https://orfeo.belnet.be/handle/internal/1 2026-05-26T21:05:53Z 2026-05-26T21:05:53Z De Keyser, J. Edberg, N.J.T. Henri, P. Rothkaehl, H. Della Corte, V. Rubin, M. Funase, R. Kasahara, S. Snodgrass, C. https://orfeo.belnet.be/handle/internal/14752 2026-05-23T05:38:23Z

Finding the optimal flyby distance for the Comet Interceptor comet mission De Keyser, J.; Edberg, N.J.T.; Henri, P.; Rothkaehl, H.; Della Corte, V.; Rubin, M.; Funase, R.; Kasahara, S.; Snodgrass, C. The Comet Interceptor mission will attempt to fly by a yet undetermined target comet. The conditions of this flyby will remain largely unknown up to the selection of target and possibly even the moment of encounter. A detailed trajectory design phase, which includes verification of the technical limitations implied by the flyby geometry, precedes target comet selection, so the flyby velocity and the details of the geometry are known in advance. Solar irradiance and the neutral gas expansion speed can be estimated reasonably well. However, the comet outgassing rate, the dust production rate, and the solar wind conditions are only known within broader uncertainty margins. The present contribution aims to optimally choose the distance of closest approach based on a simplified formalism that expresses, on one hand, the science return to be expected as a function of the closest approach distance, and, on the other hand, the risks implied by a close approach. This is done by performing Monte Carlo simulations over a large sample of possible flyby configurations, based on the expected probability distributions of the gas and dust production rates and the solar wind conditions, and for different closest approach distances. For small flyby distances, a spacecraft can study the nucleus, the neutral gas coma, and the induced magnetosphere from up close, benefiting the science return. There is a trade-off to be made against the cometary dust collision risk, which becomes larger close to the nucleus. The change of the optimal flyby distance with gas and dust production rate, solar EUV flux, and flyby speed is discussed. The conclusion is that the Comet Interceptor main spacecraft and its two daughter probes – within the limitations of the approximations made – would benefit from a target comet with a gas production rate of 1028-1029 molecules·s-1, a low dust-to-gas ratio, a high solar EUV flux, and a slow flyby speed (De Keyser et al., 2024, https://doi.org/10.1016/j.pss.2024.106032), for which the optimal closest approach distance (somewhere between 300 to 2000 km for the mother spacecraft) would yield a good science return at a limited risk.

Guendouz, N. Viatte, C. Zeng, Z.-C. Boynard, A. Safieddine, S. Standfuss, C. Turquety, S. Van Damme, M. Clarisse, L. Coheur, P. Sheng, M. Prunet, R.P. Clerbaux, C. https://orfeo.belnet.be/handle/internal/14751 2026-05-21T13:51:28Z

Monitoring Atmospheric Ammonia From Geostationary Orbit: Contributions of GIIRS-B and IRS Remote Sensors Guendouz, N.; Viatte, C.; Zeng, Z.-C.; Boynard, A.; Safieddine, S.; Standfuss, C.; Turquety, S.; Van Damme, M.; Clarisse, L.; Coheur, P.; Sheng, M.; Prunet, R.P.; Clerbaux, C. Ammonia (NH3) is a short-lived atmospheric pollutant with significant environmental and health impacts. Monitoring NH3 remains challenging, as diurnal variability at local scales is still poorly documented. In this study, we analyze two years (July 2022–June 2024) of NH3 total columns from the Geostationary Interferometric Infrared Sounder onboard China's FengYun-4B (GIIRS-B) over East Asia. After applying quality and uncertainty filters, we find good agreement with Infrared Atmospheric Sounding Interferometer (IASI) morning observations (weighted Pearson R = 0.64) and identify relationships between NH3 and skin/land surface temperature over five major NH3 hotspots, suggesting contributions from agriculture (urea fertilizer use), livestock, and secondary urban sources. GIIRS-B's high temporal resolution reveals a clear bimodal diurnal pattern, with NH3 enhancements in the early morning and mid-afternoon in four regions. A dedicated analysis of GIIRS-B NH3 retrieval uncertainties provides a realistic physical benchmark for geostationary infrared observations. Using radiative transfer simulations (4A/OP) driven by atmospheric chemistry model outputs (CHIMERE), we evaluate the potential of the European InfraRed Sounder (IRS) onboard MTG-S to retrieve NH3 at sub-daily resolution. IRS uncertainties are generally larger and more variable than those of IASI, but under favorable thermal-contrast conditions they can become comparable. GIIRS-B and IRS exhibit consistent diurnal uncertainty patterns with nighttime maxima and daytime minima, confirming the realism of the IRS performance assessment. These results highlight the added value of geostationary sounders for improving NH3 emission monitoring, source attribution, and diurnal process understanding in support of future European air-quality regulations.

Verhoelst, T. Compernolle, S. Lambert, J.-C. Fierens, F. Vanpoucke, C. https://orfeo.belnet.be/handle/internal/14748 2026-05-19T13:05:25Z

LEGO-BEL-AQ: luchtvervuiling in België meten vanuit de ruimte Verhoelst, T.; Compernolle, S.; Lambert, J.-C.; Fierens, F.; Vanpoucke, C.

Verhoelst, T. Compernolle, S. Lambert, J.-C. Fierens, F. Vanpoucke, C. https://orfeo.belnet.be/handle/internal/14747 2026-05-19T13:04:44Z

LEGO-BEL-AQ Low-Earth and Geostationary Observations of Belgian Air Quality Verhoelst, T.; Compernolle, S.; Lambert, J.-C.; Fierens, F.; Vanpoucke, C. As in most European countries, Air Quality (AQ) monitoring in Belgium has hitherto been relying mostly on in-situ measurements of near-surface concentrations, with geographical gaps between observations filled in with numerical models. However, a new global constellation of satellite sounders is being built to support detailed monitoring of AQ on the different relevant scales. As part of the Copernicus programme, the EC is contributing with Sentinel-5 Precursor TROPOMI (Low-Earth Orbit, LEO, since 2017) and the upcoming Sentinel-4 (geostationary, GEO) and Sentinel-5 (LEO) missions. While offering near-contiguous observations of the entire domain (cloud cover permitting), observations from space also imply substantial challenges in terms of (1) spatial resolution (e.g., to resolve the Low Emission Zones), (2) relation between observed vertical column amounts and near-surface concentrations, and (3) synergistic use of multiple satellites with different observing geometries. The aim of the LEGO-BEL-AQ project was to facilitate the use of this new-generation satellite AQ data by institutional policy makers and other stakeholders by advancing on the challenges identified above, with specific applications to the Belgian domain. Specifically, we demonstrated that policy-relevant features in the NO2 distribution over major cities can be obtained with superresolution techniques, trading short-term temporal for spatial information. A synergistic use of satellite and in-situ data (using Regression Kriging) allows a pragmatic conversion from tropospheric columns to near-surface concentrations over the complete Belgian domain, and consequently also a confrontation to, e.g., the WHO annual exposure limit guideline of 10 microgram/m3 at the level of municipalities. Reaching the stakeholders implies a need for specific tailoring of the data and their presentation, and the use of communication channels not usually followed by the Earth Observation community.