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 remain uncertain. In this study, soot aerosols with varying 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). The MAC, MEC, SSA, and AAE show a variability between the different types of soot with varying EC/TC ratios. The MAC (MEC) at 550 nm increases for increasing EC/TC, with values of 1.0 (1.4) m2 g-1 for EC/TC Combining double low line 0.0 (BrC-dominated soot) and 4.6 (5.1) m2 g-1 for EC/TC Combining double low line 0.79 (BC-dominated soot). The AAE and SSA (550 nm) decrease from 3.79 and 0.29 (EC/TC Combining double low line 0.0) to 1.27 and 0.10 (EC/TC Combining double low line 0.79). Combining present results for soot from propane combustion with literature data for flame soot from diverse fuels supports a generalised exponential relationship between particle EC/TC and its MAC and AAE values (MAC550Combining double low line(1.3±0.05) e(1.8±0.1)ECTC; AAECombining double low line(0.73±0.12)+(3.29±0.12) e-(2.32±0.30)ECTC), which represents the optical continuum of spectral absorption for soot with varying maturity. From this, it is possible to extrapolate a MAC of 7.9 and 1.3 m2 g-1 (550 nm) and an AAE (375-870 nm) of 1.05 and 4.02 for pure EC (BC-like) and pure OC (BrC-like) soot. The established relationship can provide a useful parameterisation for models to estimate the absorption from combustion aerosols and their BC and BrC contributions.