Coated Soot Amplifies Climate Effect: Biomass Smoke Warms More Than Urban Emissions

New analysis: Coating makes the difference

A recent evaluation in Nature Geoscience combines global measurement series with radiative models and reaches a clear conclusion: deposits of other substances on black carbon (soot) particles amplify their warming effect. The effect is particularly strong for smoke from biomass fires. Under comparable conditions, it leads to a two- to threefold higher radiative forcing at the top of the atmosphere (TOA) than urban emissions.

Why soot coatings increase warming

The optical core-shell effect is decisive: transparent or semi-transparent coatings focus incident light more strongly into the absorbing soot core, increasing effective absorption. At the same time, coatings alter the particles' scattering properties and thus the cooling contribution from backscattering. Overall, the radiation balance shifts in favor of additional warming.

Biomass versus urban sources

In smoke from open biomass and wildfires, soot particles are more often found embedded in thickly coated mixtures than in typical urban exhaust. Causes include composition, residence time and secondary aerosol formation, which preferentially deposits organic material onto soot. Result: for the same mass of soot, the TOA radiative forcing from biomass smoke is on average two to three times higher than from urban sources.

Method: observations linked with models

The research team used observed particle properties as input for optical and radiative transfer models. This allowed them to estimate the contribution of coated soot particles to warming with regional detail and global consistency—an explanatory power that single measurement campaigns or pure modeling studies rarely achieve on their own.

Implications for climate models and policy

Many global climate models represent aerosols primarily by mass and simplified properties. The results show that this approach can underestimate the warming effect of black carbon—especially in regions with intense biomass burning. For more robust projections, the internal mixing state of particles and their temporal evolution should be represented more explicitly. Politically, measures against open biomass burning (such as slash-and-burn, brush and forest fires, and inefficient cookstoves) move further into focus. Emission reductions in urban areas remain important but have a relatively weaker effect on the shortwave radiative forcing.

Additional impacts beyond radiative effects

Increased absorption affects atmospheric stability, cloud formation and precipitation patterns. When soot deposits on snow or ice, albedo decreases further—accelerating melt processes and potentially driving additional regional warming.

Open questions and next research needs

Unresolved are the rates and mechanisms of coating formation under different air quality conditions and the roles of specific chemical components. Long-term measurement programs and the integration of detailed aerosol processes into models are important. Effective measures require regional data on emission sources, reaction pathways and transport.

Context for practice and the public

Soot is not all the same: origin and mixing state largely determine its climate impact. Reducing open biomass burning therefore provides a double benefit—improving air quality and curbing a disproportionately strong contribution to warming.