Retrieval of refractive index and water content for the coating materials of aged black carbon aerosol based on optical properties: a theoretical analysis

Abstract. Water content in the coatings of aged black carbon (BC) aerosol can be reflected through the complex refractive index. In this study, the retrieval of the refractive index and water content for non-absorbing coatings of BC aerosol during hygroscopic growth (RH = 0 %–95 %) based on scattering and absorption properties is theoretically investigated. Optical properties of morphologically realistic fractal BC aerosols are simulated using the multiple-sphere T-matrix method (MSTM), the optical equivalent refractive index of coating material is retrieved based on the Mie theory, and the water content in coatings is further retrieved using effective medium theory. Results show that the scattering property performs best in retrieving the refractive index and water content. The retrieval errors of the refractive index of heavily aged BC aerosols are less than 10 % at high relative humidities (RHs), while partially coated BC and thinly coated BC have larger errors. The regularity of retrieved water content is similar to that of the refractive index retrieved, and the retrieved water content errors range from 2 % to 63 % for heavily coated BC. This study provides a helpful optical method to obtain the water content of BC coatings.