Simulations of light absorption of carbon particles in nanoaerosol clusters

Black carbon soot (BS) is considered to be the second most contributing organic matter next to carbon dioxide for the global warming effect. There is, however, so far no consensus on the quantitative warming effect due to the increased distribution of black carbon in the atmosphere. A recent report (Science 2012, 337, 1078) suggests that due to BS there is only a few percentage enhancement in absorption of BS-immersed aerosols. To get proper interpretation of the available experimental data, it becomes essential to obtain details of the microscopic origin of the absorption and scattering processes of the aerosol clusters due to the presence of soot. However, so far, due to the large spatial scale and the need for a quantum mechanical description of the particles involved in the absorption and scattering, this quest has posed an insurmountable challenge. In the present work we propose the use of a multiscale integrated approach based on molecular dynamics and a quantum mechanical-molecular mechanical method to model the optical property of molecules immersed in nanosized aerosol particles. We choose fluoranthene (FA) with varying cis-pinonic acid (CPA) impurity concentration as an illustrative example of application. We observe that normally FA tends to be on the surface of the nanoaerosols but in the presence of CPA impurities its spatial location changes to a core aggregate to some extent. We find that the absorption maximum is only slightly red-shifted in the presence of increased CPA concentrations and that the oscillator strengths are not altered significantly. The comparable values for the oscillator strengths of all the low energy excitations suggest that the absorption enhancement of the aerosol due to BS will not be substantial, which is in line with the recent experimental report in Science.