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

Soot aerosol generated from the incomplete combustion of biomass and fossil fuels is a major lightabsorber; 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-totalcarbon 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=TCD0.0 (BrC-dominated soot) and 4.6 (5.1)m2 g􀀀1 for EC=TCD0.79 (BC-dominated soot). The AAE and SSA (550 nm) decrease from 3.79 and 0.29 (EC=TCD0.0) to 1.27 and 0.10 (EC=TCD0.79). 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 itsMAC and AAE values (MAC550 D (1:3_0:05) e (1:8_0:1)_ECTC_; AAE D (0:73_0:12)C(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 1.3m2 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.