the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 06 Aug 2025
Impact of stray light on greenhouse gas concentration retrievals and emission estimates as observed with the passive airborne remote sensing imager MAMAP2D-Light
Abstract. MAMAP2D-Light is an airborne passive remote sensing push-broom spectrometer developed at the Institute for Environmental Physics at the University of Bremen to measure atmospheric methane (CH4) and carbon dioxide (CO2) column anomalies to quantify point-source emissions in the 1.6 μm-band. In its initial version, as flown in 2022, a significant stray light level of ~4 % of the measured signal has been observed, causing apparent error patterns in the retrieved CO2 and CH4 column anomalies. In this paper, we report the successful application of a stray light correction developed for the instrument. Measurement data collected during an airborne campaign in 2022 in Canada offer the unique opportunity to investigate the end-to-end impact of stray light and its correction on the retrieved CO2 and CH4 column anomalies, as well as the retrieved emission rates. Stray light caused apparent error patterns in the retrieved column anomaly maps. In nearly all cases, applying the CH4/CO2 proxy method reduced the stray-light-related column errors below the column noise, leading to comparable final emission rate estimates for proxy-only and stray-light-corrected data. In this paper, we additionally investigate the special scene contrast conditions under which the correction by applying the proxy method is no longer sufficient. Following the initial campaign in 2022, the stray light was reduced by ~ 75 % by the implementation of a hardware modification from 2023 onward.
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RC1: 'Comment on egusphere-2025-2953', Anonymous Referee #2, 22 Aug 2025
The manuscript assesses straylight of a custom-built imaging spectrometer and its influence on retrievals of GHG point source emissions using the recorded, straylight-contaminated spectral data. The general topic matches the scope of AMT, but I have substantial concerns accepting it for publication.
The manuscript treats a stray light problem, which was caused by reflections at the entrance slit of the spectrometer. This is mentioned in Section 8 on page 25, at the end of the paper. At the beginning of page 5, the authors mention that this slit was not in use during the measurements. Consequently, the manuscript treats an issue that was caused by one specific instrument component, which was not necessary for operation in the first place.
The manuscript does not discuss to what extent the stray light is specific to this instrument component and how general it can be transferred to straylight problems in other instruments and their impact. The relevance of the remaining straylight on measurements taken after the hardware correction (i.e. the removal of the slit, which was not in use) is not discussed.
Consequently, besides highlighting the importance of a careful spectrometer design, it remains unclear to me, what scientific question is addressed by the paper.
Without substantial revisions that address the points listed below, I must strongly recommend against publication of this manuscript in AMT.(1) It should be clearly stated at the beginning of the manuscript (i) who is addressed by the paper, (ii) what is the general scientific question, and (iii) what is the added value of the new stray light correction compared to the existing procedures.
(2) The cause of the stray light has to be clarified from the beginning of the manuscript. It should be evaluated how general or component-specific the treated stray light issue is and how general the proposed procedures can be applied to lower stray light levels.
(3) Based on your analysis, do you expect other similar scientific instruments to face the same straylight issues? Do their straylight correction schemes require improvement and why?
(4) The proposed straylight correction should be applied to the data recorded after the hardware modification and evaluated against established stray light correction approaches. How does the remaining straylight influence impact the GHG retrievals?
(5) The authors mention an order sorting filter in the setup. This is an essential component when it comes to straylight. Straylight is minimized by a filter, which only transmits the spectral range used for the spectral retrieval. Is this the case?Citation: https://doi.org/10.5194/egusphere-2025-2953-RC1 -
RC2: 'Comment on egusphere-2025-2953', Anonymous Referee #3, 27 Aug 2025
This manuscript is about the impact of straylight on measurements of an air-borne push-broom spectrometer and CH4, CO2 column retrievals from these spectra. The authors build a stray light correction and propagate its influence through to L4 (CH4 point source emission rates). As such, the paper addresses relevant scientific questions within the scope of AMT. However, in my opinion, the manuscript does not provide sufficient scientific novelty to merit the publication in its current form. The authors essentially a) report on straylight overwhelmingly due to an outdated instrument configuration, b) borrow the methodology of Tol et al. (2018) to correct for it and, c) apply their method to methane point source observations, where the stray light correction + CH4 column retrieval offers almost no added value over the CH4/CO2-proxy retrieval for subsequent emission quantification.
1) You correctly note that mixed plumes of CO2 and CH4 are challenging for the proxy retrieval. Do you have observations of such plumes at hand? If yes, you should use these measurements to discuss the impact of straylight. This would add more scientific mass for the manuscript.
2) It would be more interesting for readers to know about the stray light correction of the latest sensor hardware.
3) It is unclear to me what new information this manuscript contains, concerning a) performing stray light characterization measurements, b) stray light correction algorithms, c) the relevance of stray-light correction for spectrometers measuring CO2 and CH4 in the SWIR-1 in the context of GHG emission quantification.
Citation: https://doi.org/10.5194/egusphere-2025-2953-RC2 - RC3: 'Comment on egusphere-2025-2953', Anonymous Referee #1, 27 Aug 2025
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