Extreme Wildfires in a Changing Climate: Global Observations, Chemical Ageing and Impacts on Atmospheric Composition

Extreme wildfires are an increasingly important component of the Earth system. Drought, heatwaves and favourable fire-weather conditions can trigger severe events that release large amounts of trace gases, greenhouse gases and aerosols. Once injected into the free troposphere, wildfire plumes can be transported over thousands of kilometres and undergo substantial chemical transformation through dilution, mixing and photochemistry. Tropospheric ozone is a key outcome of this ageing: unlike primary fire pollutants, it can be either produced or destroyed within transported plumes, yet the conditions controlling this response remain poorly constrained by observations. This PhD will investigate how extreme wildfire plumes evolve chemically during long-range transport and how their occurrence, transport pathways, chemical ageing and atmospheric impacts are modulated by climate variability and extreme weather. The project will exploit the unique multi-decadal and vertically resolved IAGOS aircraft dataset, primarily using O₃ and CO observations, complemented by NOx and, where available, CO₂ and CH₄. These measurements will be combined with SOFT-IO/FLEXPART air-mass histories, satellite observations of fires and atmospheric composition, meteorological and climate datasets, and statistical and explainable machine-learning approaches. The work will follow three main objectives. First, a global database of wildfire-influenced air masses will be established by combining chemical signatures, Lagrangian transport histories and satellite fire observations. Machine-learning methods may support plume detection, classification and identification of recurrent or extreme regimes while remaining physically constrained by independent transport and fire information. Second, the chemical ageing of wildfire plumes will be quantified as a function of transport time, source region, altitude, meteorology, mixing and NOx abundance. O₃–CO relationships, including ΔO₃/ΔCO, will be used to identify conditions associated with ozone enhancement or depletion. For selected regimes, collaboration with Forschungszentrum Jülich may extend the observational analysis through simplified chemical box-model sensitivity experiments to investigate plausible oxidative and ozone-production pathways. Third, the multi-decadal record will be used to determine how climate variability and meteorological extremes affect wildfire occurrence, plume injection, transport and chemical evolution. ENSO and other regionally relevant climate modes will be analysed together with heatwaves, drought, compound hot–dry extremes and fire-weather conditions. The originality of the PhD is to connect, within a single observational framework, the complete climate–fire–transport–chemistry chain. It will provide a global assessment of wildfire plume ageing and ozone evolution and determine how a changing climate may modify the atmospheric impact of extreme fires. École doctorale : SDU2E - Sciences de l'Univers, de l'Environnement et de l'Espace Direction : Bastien SAUVAGE Financement : MSCA COFUND BEST