Lensing Effect of Black Carbon With Brown Coatings: Dominant Microphysics and Parameterization

Absorption enhancements of polydisperse black carbon (BC) aggregates with brown coatings, decomposed into those due to lensing effect (E-len) and blocking effect (E-blo), are numerically studied based on recent observations with an accurate multiple-sphere T-matrix method. Both E-len and E-blo of BC with brown coating are predominantly determined by coated volume fraction of BC in ultraviolet and visible spectral regions, and the impacts of other microphysical parameters of coated BC are trivial if BC is no more than half encapsulated. If more volume fraction of BC is coated, scattering coating can focus more photons onto BC core with stronger lensing effect whereas absorptive coating blocks more photons originally induced by lensing effect onto BC core. The E-len may not be further enhanced with increased shell/core ratio after reaching a threshold, whereas the E-blo has sizable wavelength dependence and becomes close to the E-len in ultraviolet region. Moreover, the lensing and blocking effects of BC with brown coatings are simply parameterized, and the convenient treatments are expected to find wide applications for BC absorption enhancements due to coating. Plain Language Summary Black carbon (BC) absorption enhancement due to coating is an important factor for observation-model discrepancy, whereas the results of absorption enhancement of coated BC from experimental and theoretical studies are generally conflicting. Thorough understanding of the absorption enhancements of BC with brown coatings influenced by various BC microphysics and proposing their simple parameterizations for applications are a challenging and interesting topic. In our study, an exact multiple-sphere T-matrix method is employed to thoroughly examine the effects of observation-based BC microphysics on absorption enhancements of BC with brown coatings modeled with more realistic geometries, which are decomposed into lensing effect and blocking effect due to absorptive coating. The results reveal that coated volume fraction of BC is dominant in determining both lensing and blocking effects of BC with brown coatings in ultraviolet and visible regions whereas other BC microphysics are their perturbations, which plausibly give a reconciliation of conflicting results that largely variable BC absorption enhancements are seen. Simple parameterizations of lensing and blocking effects of BC with brown coatings are then proposed for a quantitative understanding and further application. The findings are useful to the communities for understanding BC absorption enhancements and a more accurate prediction of climate.