Wildfires: nanoparticles reveal combustion conditions

A major challenge: understanding fires to better protect the environment
Wildfires are increasing worldwide, exacerbated by climate change. A report by the United Nations Environment Programme (UNEP) anticipates a 30% rise in extreme fires by 2050. Beyond visible damage, these fires profoundly transform soils and release potentially toxic metallic elements, particularly in the form of nanoparticles. These particles, smaller than 100 nm, have a large reactive surface area and can disperse through the atmosphere, soils, and waterways, carrying heavy metals or organic contaminants with them.
Despite their importance, the nature and concentration of these incidental nanoparticles in ash and charred wood had been very little studied.

Controlled experiments and State-of-the-Art analyses
The team burned laboratory samples of heather (Erica scoparia), the dominant species of Mediterranean heathlands, at three different temperatures (400, 550, and 800 °C) and for varying durations (5, 15, and 30 minutes), thereby reproducing the main fire regimes, from smouldering fires to intense crown fires. The combustion residues were then analysed using two original and complementary methods:
Single-particle inductively coupled plasma time-of-flight mass spectrometry (spICP-TOFMS), hosted on the PARI platform at IPGP, which enables characterisation of the elemental composition of each nanoparticle present in combustion residue leachates.
Magnetic measurements (susceptibility and saturation magnetisation), which provide information on the nature and concentration of iron oxides formed during combustion.

From iron to manganese: a transformation driven by temperature
The results reveal a radical shift in nanoparticle composition depending on combustion conditions. Fresh and low-temperature (<400 °C) burned samples release particles predominantly rich in iron (more than 50%). In contrast, high-temperature combustion (800 °C) releases manganese nanoparticles (more than 95%). Between these two extremes, at 550 °C, an original population of bimetallic iron–manganese (FeMn) nanoparticles appears, reflecting complex redox interactions between these two elements.
Furthermore, magnetic measurements show that combustion intensity promotes the transformation of initial iron oxides into highly magnetic phases, such as magnetite, through the reducing action of gases released from the combustion of organic plant matter. Saturation magnetisation (Ms), normalised to the initial mass, increases systematically with temperature and combustion duration.