Reply on RC2', Jaron Hansen, 20 Oct 2021
Response to Reviewer #2
The authors thank referee #2 for the feedback given with regard to our paper “Detection of Sulfur Dioxide by Broadband Cavity Enhanced Absorption Spectroscopy (BBCEAS)". Below are specific responses to items raised in their general and specific comments.
“The detection limit (0.6ppbv) mentioned in the abstract is not obtained based on experiments but inferences, which is not acceptable. The measurement uncertainty is not mentioned and assessed. I encourage the authors to provide a more detailed characterization of this instrument and provide at least one measurement test to show the advantages of IBBCEAS or its potential compared to the UV fluorescence method.”
Further experiments have been carried out to assess the detection limit and measurement uncertainty as well as field applicability as described in the response to Reviewer #1. The results of these experiments are detailed in the revised version of the manuscript.
Major Comments:
Line 122, the LOD is obtained based on the measurement by Avantes? The plot of the time series of the baseline measurement should be presented rather than a value.
Because the Avantes proved so much worse in comparison with the Andor spectrometer/CCD combination, only the Andor data timeseries was shown. This is not unexpected as the Avantes spectrometer features a non-cooled detector, a high noise background, and an unoptimized slit resolution to wavelength range combination for this application. For the revision, all mention of the Avantes spectrometer has been removed to clarify the presentation of the results and focus on the optimal set up.
The measurement uncertainties are not discussed in the manuscript.
Discussion of measurement uncertainty is added to the section Noise Evaluation which includes discussion of absorption cross-section uncertainties and other measurement variables (uncertainty in R, etc). The measurement uncertainty is limited by the uncertainty of the reference cross-sections: 5% as given for the Rufus cross-section.
“In the paper there is no hint that the application of BBCEAS has a major advantage compared to other methods that are used for atmospheric detection of SO2.”
We discussed other techniques in detail in the Introduction including interfering species specifically for fluorescence-based detection instruments. We have updated this section to add more details to this effect and elaborated the advantages of direct absorption measurements with DOAS analysis in the results and conclusion sections. The advantages of BBCEAS come with simplified light sources (LED compared to lasers or flash lamps), calibration standard free calibration, absence of interfering species, and long-term stability.
“The investigation of the applicability of the instrument in ambient air is missing”
Originally, ambient measurements were not completed due to the lack of variability of SO2 in the vicinity of the locations of our testing (Richfield and Provo, UT, historical data for the last 6 months shows almost no measured concentrations in excess of 1 ppbv over that period). In response to the reviewers, ambient data was acquired to assess fitting of interfering species (such as NO2) as well as performance in an ambient matrix. SO2 was injected into the inlet line to provide varying SO2 levels to measure. The description of these experiments was added to the Experimental, Results and Conclusion sections.
Field application should be added to prove the feasibility of this instrument.
A field application experiment has been added to the paper, with appropriate additions to the Experiment, Results, and Conclusions sections.
I believe the data used in Figure 6 was based on the time series from Figure 4, if the intercomparison measurement is conducted by 43c simultaneously, this plot can be revised as a scatter plot rather than used six average values. Even if the six points were used to do the intercomparison, the measurement error should be given.
The description of the calibration (intercomparison) experiment has been repeated and replaced with both data from a TECO 43i-TLE and the 43c instrument which were calibrated with the dilution calibrator. The real time data from these instruments is also incorporated and described for the ambient measurement comparison.
Why this instrument do not need the purge flow, are there some new designs of the construction of the cavity to protect the high reflectivity mirror, or some other reasons? Is the filter used in the experiment tests?
Several recent papers (Barbero et al., 2020, added to the instrument description) have described the lack of need for purge volumes in BBCEAS instruments. In this instance, a particle filter is always employed upstream of the cavity to remove deposition by particles on the mirror surfaces. In the current experiment no decay of the mirror reflectivity over time was observed to justify the added uncertainty (relative to sample length) and instrument complexity of purge volumes.
Technical Comments:
Line 29, second, and sec should be unified throughout the manuscript.
All instances of sec have been changed to second.
Figure 1, all the lines should be colored in black except BrO.
Figure 1 has been updated.
Line 109, how many hours rather than “several”?
The range was given because two separate experiments with different detectors were carried out. This has been clarified in the manuscript as the Avantes spectrometer data has been removed.
Line 106 lpm, please give the full name when first appear in the text.
The text has been updated.
Why other absorbers like HCHO, BrO are not considered in the spectrum fitting?
Other absorbers were not fit since only SO2 was supplied in this experiment. Other species (NO2) were included for the added ambient measurements. The relative strength of the differential absorption cross-sections of HCHO, BrO, and NO2 relative to SO2 in the wavelength range are: 0.69, 157, and 0.56 respectively. BrO is extremely short-lived and reactive, so it would not be detected in any of the configurations used in this work. While NO2 can be detected, there are better cavity configurations and wavelength ranges with better detection limits (360 nm, 405 nm, 455 nm, etc.)
