Simulation chamber study of the spectral optical properties of flame soot and their link to composition

Carbonaceous soot aerosol formed during the incomplete combustion of fossil fuels, biofuels, and biomasses is an important light-absorbing species containing both Black and Brown Carbon (BC, BrC). Despite being key climate forcers, soot and its BC and BrC components are still difficult to represent in models due to persisting uncertainties of its spectral optical properties, such as the complex refractive index, mass absorption/scattering/extinction cross-sections (MAC/MSC/MEC, in m²g^–1) and the single scattering albedo (SSA). In particular, the dependence of optical properties on the variable composition, morphologies, and mixing state of the atmospheric soot remains poorly understood.

In order to advance on this topic, a set of experiments was performed using the 4.2 m³ CESAM simulation chamber on soot aerosol generated from a propane diffusion flame. Experiments were conceived to mechanistically investigate the dependence of soot spectral optical properties on 1/ combustion conditions and varying particle composition, and 2/ different aging processes. To investigate point 1/ the soot aerosols were generated under different combustion conditions covering both fuel–lean and fuel-rich conditions, resulting in particles with varying sizes and elemental/organic carbon (EC, OC) content. For investigating point 2/, the EC-richer soot was subjected to simulated atmospheric aging including exposure to humidity, radiation, and additional gaseous phases (O₃, SO₂), also inducing the formation of a coating by a second scattering aerosol phase produced via the photo-oxidation of SO₂ or the ozonolysis of α-pinene.

The datasets retrieved from the chamber experiments permitted to analyse the dependence of the soot spectral absorption on their BC and BrC particle’s content, resulting in predictive relationships to use in models. Systematic simulation chamber experiments showed that the MAC has a variability associated with the EC/TC ratio in soot. The MAC at 550 nm increases for increasing EC/TC, with values of 1.0 m²g^-1 for EC/TC=0.0 (BrC-dominated soot) and 4.6 m²g^-1 for EC/TC=0.79 (BC-dominated soot). The Absorbing Angstrom Exponent (AAE) and the SSA at 550 nm decrease from 3.79 and 0.29 (EC/TC=0.0) to 1.27 and 0.10 (EC/TC=0.79). A combination of our results for propane soot with literature data for laboratory flame soot from diverse fuels supports a generalized exponential relationship between particle EC/TC and its MAC and AAE values, representing the spectral absorption of soot with varying maturity to lie in an optical continuum. From this, we extrapolate a MAC of 7.9 and 1.3 m²g^-1 (550 nm) and an AAE (375–870 nm) of 1.05 and 4.02 for pure EC (BC-like) and OC (BrC-like) propane soot. The established relationship can provide a useful parameterization for models to estimate the absorption from combustion aerosols and their BC and BrC contributions. Results from this analysis are presented in the Heuser et al. paper available at https://egusphere.copernicus.org/preprints/2024/egusphere-2024-2381/.