Numerical Investigation On The Effects Of Complex Mixing State On The Black Carbon Absorption Properties

Black carbon（BC）,a kind of trongly absorbing aerosols,is the second anthropogenic global warming factor only after carbon dioxide.Due to the complex aggregate structure,BC are commonly mixed with other aerosols in the ambient atmospheres,and their absorption can be further enhanced due to such mixing.However,due to the complex changes and differences in the size distribution,shape,and mixing state of the particels,there are still large uncertainties in estimating the absorption enhancement.This study develops a numerical model to constrain the effects of internal mixing on the absorption enhancement.Different from previous models that use a particular or a type of particle geometry,our model accounts the randomness of particle geometries and mixing states of aerosols in the atmosphere to better represent BC microphysical properties.e.g.,refractive indices,size,morphologies,mixing position,coating amount,and so on.Our results indicate that the general“core-shell”model overestimate the absorption enhancement about 36%,and our model give more reasonable results of 2～2.2compared with the observations.This model can be used to parameterize the optical properties of black carbon in aerosol chemistry or climate models,and provide a theoretical basis to accurately characterize aerosol properties.If other absorbing aerosols such as brown carbons（Br Cs）are mixed with BC particles,there would be more uncertainties on their absorption properties.Thus,this study also focuses on mixtures of BC and Br C,i.e.,two absorbing carbonaceous aerosols with different absorption spectral variations,and reveals the influences of mixing states on their absorption,especially the spectral variation and enhancement.Three typical mixing states（including internally mixed,partially mixed and externally mixed cases）are considered in the light scattering simulations.Partially mixed particles result in absorption property more similar to the externally mixed ones.The absorption Angstrom exponent（AAE）of internally mixed mixtures would increase to over2 with Br C volume fraction increasing to over～70%,while it is always smaller（by as large as0.1）than those of the partially/externally mixed particles.Different from non-absorbing coating that results in obvious lensing effects,internally mixed BC by Br C coating may not further enhance BC absorption at shorter wavelengths,and the absorption may even be slightly weakened due to the“shielding effect”of the coating.Absorption enhancement E_BC of the internally mixed particles is sensitive to coating absorptivity and amount,and becomes close to or smaller than 1 as the coating becomes more absorbing.Furthermore,the mixing states of two absorbing aerosols would surely influence absorption attribution as well as downstream estimations of BC heating effects,and should be better modelled accurately and paid more attention in observations in the future.