Absorption closure in highly aged biomass burning smoke

The optical properties of black carbon (BC) are a major source of uncertainty in regional and global climate studies. In the past, detailed investigation of BC absorption has been hampered by systematic biases in the measurement instrumentation. We present airborne measurements of aerosol absorption and black carbon microphysical properties in highly aged biomass burning plumes measured over the southeast Atlantic ocean during CLARIFY-2017, using a suite of novel photoacoustic spectrometers to measure 5 aerosol absorption at 405 nm, 514 nm, and 655 nm, and a single-particle soot photometer to measure the BC mass concentration, size, and mixing state. These measurements are of sufficient quality and detail to provide constraint on optical schemes used in climate models for the first time in biomass burning plumes far from source, an aerosol environment that is one of the most important climatically. The average absorption Angstrom exponents (AAE) were 1.39 over the wavelength range 405–514 nm, and 0.94 over the range 514–655 nm, suggesting brown carbon (BrC) contributed to 10 ± 2 % of absorption at 405 nm. The effective OA mass absorption coefficient (MAC) was 0.27 ± 0.08 m2 g−1 at 405 nm. The BC particles were universally thickly-coated, and almost no externally-mixed BC particles were detected. The MAC of BC was also high, with equivalent absorption enhancements of around 1.8 at all three wavelengths, suggesting that the thick coatings acted as a lens that enhanced light absorption by the BC. We compared the measured MAC and AAE values with those calculated using several optical models and absorption parametrisations that took the measured BC mass and mixing state as inputs. Homogeneous grey sphere Mie models were only able to replicate MAC for some low (real and imaginary) values of the complex BC refractive index (mBC) at the shortest wavelength, but they would have to use unrealistically low values of mBC to accurately replicate AAE. A core/shell Mie model was able to generate good agreement for MAC in the green/red end of the visible spectrum for most values of mBC. However, there are no possible values of mBC that produce MAC values that agree with our observations at all three wavelengths, due to a wavelength-dependent underestimation of the MAC of the underlying BC core. Four semi-empirical parametrisations from literature were also tested, linking BC mixing state to either MAC or absorption enhancement. Two of these schemes produced results that agreed within a few percent of the measured MAC at all three wavelengths, and AAE agreed well when discounting the effects of BrC. Our results uniquely demonstrate the validity of absorption parametrisations, as well as the failings of Mie calculations, in this highly aged environment. We recommend future work should conduct similar analyses in environments where BC has different properties, and investigate the impact of implementing these types of schemes within climate models, as well as developing equivalent schemes for light scattering by soot particles at visible wavelengths.