(117b) The Impact of Carbonaceous Nanoparticles to Climate through Their Composition and Morphology

Combustion-generated carbonaceous (soot) nanoparticles have the third strongest contribution to global warming and at the same time the highest uncertainty among all such contributors. Even though soot is made of agglomerates of polydisperse single and chemically-bonded (aggregated) primary particles, its optical properties are still estimated by most climate models using Mie theory for spheres neglecting soot morphology and composition and impeding the accurate estimation of soot direct radiative forcing and climate impact.

Here, the Rayleigh-Debye-Gans (RDG) theory is revised using discrete element modeling for surface growth and agglomeration [2] and discrete dipole approximation [3] to determine the direct radiative forcing of soot agglomerates accounting for the composition, necking (aggregation) and polydispersity of their constituent primary particles. For example, nascent soot single or aggregated primary particles identified in the nucleation mode of engine emissions (mobility diameter, d_m = 7-30 nm) have up to 35 % smaller absorption cross sections than large mature soot agglomerates found in the accumulation mode (d_m = 50-300 nm) due to their smaller C/H ratio. Furthermore, neglecting the fractal-like soot morphology results in 66 % smaller absorption and 6 times larger scattering cross-sections than those of mature soot agglomerates. The nascent and mature soot scattering/absorption cross-sections derived here by the revised RDG theory accounting for primary particle composition, aggregation and polydispersity are in excellent agreement with light scattering [4] and laser induced incandescence measurements in premixed flames and result in an average direct radiative forcing of 0.63 ± 0.05 W/m². This estimate is within those of employed in climate models but its uncertainty is 90 % lower than that from Mie theory. Employing this value, the soot contribution to global warming is reduced to 27.5 ± 2.5 % compared to the 19-58 % given today by climate models.

References:

[1] Kelesidis, G. A., Goudeli, E., & Pratsinis, S. E. (2017). Carbon 121, 527-535.

[2] Kelesidis, G. A., Goudeli, E., & Pratsinis, S. E. (2017). Proc. Combust. Inst. 36, 29-50.

[3] Kelesidis, G. A., & Pratsinis, S. E. (2019). Proc. Combust. Inst. 37, 1177-1184.

[4] Kelesidis, G. A., Kholghy, M. R., Zuercher, J., Robertz, J., Allemann, M., Duric, A., & Pratsinis, S. E. (2019). Powder Technol., doi.org/10.1016/j.powtec.2019.02.003.