Is Black Carbon an appropriate generalization in climate studies?

Abstract

Soot aerosols produced during incomplete combustion processes are agglomerates composed of quasi-spherical carbonaceous primary particles with markedly different arrangements (shape, size, and internal structure) and varying compositions. Although they have been considered to be one of the main contributors to climate change, it remains difficult to quantify their global warming potential (GWP) precisely. Exact knowledge of the optical properties of soot agglomerates (black carbon -BC) would be necessary to fairly establish a CO2 equivalent GWP. BC warming effects depend on their size and shape, and despite their limited residence time in the atmosphere, their high radiative forcing would lead to GWP ~ 2000. The quantification of BC concentrations in the atmosphere is often based on optical measurements, which usually lack accuracy. Our calculations on the direct radiative forcing of BC showed that the irregularity of the agglomerate, and their size markedly affect the cooling/heating capacity due to extreme variations of its absorbing and scattering characteristics. In addition, the estimation of the incremental radiative forcing of aerosols is challenging since it is highly dependent on local parameters such as cloudiness, surface albedo, aerosol concentration, etc., which are highly variable worldwide. Based on that, we do not recommend using a unique CO2 equivalent GWP for BC.