the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 02 Apr 2025
Global Optimal Estimation Retrievals of Atmospheric Carbonyl Sulfide Over Water from IASI Measurement Spectra for 2018
Abstract. Carbonyl sulfide (OCS) is consumed by vegetation during photosynthesis in a one-way hydrolysis reaction, making measuring OCS vegetative uptake a means of inferring and quantifying global gross primary productivity. Recent studies highlight that uncertainties in OCS surface fluxes remain high and the need for satellite datasets with better spatial coverage are required. Here OCS profiles are retrieved using measured radiances from Infrared Atmospheric Sounding Interferometer (IASI) instruments onboard the MetOp-A and MetOp-B satellites, and an adapted version of the University of Leicester IASI Retrieval Scheme (ULIRS). We focus on oceanic and inland water regions for the example year 2018, using an optimal estimation approach for selected microwindows in the 2000–2100 cm⁻¹ wavenumber range. ULIRS information content exceeds one between ±50° latitude and a peak in vertical sensitivity around 6 – 10 km (500 – 300 hPa) in the troposphere. Diurnal variations are limited to ±2 %, showing larger total column amounts at the daytime overpass. The IASI OCS measurements show a correlation of at least 0.74 at half the ground-based flask measurement sites compared. Results also agree with the University of Leeds TOMCAT 3-D chemical transport model simulations within ±5 % throughout most tropical regions. This study demonstrates the ability of the IASI instrument to accurately measure OCS in the troposphere and observe a reasonable seasonal cycle indicative of being driven by photosynthesis. Further data acquisition is recommended to provide insights into inter-annual variability and seasonality of OCS, and for further application in OCS flux estimation.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this preprint. The responsibility to include appropriate place names lies with the authors.- Preprint
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RC1: 'Comment on egusphere-2025-1073', Anonymous Referee #1, 09 Jun 2025
The study addresses the problem of estimating atmospheric OCS from IASI radiances over the ocean and inland water bodies. The analysis is global and considers the year 2018. The retrieval methodology utilizes a suitable forward model and employs Optimal Estimation to address the inverse problems. The study is well-organized, featuring a detailed analysis that compares IASI retrievals with other OCS products, as well as in situ and remote sensing platforms.
In general, I enjoyed reading the paper, which addresses the issue of OCS retrievals from IASI with a high degree of maturity. Therefore, my remarks are minor, although they address some technical points that I think need clarification to strengthen the good work done by the authors.
Correlation between DOF and surface temperature. The correlation is explained by the supposed high thermal contrast, which, according to the authors, is higher in the tropical region. I do not like this explanation. In general, the thermal contrast is vital for day soundings over land and the ocean, especially in tropical areas where it is almost zero. Also, remember that for the sea, the emissivity is approximately 1 everywhere in the spectral domain of interest. Therefore, the thermal contrast is irrelevant. In effect, from Fig. 4 we see that the AK close to the surface is nearly zero. The correlation is primarily an effect of the larger signal present in the tropics; therefore, the correlation with surface temperature is trivial and does not add significantly to the problem. I would spend only a few words about this trivial effect, trying not to invoke thermal contrast, which is again irrelevant.
A more interesting point on which I would like to draw the author's attention is the possible seasonality in the DOF, which is driven by the normal dependence of weather conditions on the season rather than biological effects. The problem is addressed extensively in the paper doi: 10.1117/12.2599761, which the authors should consider and use to demonstrate that the OCS retrieved is not affected by spurious behavior driven by the background modulation with the season, rather than by biogenic activity leading to the natural sources and sinks of OCS. This modulation effect is expected to be negligible within the tropical belt, where there are no seasons, but could have an impact on mid- and high-latitude areas and stations, as shown in Fig. 12.
Citation: https://doi.org/10.5194/egusphere-2025-1073-RC1 -
RC2: 'Comment on egusphere-2025-1073', Giuliano Liuzzi, 12 Jun 2025
The paper presents a comprehensive analysis of OCS retrievals with IASI and a detailed comparison with modeling and ground-based datasets. I think that the paper is generally well-written and contains a lot of information which is of interest to a broad audience. The role of OCS in the atmosphere has been emerging as key to many aspects of global interest lately, therefore any effort devoted to obtaining long-term, global records of OCS abundances is relevant and timely. This manuscript makes no exception.
Besides the positive impression I had about this work, I only have a few minor comments which would help in improving further the quality of the paper:
1) The introduction section is very detailed and includes a lot of information. For the purpose of showing sources and sinks of OCS, I think that the manuscript should include an infographics with fluxes and sources/sinks and related processes. It certainly would take some time, but it would be a great addition to the paper, and would bolster the readability of the introduction.2) Lines 202-205: Rather than talking about the fact that the signal due to background OCS is above noise level, it would be more useful to say how big of an OCS variation would yield a detectable signal by IASI.
3) Lines 225-226: Is this because of specific contamination from some gases? In any case, contamination is always present, and this sentence probably can be removed.
4) Line 226: Please note that IASI apodized resolution is 0.5 cm-1, while the sampling is 0.25 cm-1. There is no ambiguity about that.
5) Line 264: I was wondering if the authors considered using a covariance for temperature profiles built using a representative dataset of profiles (such as ERA5) instead of a Markovian matrix with a uniform variance across all altitudes.
6) Liine 275: Do you use any inflation factor for the IASI noise to account e.g. for eventual spectroscopic biases? If so, please specify. In any case, I would comment on any choice made as this is typically a relevant point in any retrieval scheme based on Optimal Estimation.
7) Caption of Figure 4: the coordinates of point (b) as specified here are not in the indian ocean, please check the coordinates (perhaps 64.1 E?)
8) Lines 332-333: I would probably expand a little bit explaining why, as the concept that a correlation between chi square and surface temperature does not translate into a bias in the retrieved column is not obvious.
9) Line 494: that altitude range (> 5 km) is above the airmass that contains the large majority of OCS total column. Given that, it would be very beneficial if the manuscript would comment on the differences in light of the IASI retrievals presented.
Citation: https://doi.org/10.5194/egusphere-2025-1073-RC2 - RC3: 'Comment on egusphere-2025-1073', Anonymous Referee #3, 15 Jun 2025
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