Light Absorption Properties of Atmospheric Black Carbon

Atmospheric black carbon (BC) is a potent absorber of sunlight, exerting a significant warming influence on the climate system. Generated by incomplete combustion of fossil fuels and biomass, BC exists as fractal aggregates of carbonaceous spherules whose optical behaviour depends on size, morphology and chemical environment. Freshly emitted BC typically presents a ‘lacy’ agglomerate form with high porosity and strong absorption per unit mass. As particles age, they acquire coatings of organic and inorganic materials, altering refractive index contrasts and enhancing absorption through the ‘lensing’ effect. The degree of enhancement varies with coating thickness and composition, and is quantified by parameters such as the mass absorption coefficient, absorption Ångström exponent and single-scattering albedo. Heterogeneity in mixing state—comprising internally mixed, externally mixed or partially coated BC—and diversity in core-shell structures challenge global models and remote-sensing retrievals. Improved understanding of BC light absorption is essential for refining climate projections, guiding emission-control strategies and evaluating health impacts arising from radiative and pulmonary effects.

Research from Nature Portfolio

Recent studies have developed a unified theoretical framework that connects dynamic processes of coating formation to distributions of coating thickness on BC cores. This framework reveals a universal self-similar law governing size distributions across diverse environments, and it has been integrated into global and regional climate models to improve estimates of BC radiative forcing. Complementary work employing multiple-mixing-state aerosol microphysics models demonstrates that resolving particle-scale diversity in size and mixing state amplifies uncertainties in present-day direct radiative effects by five to sevenfold compared with single-state representations. Foundational research has further shown that neglecting compositional diversity at the particle level can lead to a twofold overestimation of absorption enhancement, and the explicit resolution of population-averaged composition diversity brings modelled absorption into closer agreement with ambient observations.

Light Absorption Properties of Atmospheric Black Carbon publication trend

The graph below shows the total number of articles in light absorption properties of atmospheric black carbon across all publications each year (not limited to Nature Index journals).

Technical terms

Mixing state: The arrangement and distribution of BC and non-BC components within aerosol particles, influencing optical and hygroscopic properties.

Coating thickness: The radial extent of non-absorbing or brown carbon materials enveloping a BC core, critical for absorption enhancement.

Mass absorption coefficient (MAC): The absorption cross-section of BC per unit mass, reflecting its efficiency in converting light into heat.

Single-scattering albedo (SSA): The ratio of scattering to total extinction (absorption plus scattering), indicating the relative contribution of scattering to light attenuation.

Absorption Ångström exponent (AAE): A wavelength-dependent exponent that describes how absorption varies across the spectrum, sensitive to composition and mixing.

Fractal dimension (Df): A quantitative metric describing the complexity and compactness of BC aggregates, affecting optical interactions.

References

1. Unified theoretical framework for black carbon mixing state allows greater accuracy of climate effect estimation. Nature Communications (2023).
2. Black carbon radiative effects highly sensitive to emitted particle size when resolving mixing-state diversity. Nature Communications (2018).
3. Black carbon absorption at the global scale is affected by particle-scale diversity in composition. Nature Communications (2016).
4. Photochemical Aging Induces Changes in the Effective Densities, Morphologies, and Optical Properties of Combustion Aerosol Particles. Environmental Science and Technology (2023).
5. Microphysical properties of atmospheric soot and organic particles: measurements, modeling, and impacts. npj Climate and Atmospheric Science (2024).
6. Lifecycle of light-absorbing carbonaceous aerosols in the atmosphere. npj Climate and Atmospheric Science (2020).