(57d) Enhanced Light Absorption and Radiative Forcing By Black Carbon Agglomerates

The climate models of the Intergovernmental Panel on Climate Change (IPCC) list CO₂, CH₄ and black carbon (BC), as the most potent contributors to global warming based on their radiative forcing (RF) impact. Among them, the BC contribution comes with the highest uncertainty (~ 90 %). Examining closely these models, it becomes apparent that they might underpredict significantly the direct RF for BC, largely due to their assumed spherical BC morphology (Kelesidis & Pratsinis, 2021). In specific, the light absorption and direct RF of BC agglomerates are enhanced by light scattering between their constituent primary particles (Kelesidis et al., 2020) as determined by the Rayleigh-Debye-Gans theory interfaced with discrete dipole approximation (Kelesidis & Pratsinis, 2019) and recent relations for the refractive index and lensing effect. The resulting light absorption agree very well with the observed absorption aerosol optical depth of BC. ECHAM-HAM simulations accounting for the realistic BC morphology and its coatings reveal high direct RF = 3 - 5 W/m² in East, South Asia, sub-Sahara, western Africa and the Arabian peninsula (Fig. 1). These are in agreement with satellite and AERONET observations of RF and indicate a regional climate warming contribution by 2.4 - 4 ^oC, solely due to BC emissions.

References:

Kelesidis, G. A., and Pratsinis, S. E. 2019. Soot light absorption and refractive index during agglomeration and surface growth. Proc. Combust. Inst. 37:1177-1184, doi.org/10.1016/j.proci.2018.08.025.

Kelesidis, G.A., Kholghy, M.R., Zuercher, J., Robertz, J., Allemann, M., Duric, A., and Pratsinis, S.E. 2020. Light scattering from nanoparticle agglomerates. Powder Technol. 365:52–59, doi.org/10.1016/j.powtec.2019.02.003.

Kelesidis, G. A., and Pratsinis, S. E. 2021. Determination of the volume fraction of soot accounting for its composition and morphology. Proc. Combust. Inst. 38:1189-1196, doi.org/10.1016/j.proci.2020.07.055.

Figure 1. Global map of the direct RF of coated BC agglomerates estimated by ECHAM-HAM.