Investigation on the linkage between mystery of excess atmospheric absorption and state of mixing of aerosols

Abstract

In order to understand the response of the Earth–atmosphere system to anthropogenic aerosol forcing, it’s essential to know the relative impact of the various aerosol species on the radiation budget. Most of the models used for estimating direct radiative forcing have assumed that various aerosol species are either externally mixed (where each species is considered individually) or internally mixed (well mixed internally, where properties of individual species are volume-averaged). However, it’s possible that one aerosol species is coated over another species to form a core–shell structure, and the resulting radiative impacts can be significantly different from those of externally mixed or internally mixed cases. Recent investigations have suggested that clear-sky atmosphere absorbs more shortwave radiation than predicted by radiative transfer models. The magnitude of discrepancy varies from one location to another. This is popularly known as ‘excess’ atmospheric absorption. Our study suggests that changes in the mixing state of black carbon aerosols may be one of the possible causes of ‘excess’ atmospheric absorption reported by many investigators. We show that past estimates of climate forcing due to anthropogenic black carbon aerosols represent a lower bound, and actual values may be larger than the current estimates. There have been many studies dealing with Mie computation calculations for a layered sphere, but most of them make various kinds of approximations with respect to size, outer medium refractive index, and number of terms in the infinite series representation of various optical parameters. We have developed an analytic method dealing with Mie computations for a spherical shell and have tried to address the limitations put forth by previous methods. It has been tested for QextQ_{\text{ext}}Qext? (extinction efficiency factor) with previous algorithms (in our study, Wiscombe’s algorithm) and further generalized with respect to limitations of previous methods. We have also presented its stability check for all sizes (both ultra-small and ultra-large), and it has been shown that error propagation is constant with increasing series terms in the calculations of various optical parameters, unlike earlier studies which make critical approximations with respect to the number of terms in the series beyond which error adds up with increasing terms in the series.