Radiative absorption enhancements by black carbon controlled by particle-to-particle heterogeneity in composition

Black carbon (BC) absorbs solar radiation, leading to a strong but uncertain warming effect on climate. A key challenge in modeling and quantifying BC’s radiative effect on climate is predicting enhancements in light absorption that result from internal mixing between BC and other aerosol components...

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Bibliographic Details
Published in:Proceedings of the National Academy of Sciences - PNAS 2020-03, Vol.117 (10), p.5196-5203
Main Authors: Fierce, Laura, Onasch, Timothy B., Cappa, Christopher D., Mazzoleni, Claudio, China, Swarup, Bhandari, Janarjan, Davidovits, Paul, Fischer, D. Al, Helgestad, Taylor, Lambe, Andrew T., Sedlacek, Arthur J., Smith, Geoffrey D., Wolff, Lindsay
Format: Article
Language:English
Subjects:
Absorption
absorption enhancement
aerosol mixing state
Aerosols
Approximation
Black carbon
Climate effects
Climate models
Climate prediction
Composition effects
direct radiative forcing
Electromagnetic absorption
ENVIRONMENTAL SCIENCES
Heterogeneity
Laboratories
Mathematical analysis
Physical Sciences
Solar radiation
Spherical shells
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Summary:Black carbon (BC) absorbs solar radiation, leading to a strong but uncertain warming effect on climate. A key challenge in modeling and quantifying BC’s radiative effect on climate is predicting enhancements in light absorption that result from internal mixing between BC and other aerosol components. Modeling and laboratory studies show that BC, when mixed with other aerosol components, absorbs more strongly than pure, uncoated BC; however, some ambient observations suggest more variable and weaker absorption enhancement. We show that the lower-than-expected enhancements in ambient measurements result from a combination of two factors. First, the often used spherical, concentric core-shell approximation generally overestimates the absorption by BC. Second, and more importantly, inadequate consideration of heterogeneity in particle-to-particle composition engenders substantial overestimation in absorption by the total particle population, with greater heterogeneity associated with larger model–measurement differences. We show that accounting for these two effects—variability in per-particle composition and deviations from the core-shell approximation—reconciles absorption enhancement predictions with laboratory and field observations and resolves the apparent discrepancy. Furthermore, our consistent model framework provides a path forward for improving predictions of BC’s radiative effect on climate.
ISSN:0027-8424
1091-6490
DOI:10.1073/pnas.1919723117