| 04 Dec 2025
The sensitivity of the Far-infrared Outgoing Radiation Understanding and Monitoring (FORUM) mission to dust aerosols: a pseudo-observations analysis
Abstract. The Earth Explorer 9 Far-infrared Outgoing Radiation Understanding and Monitoring (FORUM) mission will bring the first satellite instrument capable of observing the Earth's far infrared (FIR) spectra with a high spectral resolution. Using an ad-hoc pseudo-observations (PO) simulator, we generate FORUM PO that we use to evaluate its sensitivity to dust aerosols. We compare the sensitivity of the FIR (15–100 µm) part of the spectra to that of the mid infrared (MIR, 2.5–15 µm) as observed by the Infrared Atmospheric Sounder Interferometer – New Generation (IASI-NG) instrument. Different dust scenarios are tested, in terms of mineralogy, source region and background atmosphere, burden and vertical distribution. The simulations in this study show a clear FIR spectral signature of dust in the Earth's atmosphere, and a corresponding dust sensitivity in the FIR spectra. The analysis of spectral signatures and Jacobians suggests that the FORUM observations are expected to bring an added value to dust observations from satellite, with respect to MIR-only, especially in cases of large burdens of mid-to-long-range transported plumes of dust in the free and upper troposphere. At these conditions, the FIR contributes up to the 50 % of the overall infrared sensitivity, and the impact of interfering species, like water vapour, is limited. More quantitative estimations of the inherent information content are needed to corroborate our results.
This manuscript presents a sensitivity study of dust aerosols’ minerology, loading, vertical distribution, and the background atmosphere to FIR (15-100 μm) observation from FORUM mission and compares the sensitivity to the MIR (2.5-15 μm) observation from IASI, using a pseudo-observation simulator. It demonstrates a significant FIR spectral signature of dust (up to 50% of the longwave spectrum), which can complement the MIS observation, especially for relatively thick dust in the free and upper troposphere. The manuscript is well-written with detailed demonstrations of the methodology. The science merits publication in AMT, I have some minor suggestions listed as follows.
Line 102: The PO is limited at nadir viewing angle, which should be recognized in the last section, as it is relatively important.
Line 107: The two atmospheric profiles representing tropical and polar regions should be shown in either the manuscript or the supplementary documents, especially the water vapor profile, which is directly related to Section 3.3.
Line 115: What is the assumption of the two vertical distributions of dust (e.g., normal distribution with a central height)? The plot for dust vertical distributions is needed in either the manuscript or the supplementary documents.
Line 120: As this study is using a PO for the FORUM observations, why not use a more realistic particle size distribution (SD) (e.g., from in-situ measurements as presented in Formenti and Di Biagio (2024)) instead of using a monomodal lognormal size distribution?
Line 160: Using Mie calculation with spherical assumption for dust needs to be justified, for example, by providing the evidence (or direct references) that dust optical properties are insensitive to particle shape in MIR and FIR spectrum.
Line 173 and Table 1: At which wavelength is the AOD used as the input for the RTM in this study? It should be presented in text and in Table 1. It is also a bit confusing for “average value in the FORUM spectral range” in Table 1. Does it mean the spectral AODs in the entire FORUM spectral range are constant? If yes, it would not be appropriate, and it is suggested to follow the spectral extinction coefficients based on the CRI and SD. If not, the AOD at which specific wavelength is needed.
In Section 3.2, the Jacobian calculated in this study is the radiance to the number concentration, which is optically to the AOD with the fixed particle size distribution assumption. Therefore, the sensitivity of dust in MIR and FIR is with respect to the aerosol burden, which is similar in terms of spectral behaviors in MIR and FIR. Therefore, using either MIR or FIR could have similar spectral radiance sensitivity to the AOD or number concentrations. The FIR does not provide additional information content in terms of retrieving AOD. Therefore, the contribution of FIR to the total sensitivity in infrared is not appropriate to be simply divided by the add-up sensitivity, as the information content between them is similar.
Based on Figures 3 and 4, the FIR sensitivity to dust CRI is more distinguishable from the CRI sensitivity in MIR. Thus, it is suggested to calculate Jacobians with respect to the different mineral fractions (e.g., Quartz and Feldspar) to further look at the sensitivity contribution of the FIR.
According to the size parameter differences in different wavelengths with the same particle SD, the sensitivity of spectral radiances to dust particle SD would also be different between FIR and MIR, which is also suggested for the authors to consider.
Reference:
Formenti, P. and Di Biagio, C.: Large synthesis of in situ field measurements of the size distribution of mineral dust aerosols across their life cycles, Earth Syst. Sci. Data, 16, 4995-5007, 10.5194/essd-16-4995-2024, 2024.