Comparative study of black carbon mixing state characterization: Evaluating Only-SP2 and CPMA-SP2 techniques for...

Black carbon (BC) mixed with non-BC components strongly absorbs visible light and leads to uncertainty in assessing the absorption enhancement (E_abs) and thus radiative forcing. Traditional Single-Particle Soot Photometer (SP2) combined with the leading-edge only fitting (the only-SP2 method) derives BC's mixing states through Mie scattering calculations. However, errors exist in retrieved optical diameter (D_opt) and M_R due to the assumption of the ideal spherical core-shell structure and the selection of the calculation parameters like density and refractive index (RI) of the components. Here, we employed a custom-developed tandem CPMA-SP2 system, which classifies fixed-mass BC to characterize the mixing state, then compared with the only-SP2 method in quantifying the mixing state and E_abs. The field measurements show that the SP2 demonstrates variability in assessing the mixing state of BC in different aging states. The thickly-coated particles with small core approaching the internally mixed state are more sensitive to the change of calculated RI. The D_opt decreases with the RI increasing, indicating that this method accurately measures both D_opt and E_abs when a reasonable refractive index is selected for calculation. However, for thinly-coated particles with moderate or large core, this method results in significant deviations in the computed E_abs (errors up to 15 %). These deviations may be caused by the various shapes of BC and systematic errors. Our results provide valuable insights into the accuracy of the SP2-retrieved D_opt and M_R based on Mie calculations and highlight the importance of employing advanced techniques for further assessment of BC's mixing state.