Line 120, so which one of the cross-sections is used in the instrument characterization in the following sections?
The text has been updated to read: “The Rufus et al. absorption cross-section was used for fitting because it yielded a 20% lower residual than the Bogumil et al. (2003) cross-section.
Figure 3, please label panel (A, B, C)
Figure 3 has been updated with panel labels.
Figure 4, please delete “:”.
: has been deleted
Line 127, the integration time of 2 seconds is not consistent with that mentioned in Section 2.2.
The section has been updated to remove the discussion of the Avantes spectrometer.
Citation: https://doi.org/10.5194/amt-2021-172-AC2
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RC3: 'Comment on amt-2021-172', Anonymous Referee #3, 15 Jul 2021
Thalman and Hansen describe a promising new broadband cavity-enhanced absorption spectrometer for quantification of SO2 in air in the 305-312 nm region. Overall, I think this paper was perhaps submitted prematurely as some needed experimental work and a (critical) discussion of the results were absent. In my opinion, this paper should be rejected at this time, but I would like to see the authors submit a revised version once more data have been collected and the manuscript has been revised.
Major/general comments:
The authors present what looks like a promising instrument, but the manuscript is lacking a few key elements:
(1) Since the scope of this journal is on atmospheric measurement techniques, I would have expected to see some sample ambient air measurements with this new instrument, in parallel to a trusted reference method. Are there interferences in ambient air, for example? The authors state that other species were not included in the fit, which may be OK for a laboratory comparison, but does this approach hold up for ambient air measurements?
(2) What can the authors say about the stability of this instrument? The LED manufacturer shows a stability data over a 2,000-hour life span on their web site - does this imply that the LED will have to be replaced after 3 months of use?
(3) A critical discussion of the results needs to be added. How does the performance of this instrument compare to other instruments, including commercially available ones? Is it better, worse, and why? What was the dynamic range, response time, power requirements, size compared to (many) other instruments out there? What has this paper contributed to the problem of SO2 measurements? Does this new instrument provide a faster, more sensitive, or more accurate data? What needs to be done to be improve matters further? Are any parts of the design advantageous or trouble (such as the 3D-printed cages, or the materials used for printing)? Since two spectrometers were evaluated, what are the advantages of the Avantes over the Andor (and vice versa), and are the observations consistent with what might have expected from the manufacturer specs?
Specific comments:
line 3 LOD is stated as 3.6 ppbv on line 122, but 0.6 ppbv on line 3.
lines 20 Consider consolidating all this information in a Table.
line 53 3D-printed cages are not very common. Please discuss (in the discussion section) pros and cons and the performance (do they last?) of the printed cages.
line 57 "The LED was temperature controlled with a Peltier cooler". More detail is needed here. How was the LED mounted/secured? How/where was the Peltier element attached? What model Peltier element was used? How/where was the temperature measured?
line 64 "260 - 820 nm" This huge range seems to be mismatch for this application - consider exchanging the grating in future.
line 67-69 "The Avantes spectrometer was not ideal" - consider moving this sentence to the discussion section.
pg 3 / Figure 2 - Panel A is of insufficient resolution.
line 72 "The material of choice was changed for various structural elements depending on the structural and design requirement." Please provide more details here. What are the structural and design requirements? Changed how?
line 94-95 This is not a grammatically correct sentence.
line 102 how were temperature and pressure measured?
line 104 What was the manufacturer-specified uncertainty of this standard?
line 120 Strike duplicate author names (twice); replace "rather that" with "rather than"
line 134 "Known interferences ... are avoided using the BBCEAS method". Please provide data to show that this is indeed the case.
line 136 "Improvements" - is this future work?
line 150 "Data for the experiments and figures are available in the Supplementary Material". There was no supplementary material uploaded.
Citation: https://doi.org/10.5194/amt-2021-172-RC3 -
AC3: 'Reply on RC3', Jaron Hansen, 20 Oct 2021
Response to Reviewer #3
Thalman and Hansen describe a promising new broadband cavity-enhanced absorption spectrometer for quantification of SO2 in air in the 305-312 nm region. Overall, I think this paper was perhaps submitted prematurely as some needed experimental work and a (critical) discussion of the results were absent. In my opinion, this paper should be rejected at this time, but I would like to see the authors submit a revised version once more data have been collected and the manuscript has been revised.
Major/general comments:
The authors present what looks like a promising instrument, but the manuscript is lacking a few key elements:
(1) Since the scope of this journal is on atmospheric measurement techniques, I would have expected to see some sample ambient air measurements with this new instrument, in parallel to a trusted reference method. Are there interferences in ambient air, for example? The authors state that other species were not included in the fit, which may be OK for a laboratory comparison, but does this approach hold up for ambient air measurements?
