Spectral optical properties of soot: laboratory investigation of propane flame particles and their link to composition

Abstract. Soot aerosol generated from the incomplete combustion of biomass and fossil fuels is a major light-absorber, however its spectral optical properties for varying black carbon (BC) and brown carbon (BrC) content remains uncertain. In this study, propane soot aerosols with varying size, maturity, and composition, i.e. elemental to total carbon ratio (EC/TC), have been studied systematically in a large simulation chamber to determine their mass absorption, scattering, and extinction cross sections (MAC, MSC, MEC), single scattering albedo (SSA), and Absorption and Scattering Ångström Exponents (AAE, SAE). Apart from the MSC, all other parameters show a variability associated with the soot 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 AAE and 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 its BC and BrC contributions.