IASI-PCA: detection of extreme atmospheric events

Air quality is essential for the health and well-being of both human populations and ecosystems. However, in the last decades, atmospheric composition has been increasingly disrupted by extreme events. Pollutants resulting from wildfires, volcanic eruptions, dust storms or anthropogenic pollutions episodes can have significant impacts on human health and can profoundly alter the functioning and resilience of ecosystems. Consequently, the need for rapid and efficient detection and monitoring of extreme atmospheric events become essential for air quality surveillance and forecasting.

Developed by CNES and flying on board the EUMETSAT Metop satellites, IASI (Infrared Atmospheric Sounding Interferometer) is a high-resolution Fourier-transform infrared spectrometer scanning the whole globe twice a day (day and night). It covers the spectral range from approximately 645 to 2760 cm^-1 with 8,461 spectral channels and provides highly accurate measurements of the thermal infrared radiation emitted by the Earth and its atmosphere. These observations enable the retrieval of atmospheric temperature and humidity profiles, as well as the detection and quantification of a wide range of trace gases.

Processing the large volume of IASI data is challenging. Exploiting the potential of IASI for monitoring extreme events therefore requires the development of automated methods. HYGEOS and SPASCIA have investigated the application of Principal Component Analysis on atmospheric spectra to identify extreme events. These latter are defined as data outliers relative to the natural variability of the atmosphere. The novelty is to take advantage of PCA ability to target reconstruction anomalies arising from unusual spectral features in the original measurements, which can be associated with rare or extreme events. The objective is to detect quickly anomalous spectral signatures associated with some trace gases (CO, CO₂, NH₃, SO₂, volatile organic compounds including C₂H₂, C₂H₄, CH₃OH, HCOOH, HCN) and one carbonate mineral (calcite CaCO₃).

IASI-PCA methodology to detect extreme atmospheric events

The performance of the PCA-based detection method has been demonstrated for a large variety of atmospheric events. Vu Van et al. (2023) have proven the relevance of the IASI-PCA approach for the detection of volcanic eruptions. In Pipien et al. (2026), authors show uses cases and statistical analysis of wildfire and dust plumes detection. They also present the framework of an operational processing chain able to deliver a robust and confident first-level characterization of the detected events, suitable for routine monitoring and for rapid user access. Now, the IASI-PCA software is deployed on the ICARE infrastructure. The resulting products are available through the AERIS / ICARE data portal. Furthermore, users can perform the detection of species of interest via the interactive tool available on the ICARE visualisation portal. Below two examples: detection of calcite and detection of sulphur dioxide.

The first calcite (CaCO3) plume is observed on 17^th April 2024 over the northeastern Sahara. It then moves eastward towards Anatolia. The plume cannot be tracked for two days (21^st and 22^nd April), before reappearing over the Aegean Sea on 23^rd April. The plume then moves over Crimea and Ukraine on 24^th April, and subsequently reaches southern Russia, near the Georgian border. On 25^th April, the plume is observed along the coast of the Caspian Sea. In the following days, it is no longer detected.

The eruption of Krakatoa volcano (Indonesia) in September 2026

The major explosive activity of the Krakatoa volcano occurred during the night of 4^th–5^th September 2026, with further eruptive phases in the following days. The plume emitted by the volcano can be observed over the following days thanks to the detection by PCA of anomalous SO₂ concentrations in the atmosphere. The reconstructed spectrum differs significantly from the measured spectrum in the SO₂ absorption spectral regions.

This work is a great achievement of a longstanding cooperation between HYGEOS and SPASCIA. Originally, SPASCIA was the expert in the IASI instrument, thermal infrared measurements, and spectroscopic aspects related to the detection of atmospheric species. HYGEOS contributed its expertise in the analysis of satellite measurements, the development of scientific algorithms, the industrialization of prototypes, as well as its capabilities for processing large volumes of data. Over the years, the collaboration has evolved toward an almost equal workload distribution between the two partners. Thus, HYGEOS has played a significant role to improving the detection method, in particular through the selection and filtering of the reference database of IASI spectra, the consideration of instrumental noise, species detection, and event classification.

For more than 10 years now, both companies are aligned with their core values of collaboration, transparency and equity. Their collaboration is continuing to develop and expand atmospheric composition monitoring capabilities, both through the extension of PCA to the solar spectral range using Sentinel-5P and through the exploitation of the enhanced spectral capabilities of IASI-NG to investigate the detection of additional atmospheric species.