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. The mixing states of coated black carbon particles (BC) are essential for assessing their optical properties and radiative effects. The single particle soot photometer (SP2) measures BC mixing states based on the assumption that the refractive index (RI) of the coating material is constant 1.50 + 0i. However, atmospheric BC aerosols may be coated by diverse materials with various refractive indices, therefore, the unrealistic assumption of coating RI would bring errors in the measured mixing states. For the evaluation of the effects of coating materials on the measurement accuracy of BC mixing states, from the perspective of numerical simulation, typical materials: sulfate, non-absorbing organic carbon (OC), and brown carbon (BrC) are selected as coatings of BC in this study. The fractal closed-cell model and coated-aggregate model are constructed to represent BC with thin and thick coatings, multiple sphere T-matrix (MSTM) simulation results are used to study the optical properties of realistic coated BC, and the mixing states are optically retrieved based on Mie theory according to the retrieval principle of the SP2. Results showed that the SP2 would underestimate mixing states for fractal BC with BC core diameters larger than about 200 nm, and the relative errors of retrieved mixing states for BC coated by sulfate and OC are evidently larger than those for BC coated by BrC. Furthermore, when the SP2 measured mixing states are employed to estimate the absorption enhancement (Eab) and radiative forcing of BC aerosols, the estimation error of Eab for BC with BrC coating is significantly larger than that with sulfate and OC coatings, and the estimation Eab could be up to 6.19 times higher than the inherent value of fractal BC. Compared with fractal BC, the radiative forcing estimation error at 1064 nm for BrC-coated BC can reach −88.73 %, while the estimation errors of BC coated by the other two materials range from −65.22 % to −37.50 %. This study highlights that the coating materials and fractal morphology should be considered when the SP2 is used to measure the mixing state of BC.