the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 09 Jan 2025
Assessment of laboratory O4 absorption cross-sections at 360 nm using atmospheric long-path DOAS observations
Abstract. The atmospheric absorption of the oxygen collision complex O2-O2, in the following referred to as O4, can be used to derive properties of aerosols and clouds from remote sensing observations. In recent years, inconsistencies between the measured atmospheric O4 absorption and radiative transfer simulations were found for Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements. In the presented study, over two years of observations from a long-path (LP-) DOAS instrument deployed at the German research station Neumayer, Antarctica, are analysed. While MAX-DOAS instruments measure spectra of scattered sunlight at different elevation angles, LP-DOAS utilises an artificial light source and the atmospheric absorptions are measured along a fixed (and well-defined) light path close to the surface. Further, the pristine measurement location allows to investigate the relation between measured and modelled O4 absorption over a large range of temperatures (-45 °C to +5 °C). Overall good agreement is found between the retrieved O4 absorption cross-sections covering the absorption band at 360 nm and laboratory measurements. While the best agreement is obtained for the Finkenzeller and Volkamer (2022) cross-sections, deviations at cold ambient temperatures (below ca. -25 °C) are observed for the Thalman and Volkamer (2013) cross-sections. Other O4 absorption bands could not be investigated because these are not (fully) within the spectral range of the measured spectra. This study strongly supports the accuracy of commonly used O4 absorption cross-sections in DOAS analyses, while more work is needed to understand the earlier reported inconsistencies in MAX-DOAS observations.
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Status: open (until 14 Feb 2025)
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RC1: 'Comment on egusphere-2024-3881', Henning Finkenzeller, 14 Jan 2025
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Lauster and colleagues present a study that investigates the accuracy of two O2-O2 CIA cross sections with long-path DOAS measurements. The measurements are carried out at an arctic site that experiences large temperature variations and is otherwise clean. Calculated oxygen abundances are compared to retrieved O2-O2 CIA, with varied fit settings. The study confirms that the cross sections are overall accurate and not the cause for scaling factors frequently needed in MAX-DOAS. The more recent Finkenzeller & Volkamer cross section seems to produce more accurate results for values of O2-O2 CIA, especially regarding the temperature sensitivity.
This scope of this study is nicely set up. The data set lends itself to this type of analysis. The logic of the analysis is appropriate. The figures are adequate. Overall, I am convinced by the work and the results outlined. I enjoyed reading the manuscript and I am looking forward to seeing this manuscript published. However, I have a couple points where I would ask the authors to refine the manuscript.
I do believe that referring to O2-O2 collision-induced absorption as “collision complex” or “collision pair” is wrong, and “O4”is misleading. It creates a wrong conception of the physical effect. I understand that “O4” is rooted in the MAX-DOAS community, and “O2O2 CIA” doesn’t come off the lips as easily. However, I do ask the authors, particularly in light that this is a manuscript investigating our understanding of this effect, to be accurate when referring to it. Please also consider referring to the excellent monograph on collision-induced absorption by L. Frommhold. I understand that getting away from “O4” is an uphill battle, but I invite the authors to pick up this fight, and I invite the editor and other reviewers to chime in.
17: what is “atmospheric” in “atmospheric absorption”
18: See main comment. Maybe something along the lines of “O2-O2 CIA occurs proportionally to the square of the oxygen concentration.” could work.
37: What art the differences and common aspects of the two cross sections? Please help the reader to understand the difference.
90: What is the resolution of the cross sections? What is the resolution of the spectrometer? (I think this is relevant information that should be within this manuscript.)
97-99: Is this novel? If not, please reference the method.
110-118: I was left a little unsatisfied about why 8000 iterations were identified as best setting. How would the further interpretation change if another value was chosen? How does the value affect the seemingly persistent offsets in the O2-O2 CIA? This should be discussed further. Maybe a duplicate of Figure 8, but with another iteration number, could be interesting.
Table 1: The number of iterations for the high-pass filter should be 8000, rather than 4000, shouldn’t it?
Figure 4: The highlighting frame does not work well. Could be left out or improved.
Figure 5: Which cross section underlies these data? Currently not clear. The link to the table is broken in the caption.
128: How does humidity play in here? In the arctic it won’t matter much, but it should be discussed as being generally important. Is 21% sufficiently accurate? I believe 20.95% might be more accurate (0.1% difference in O2-O2 CIA).
137: Do I understand correctly that the pressure changes were not considered? A pressure change of 2% (1000 vs 980 hPa) would lead to a O2-O2 CIA change of 4%, rather substantial. Accounting for the pressure could improve the closure – or make it worse… Please consider considering the pressure, or discuss the effects of not considering it a little more.
142: The temperature dependence pertains not only to the peak absorption cross section, but also shape and integral cross section. Better to eliminate the parenthesis “i.e., the strength of its peak value”.
145: The reference to Fig A2 is misplaced, it should only be a reference to A3?!
146,147: Temperature should be 223 K, not 233 K?!
153: I disagree with the statement that the integral cross sections are independent of temperature. Thalman & Volkamer were not on point in this regard, but presumably rather intended to argue that there is no bound state (“O4”). The more recent Finkenzeller & Volkamer study found a temperature sensitivity, which is also expected from physics: “With increasing temperature closer encounters occur, which leads to stronger induced dipole moments and thus greater intensities [1].”
168: The interpolation of temperatures has been used previously. Please add (a) reference(s).
Figure A2: I was misled by the caption. It was not clear to me that the temperatures are the interesting parameter, rather than the origin of the O2-O2 CIA cross section. I suggest starting the caption with something like "Sensitivity of O2-O2 CIA to temperature. …”
Citation: https://doi.org/10.5194/egusphere-2024-3881-RC1
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