The study, published as a discussion preprint in the journal Aerosol Research, focused on a subtle but crucial property of wood smoke known as the mixing state of carbonaceous aerosols. When wood burns, it releases two main classes of carbon-based particles: black carbon, the light-absorbing soot that warms the atmosphere, and organic aerosol, a complex cocktail of carbon compounds that can include toxic polycyclic aromatic hydrocarbons, or PAHs. Whether these particles travel through the air as pure soot spheres or as soot cores coated in organic material changes nearly everything about how they behave—how much light they absorb, how readily they take up water and form clouds, how long they linger in the atmosphere, and even how toxic they are when inhaled deep into the lungs.

To probe this hidden structure, the researchers burned dry ash hardwood in a UK Ecodesign-compliant woodstove installed in a controlled test system, running the appliance under five different operating protocols: a standard burn, an overload condition with too much fuel, an underload condition with too little, a hot-reload protocol in which fresh logs were added to an already burning fire, and an open-door protocol in which the stove was opened during operation. The smoke was then fed into an impressive battery of online instruments capable of analyzing particles one by one, in real time, as they emerged from the flue.

At the heart of the measurement suite was a Single Particle Soot Photometer, an instrument that uses a powerful laser to vaporize individual soot particles and measure the light they scatter and incandesce, revealing whether each particle is a bare soot fragment or a soot core wrapped in a thick organic coating. A Differential Mobility Sizer sorted particles by their electrical mobility and thus their size, capturing the full spectrum from nanometre-scale ultrafine particles to larger aggregates. An Aerosol Mass Spectrometer simultaneously vaporized the organic fraction and weighed its chemical constituents, providing a running census of what the smoke was actually made of. Meanwhile, a Fourier-Transform Infrared spectrometer tracked carbon monoxide and carbon dioxide concentrations, allowing the team to calculate a key diagnostic called the modified combustion efficiency, or MCE, which indicates how completely the wood is burning.

From these measurements, the researchers identified three distinct combustion phases that every fire passes through: a pre-ignition phase as the fresh fuel heats up and begins to pyrolyze, a flaming phase in which the volatiles burn vigorously, and a smouldering phase as the fire dies down. Crucially, they subdivided the flaming phase into oxygen-rich and oxygen-poor, or rich, flaming, separated by a modified combustion efficiency of 0.95. This distinction turned out to be the key to understanding why certain user behaviours are so polluting. When oxygen cannot reach the fuel fast enough, combustion becomes incomplete, and the smoke fills with partially burned organic compounds, including PAHs, alongside black carbon particles that never get the chance to burn away cleanly.

The team’s results showed that stove operation affects emissions through two main mechanisms. First, several behaviours triggered the formation of ultrafine particles, defined as particles smaller than roughly 100 nanometres, which are small enough to penetrate deep into lung tissue and even cross into the bloodstream. The open-door, underload, and overload conditions all produced surges of these tiny particles, with large ultrafine peaks also observed during the pre-ignition phase when single large logs were burned. Second, some behaviours prolonged the oxygen-poor rich flaming phase. Hot-reloading, in which fresh wood is thrown onto a hot but oxygen-starved fire, and overloading the firebox both extended this inefficient stage, driving up emissions of PAHs and other harmful organics.

The mixing state measurements added a further layer of insight. Under low combustion efficiency conditions, the researchers found that a large proportion of organic matter, especially PAHs, may prevent black carbon particles from forming the tidy core-shell morphology in which soot sits at the centre of an organic coating. Instead, the particles remained more externally mixed, existing as separate soot and organic populations. This matters because the optical and hygroscopic properties of a bare soot particle differ substantially from those of a coated one, and externally mixed soot can persist longer in the atmosphere, extending its climate warming footprint. The finding also illustrates a counterintuitive point highlighted by the authors: a greater measured coating thickness on soot particles does not necessarily mean the particles are more internally mixed, since thick organic coatings under high organic loading conditions can coexist with a predominantly external mixture.

The study is not without caveats, and the peer review process has been actively probing them. Reviewers noted that multiple dilution stages were applied before the soot photometer measurements, raising questions about whether semi-volatile organic compounds might have redistributed between the gas and particle phases during sampling, potentially altering the observed coating thicknesses relative to what actually leaves the chimney. Reviewers also asked whether the removal of the stove’s catalyst during the experiments means the results represent abnormal rather than typical real-world operation, and whether comparisons with older stoves should likewise use non-ideal operating conditions to be meaningful. The authors have responded to these and other comments, and a revised manuscript is currently under review at the journal.

Nevertheless, the central message carries real weight for the millions of households that burn wood for heat. Residential wood burning has become an increasingly significant source of carbonaceous aerosols in the United Kingdom and worldwide, contributing to both climate forcing and the burden of air pollution that public health agencies warn against. The research suggests that the emissions gap between modern and older stoves may be narrower in practice than certification tests imply, because real users do not always follow the manual. Reloading a hot fire, overfilling the firebox, starving it of fuel, or opening the door to admire the flames all push the combustion chemistry toward the inefficient, oxygen-poor regime where the most harmful emissions are born.

The practical takeaways for stove owners are refreshingly simple. Using appropriately sized pieces of wood, loading the stove according to the manufacturer’s guidance, and avoiding the temptation to reload while the fire is blazing hot can all keep combustion in its cleaner, oxygen-rich mode and reduce both pollution and health risks. As cities from London to Los Angeles grapple with the contribution of domestic burning to wintertime smog, this study is a reminder that the cleanest technology still depends on the hands that operate it—and that a few small changes in habit, informed by the invisible chemistry of a single soot particle, could make a measurable difference to the air we all breathe.

Subject of Research: Mixing state and emissions of carbonaceous aerosols from an Ecodesign residential wood stove under different user operating behaviours

Article Title: Mixing state of Carbonaceous Aerosol Emissions from an Ecodesign Woodstove

Article References: Cheng, Z., Kılıç, D., Wilson, D., Lea-Langton, A., Flynn, M., Kirago, L., Shaw, M., Rickard, A., Hopkins, J. R., Bryant, D., McFiggans, G., Hamilton, J. F., Coe, H., & Allan, J. (2026). Mixing state of Carbonaceous Aerosol Emissions from an Ecodesign Woodstove. https://doi.org/10.5194/ar-2026-24

Image Credits: AI Generated

DOI: 10.5194/ar-2026-24

Keywords: wood burning, Ecodesign stove, black carbon, organic aerosol, mixing state, ultrafine particles, PAHs, combustion efficiency, residential heating, air quality, aerosol measurement, user behaviour