the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 02 Jul 2025
Open Path Incoherent Broadband Cavity Enhanced Absorption Spectrometer for in situ measurement of nitrogen oxides, iodine oxide, and glyoxal in the atmosphere
Abstract. In this work we report a compact, low-cost instrument based on Open Path– Incoherent BroadBand Cavity Enhanced Absorption Spectroscopy (OP-IBBCEAS) at 445 nm for the detection of iodine oxide (IO), nitrogen oxide (NO2) and glyoxal (CHOCHO) in the atmosphere. We provide a comparison between OP-IBBCEAS and the closed-path setup on which our instrument is based, previously described in Barbero et al. 2020. The OP system achieved a noise-equivalent absorption sensitivity (NEAS) of 5.7 × 10−10 cm−1 Hz−1/2 per spectral element, only 3 times higher than the closed-path system. Automated performance of regular measurements without absorbance ensures good long-term accuracy of the system. The OP-IBBCEAS is robust and simple to install in the field, allowing quantitative measurements of NO2, CHOCHO, and IO with precisions of ±150, ± 150, and ±7 ppt (pmol mol−1, 2σ) in 50 seconds of measurement.
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RC1: 'Comment on egusphere-2025-3115', Anonymous Referee #1, 14 Jul 2025
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Pastierovic and co-workers report the development of an open path broadband CEAS instrument based on a 445 nm LED. The open path approach has been relatively little used as it is susceptible to interferences from particle extinction in the atmosphere which reduce the instrument sensitivity. However, the open path configuration has the important advantage of reducing or eliminating inlet losses for extractive (closed path) systems. Such inlet losses are an important consideration when dealing with highly reactive species like iodine monoxide (IO) which are readily lost on inlet surfaces. The authors present an instrument with good sensitivity to NO2, glyoxal and IO, using outdoor and indoor urban measurements to test the performance of the open path system and compare it against the well-established closed path approach.
While not entirely novel, this work is technically sound and makes a useful contribution by advancing the development and considerations associated with open path CEAS configurations. It therefore could be suitable for publication in AMT.
There are several areas where the manuscript should provide greater detail and seek to provide more robust explanations of the behaviour of their instrument.
- Although open path CEAS systems are relatively uncommon, the work should be justified more strongly with additional references at appropriate places. There are several statements in the paragraph beginning L30 where the reader would expect stronger evidence for the assertions being made and more references for the particular species being detected. A reference should be provided for Eq. (1). It would be helpful to note that measurements across atmospheric simulation chambers are a forerunner of the open path IBBCEAS systems.
- The structure of the work is a bit unusual, with a table of results comparing the work with others appearing after the objectives of the work. This does not make sense to me and would fit better in the results and discussion section. Moreover, the comparison with other work would be improved by adding the values in (Hz)-1/2 as another column to Table 1 and defining the relationship to the effective pathlength.
- Instrument details should be provided or clarified:
- The cavity length is missing, and the inner diameter of 16 cm is presumably incorrect.
- It’s not completely clear what the flowrate is to the two mirrors and cavity tube. Are they all 1.3 L/min?
- It is unclear how far the purge length extends beyond the mirrors, and the extent to which the flow of the purge gas into the optical cavity space shortens the effective sample pathlength. The assumption is that the velocity of the air flow through the box relative to the purge flow rate means the latter is insignificant — is there evidence to support this assumption?
- There is a relatively big difference in going from a 4th to 8th order polynomial function for the baseline. It is unclear why such a higher level of parameterisation is required between indoor and outdoor measurements. Can the authors provide some insight into how this difference arises? Is it a temperature effect or sample flow effect or does it arise from particle extinction? How does the baseline fluctuate at some wavelength over time? Such insight would be helpful in considering the merits of the open path approach.
- Have other atmospheric absorbers been considered: methylglyoxal, ozone, water vapour?
- How many spectral elements were used in the analysis of NEAS? That is, what is M?
- Fig 4 shows an Allan deviation that is distinctly different than that for the closed path system reported in Barbero et , 2020, showing both a slower decrease in the deviation and no minimum in the values. Can the authors put forward a reason for this behaviour? Where is the principle source of noise in this system and why does the open path system not follow typical noise behaviour, even as the similar closed path system does?
- L146-9: The authors need to demonstrate that the difference in mean between the retrieved glyoxal concentrations in the open and close path configurations is statistically significant. It is unclear that a comparison of the two systems allows an inference to be made about the concentration of glyoxal in particles.
- The agreement between open and closed path configurations is very good for indoor measurements, but more variable for the outdoor measurements. There are times when the deviation between the two systems is quite large (e.g., 4/10 and 8/10). Is this purely a cavity alignment issue or does it have some other cause? Why are the LED intensity and I0 so much noisier when switching to the outdoor measurements?
- How is the intensity drop inI0 to a minimum, followed by a recovery, on 16/9 explained (Fig. 7)?
Minor corrections:
- Inline references should be formatted appropriately, with names and year of publication in brackets, e.g., “(Wu et al., 2012)”, not “Wu et al. (2012), when place at the end of sentences or phrases.
- The Methods section should include manufacturer name along with the model number
- Spaces between value and units (L78, L111)
- Other:
- L8: climate change
- L55: 5 km
- L58: attempting to correct
- L75: range between
- L76: system provides
- L79: becoming a non-negligible source of noise
- L86: over time
- L100: was made to be moved
- L146: Unclear. Maybe “Mean values of X and Y for CHOCHO for the CP and OP system respectively were observed …”
- L155: The cleanliness of the cavity mirrors…
- L159: These figure numbers seem incorrect
- L168: Drop “laser”?
Citation: https://doi.org/10.5194/egusphere-2025-3115-RC1
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