Light Absorption Properties of Organic Aerosols

Organic aerosols in the atmosphere comprise a complex mixture of carbon-containing particles that can absorb solar radiation to varying degrees. Unlike black carbon, which absorbs broadly across visible and near-infrared wavelengths, a subset of organic aerosol known as brown carbon exhibits enhanced absorption at shorter visible and ultraviolet wavelengths. Sources of brown carbon include primary emissions from biomass burning and biofuel use, as well as secondary formation via atmospheric oxidation of volatile organic precursors. The light absorption of these particles influences Earth’s radiative balance by reducing outgoing shortwave radiation and altering surface and top-of-atmosphere forcing. Photochemical ageing in the atmosphere may either bleach chromophores, diminishing absorption, or lead to chemical transformations that enhance absorptivity. Accurate representation of brown carbon optical properties in climate and air-quality models is vital to constrain its contribution to warming, photochemistry and cloud interactions on regional and global scales.

Research from Nature Portfolio

Recent studies have revealed that wildfire smoke contains a dark brown carbon fraction that dominates shortwave and visible absorption. Measurements in plumes from western-United States wildfires indicate that this water-insoluble species accounts for three-quarters of short visible-light absorption and half of long visible-light absorption. Unlike typical brown carbon, it resists daytime photobleaching and exhibits increased absorptivity following nocturnal processing. These findings underscore the need to update parametrisations in climate models to capture the persistent and strongly absorbing nature of such organic aerosol components.

Light Absorption Properties of Organic Aerosols publication trend

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

Technical terms

Brown carbon (BrC): Light-absorbing organic aerosol with stronger absorption at shorter visible and ultraviolet wavelengths than non-absorbing organic carbon.

Black carbon (BC): Strongly absorbing soot particles from incomplete combustion, with relatively wavelength-independent light absorption.

Absorption Ångström exponent (AAE): Exponent describing how aerosol absorption varies with wavelength; larger values indicate steeper increases in absorption at shorter wavelengths.

Photobleaching: Photochemical degradation of chromophoric organic matter under sunlight, leading to reduced light absorption.

Primary organic aerosol (POA): Organic particles directly emitted into the atmosphere from sources such as biomass burning and combustion.

Secondary organic aerosol (SOA): Organic particles formed in the atmosphere through oxidation and gas-to-particle conversion of volatile organic compounds.

Radiative forcing: Change in Earth’s energy balance caused by alterations in the scattering or absorption of solar radiation by aerosols.

References

1. Shortwave absorption by wildfire smoke dominated by dark brown carbon. Nature Geoscience (2023).
2. More water-soluble brown carbon after the residential “coal-to-gas” conversion measure in urban Beijing. npj Climate and Atmospheric Science (2023).
3. A global simulation of brown carbon: implications for photochemistry and direct radiative effect. Atmospheric Chemistry and Physics (2016).