Absorption amplification of black carbon internally mixed with secondary organic aerosol

Abstract

The scattering and absorption properties of black carbon (BC) particles internally mixed with secondary organic aerosol (SOA) were investigated experimentally at the large aerosol chamber facility AIDA. Diesel soot particles were coated with secondary organic compounds produced by the in situ ozonolysis of α‐pinene. It was found that the organic coating strongly affects the optical and microphysical properties of the soot aggregates. Amplification factors of the internally mixed BC of 1.8 to 2.1 compared to the specific absorption cross section of externally mixed BC were measured. These amplification factors are well reproduced by a Mie model for concentrically coated spheres over a wide range of organic coating/BC mixing ratios. Other optical properties in particular of thinly coated soot particles, namely, the single scattering albedo, the Ångstrøm exponent, and the hemispheric backscattering ratio, are less well reproduced by the model, most likely because of the restructuring and the incomplete enclosure of the porous soot aggregates.