Numerical quantitation on the effect of coating materials on the mixing state retrieval accuracy of fractal black carbon based on single particle soot photometer

Abstract. Atmospheric black carbon (BC) particles have complex mixing states, the characterization of BC mixing state is essential for assessment of the radiative effects. The single particle soot photometer (SP2) measures BC mixing state with the coating refractive index is assumed to be constant 1.50+0i, which is obviously not suitable for realistic various coating components. In this study, sulfate, non-absorbing organic carbon (OC), and brown carbon (BrC) are selected as typical coatings to numerically quantify the effects of coating material and morphology structure on the retrieval accuracy of BC mixing state. The numerical simulations of BC with thin and thick coatings are conducted using multiple-sphere T-matrix (MSTM), and the mixing state retrievals are based on Mie theory according to the principle of SP2. Results showed that the relative errors of retrieved mixing states for BC coated by sulfate and OC are larger than these for BC coated by BrC, however, the SP2 missed evidently more results of mixing states of BC aerosols coated by BrC. When the retrieved mixing states are employed to assess the absorption enhancement (E_ab) and the radiative forcing, BrC coating also leads to larger deviations in E_ab than sulfate and OC, the estimation error of radiative forcing of BrC coated BC at 1064 nm wavelength reaches -89.9 %, while the corresponding errors for sulfate and OC coatings range from -64.3 % to -38.4 %. This study highlights the coating materials of BC should be considered when monitoring the mixing state using SP2.