Ambient measurements have been added to the manuscript along with data evaluating interfering species. Additional species are fit for ambient measurements (NO2).
(2) What can the authors say about the stability of this instrument? The LED manufacturer shows a stability data over a 2,000-hour life span on their web site - does this imply that the LED will have to be replaced after 3 months of use?
This is a good point brought up by the reviewer. For the time period of this work (off and on for 6+ months), no appreciable decay in the LED intensity was observed.
(3) A critical discussion of the results needs to be added. How does the performance of this instrument compare to other instruments, including commercially available ones? Is it better, worse, and why?
Commercial viability for regulatory purposes is driven by standards for SO2 of 75 ppbv (EPA, 1-hour average) and 63 ppbv (WHO, 24 hour average). The reality is that current sensitive techniques and BBCEAS are much more than capable of measuring at these standard levels. Possible advantages in future developments for a BBCEAS include weight and power as well as cost, operation free of calibration standards, and lack of interfering species. These points have been added to the conclusions.
What was the dynamic range, response time, power requirements, size compared to (many) other instruments out there? What has this paper contributed to the problem of SO2 measurements? Does this new instrument provide a faster, more sensitive, or more accurate data? What needs to be done to be improve matters further? Are any parts of the design advantageous or trouble (such as the 3Dprinted cages, or the materials used for printing)? Since two spectrometers were evaluated, what are the advantages of the Avantes over the Andor (and vice versa), and are the observations consistent with what might have expected from the manufacturer specs?
Instrument specifications including power consumption and interferences has been added to the paper, all relative to the Thermo UV flash fluorescence instrument. While the current iteration of the instrument is a longer cavity attached to a 19” instrument rack mount box (9”x 19”x 24”), there is still plenty of room for further optimization in size, power consumption, streamlined electronics, etc.
Specific comments: line 3 LOD is stated as 3.6 ppbv on line 122, but 0.6 ppbv on line 3.
lines 20 Consider consolidating all this information in a Table.
The information presented here covers only a few techniques and focuses on the fluorescence instruments produced by Thermo, so the text provides the smallest footprint in the paper.
line 53 3D-printed cages are not very common. Please discuss (in the discussion section) pros and cons and the performance (do they last?) of the printed cages.
This is a good point and some discussion has been added about cavity construction and stability has been added to the conclusions.
line 57 "The LED was temperature controlled with a Peltier cooler". More detail is needed here. How was the LED mounted/secured? How/where was the Peltier element attached? What model Peltier element was used? How/where was the temperature measured?
The instrument description has been updated for more detail and a figure (picture) has been added to the Supplementary Information.
line 64 "260 - 820 nm" This huge range seems to be mismatch for this application - consider exchanging the grating in future.
The Avantes spectrometer was one that was available for initial use and was not ideal, thus the paper will now only include results and discussion about the data taken with the Andor spectrometer.
line 67-69 "The Avantes spectrometer was not ideal" - consider moving this sentence to the discussion section.
Per comments from the other reviewers and further experiments, the data and discussion relative to the Avantes spectrometer has been removed from the paper as it is unnecessary for the results.
pg 3 / Figure 2 - Panel A is of insufficient resolution.
An updated figure has been provided and will be proofed to ensure proper resolution before final publication.
line 72 "The material of choice was changed for various structural elements depending on the structural and design requirement." Please provide more details here. What are the structural and design requirements? Changed how?
This section has been updated to have more detail relative to choices made in 3D printing material.
line 94-95 This is not a grammatically correct sentence.
The sentence has been updated.
line 102 how were temperature and pressure measured?
Details of the pressure and temperature measurement have been added to the instrument description.
line 104 What was the manufacturer-specified uncertainty of this standard?
The manufacturer specified uncertainty was 1.4% and has been added to the description of the standard.
line 120 Strike duplicate author names (twice); replace "rather that" with "rather than"
The duplicate reference has been fixed and the sentence updated to remove the “rather that”.
line 134 "Known interferences ... are avoided using the BBCEAS method". Please provide data to show that this is indeed the case.
NO was provided to the instruments as a comparison and the results of the interference have been added to the results and discussion.
line 136 "Improvements" - is this future work?
This section has been updated to reflect “Future Work” which is pertinent to the description of the instrument and obvious improvements that were not able to be tested.
line 150 "Data for the experiments and figures are available in the Supplementary Material". There was no supplementary material uploaded.
The text has been updated to reflect the data will be available through the Scholars archive.
Citation: https://doi.org/10.5194/amt-2021-172-AC3
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AC3: 'Reply on RC3', Jaron Hansen, 20 Oct 2021
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