Impacts of variations in aerosol refractive index on the retrieving of the light-absorption and hygroscopicity of ambient black carbon-containing aerosols using SP2

Black carbon (BC) aerosols directly influence the earth's climate system by absorbing the light intensity and indirectly by interacting with clouds. The microphysical properties of BC -containing aerosols were determined by their core diameter and shell thickness, which can be derived from the measurement of a single -particle soot photometer (SP2). Traditionally, a constant real part of the shell's refractive index (RRI) was employed to retrieve the shell thickness of BC -containing aerosols using the measured scattering signals from SP2. Recent field measurements in East China show that ambient aerosol RRI varies over a wide range between 1.36 and 1.56. The influences of aerosol RRI variation on the retrieving of BC -containing aerosol shell thickness from the measurements of SP2 are investigated with simulation studies. Results show that the variation in ambient aerosol RRI can lead to a variation in the coating thickness of BC -containing aerosols by 9.4%. The corresponding uncertainties in the light absorption enhancement reach up to 29% due to the influence of aerosol RRI. The critical mean supersaturation (SS) of the BC -containing aerosol varies significantly between 0.044% and 0.055% due to the uncertainties in the derived shell thickness using a constant RRI. This study highlights the demand for the real-time measurement of ambient aerosol RRI when deriving the microphysical properties of ambient BC -containing aerosols from the SP2 measurement.