Advances in High-Precision NO2 Measurement by Quantum Cascade Laser Absorption Spectroscopy
, Philipp Scheidegger 1,2, Herbert Looser 1, André Kupferschmid 2, Maitane Iturrate 3, Céline Pascale 3, Christoph Hüglin 1 and Lukas Emmenegger 1Round 1
Reviewer 1 Report
Review of the paper applsci-1050994, entitled "Advances in high-precision NO2 measurement by Quantum Cascade Laser Absorption Spectroscopy" by Sobanski et al.
The article aims to describe a direct diode laser spectrometer based on QCL to probe atmospheric concentration of NO2. The proposed setup allows a direct detection of highly reactive species within a short integration time (1s) and with high precision (at best 0.8 pptv for a 150sec int. time).
If lifetimes of NOx compounds have been reported to be around 3.5-4 hours in power plant and megacities, such instrument then appears to be an interesting tool to study the chemistry of NOx in the atmosphere. From WMO, high quality target for the detection limit of NO2 is of 5pmol/mol for a 1h-integration time. Then, the described instrument fits requirements by proposing a 4.1 to 6.7 pmol/mol detection limit over 60 sec, which can be improved by increasing the averaging time within the range free of drifts.
Biases issues have been thoroughly investigated.
General comments:
In the 2.1.2 "Performances assessment" section, I would add a table showing the comparison between performances of the present apparatus with those reproted in the litterature based on the same technique. Of course, scaling the performances of each on the same averaging time. This would help the reader.
It could be also useful to state the detection limit in terms of molecules.cm-3 and in SI units (in parenthesis) so the reader can easily compare the performances of the instrument to the requirements and other published remote measurements used to estimate emissions and lifetimes for example.
l 69-70 : "More accurate measurements of NO2 will bring greater confidence in identified 70 trends in imissions and air quality"
l 264 "e, the GAW programm recommends"
Based on the quality of the publication, I recommend this paper for publication after minor corrections.
Author Response
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Author Response File: 
Author Response.docx
Reviewer 2 Report
The paper describes the development of a custom-made quantum cascade laser (QCLA) spectrometer for sensitive detection and accurate measurement of NO2 in the atmosphere. The performance of the instrument is compared to that of other instruments, including a commercially available QCLA spectrometer.
The presentation is clear, technical description and characterization of the instrument is accurate, and the instrument has demonstrated high performance. Nonetheless, the use of QCLA spectrometers for the detection of pollutant gases, including NO2, is no new conception as it was demonstrated long ago (Ref. 32, 34) and a QCLA spectrometer for NO2 detection is commercially available. The manuscript is all about the instrument development and caracterization. I cannot detect any new science.
I do not recommend publication in Applied Sciences. The manuscript may be more suitable for submission to a journal devoted to scientific instruments.
Author Response
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Author Response File: Author Response.docx
Reviewer 3 Report
The manuscript presented for review is an example of development work in the field of a new technique for automatic measurement of NO2 concentration in the air - quantum cascade laser absorption spectroscopy (QCLAS), presenting the results of a comparative and validation experiment.
The work is generally very good and presented in detail - congratulations!
However, if you include the comments below, it will make it even better.
Main comments:
In routine measurements of NO2 concentrations in the air, usually in parallel with the same analyzer - most often using the chemiluminescence method (CLD) - the sum of NO and NO2 (denoted as NOx) are also measured, which the authors of the manuscript mention. Measurements of NOx concentrations in the air are important from the point of view of plant protection and there is a limit value for them, resulting e.g. from the CAFE Directive (Directive 2008/50/EC of the European Parliament and of the Council of 21 May 2008 on ambient air quality and cleaner air for Europe).
Therefore, the lack of reference to the quality of NO/NOx measurements obtained by the described new measurement method should be seen as a certain deficiency in this work. It was limited only to a detailed description of the comparative and validation experiment relating to the results of NO2 concentration measurements. Meanwhile, as it can be seen from, for example, Figure 8, the QCLAS instrument also allows for simultaneous measurements of NO concentrations and it was possible to refer to the obtained results, as it is possible, for example, in the CLD method (NO can be calculated from the difference between NOx and NO2).
If such analyzes have not been conducted, it is worth referring to this problem at least in the conclusions that this is an area requiring further research. The statement "our instruments is a highly attractive alternative, both in terms of precision and selectivity, to existing instruments based on more commonly used techniques for NO2 detection" (lines 484-486), although it seems to be true, somewhat ignores the fact that these types of analyzers usually measure (or calculate) not only NO2 but also NO and NOx, and such functionality is expected by the users of these instruments (air quality monitoring services).
Other (editorial) comments:
It is advisable to introduce symbols in Figure 1 to explain all the types of mirrors used on it.
Paragraphs 159-198 seems too long. It is worth dividing it into 2 or 3 smaller paragraphs.
The description of Figures 3 and 8 should be modified so that its general title appears first, and then they find detailed explanations of items a), b) and c).
On the horizontal axis of Figure 8, it is recommended to use the date format: dd.mm (e.g. 01.02 instead of 1.2).
Author Response
Please see the attachment.
Author Response File: Author Response.docx
Round 2
Reviewer 2 Report
In my 1st report, when asked for "overall recommendation" I checked the box "reject". So I find it specious and nonsensical that the authors blame me for writing "I do not recommend publication" in my comments.
On the one hand the new version of the manuscript has not changed in a way that would mitigate my concern. On the other hand the author response did not change my overall opinion.
