Modification of scattering and absorption properties of cloud water droplets by black-carbon aerosols

It is widely recognized that contamination by black carbon may cause substantial changes in scattering and absorption properties of liquid-water clouds, with strong implications for Earth's climate system. However, accurate quantification of these changes had been a very challenging problem since direct computer solutions of the Maxwell equations for realistic models of micrometer-sized droplets with various types of soot impurities were impracticable. This unfortunate situation has finally changed owing to the recent extension of the superposition T-matrix method (STMM) to arbitrarily clustered and nested spherical domains. Hence the principal objective of this talk is to perform the first quantitative comparison of scattering and absorption properties of different types of droplet–soot mixtures. We will also analyze to what extent these properties can be reproduced by a popular effective-medium theory, viz., the Maxwell-Garnett approximation (MGA).

Our numerically exact STMM results reveal that depending on the mode of soot–water mixing, the soot specific absorption can vary by a factor exceeding 6.5. The absorption is maximized when the soot material gets quasi-uniformly distributed throughout the droplet interior in the form of numerous small monomers. The absorption is minimal when black carbon exists in the form of larger homogeneous particles mixed with cloud droplets externally or semi-externally. The presence of soot has other noticeable manifestations such as its effects on the single-scattering co-albedo (and hence cloud albedo) and the elements of the scattering matrix. It is beyond the scope of this talk to discuss the relative plausibility of the various mixing scenarios studied. However, the morphological range captured by these scenarios implies a wide range of remote-sensing and radiation-budget implications of the presence of black carbon in liquid-water clouds.

An important byproduct of our study is the first quantitative analysis of the accuracy of the MGA as applied to micrometer-sized water droplets mixed with black carbon. It appears that the only worthwhile application of the MGA is the calculation of the optical cross sections, single-scattering albedo, and asymmetry parameter for the quasi-uniform mixing scenario.