(447g) Effect of Soot Chemistry and Structure On the Uncertainty in the Atmospheric Aerosols Irradiative Forcing Component

Soot generated during combustion is composed of nanometer carbonaceous particles (black soot) that are aggregated together into submicrometer fractal aggregates. These particles can have adsorbed layers of polycyclic aromatic hydrocarbons (PAHs) or layers of condensed water, sulfates or other hydrocarbons. Soot particles can also be individual droplets of polycyclic aromatic hydrocarbons (brown soot) that can be coated with layers of water, sulfates, and other hydrocarbons. The present state of the art of light-scattering and light-absorption models provides for the multiple scattering of small primary particles typically below the Rayleigh limit (a size less than the wavelength of light) in fractal aggregates. This work extends the multiple-scattering theory to include multiple absorption, larger primary particles by using Mie theory, and more importantly adds scattering and absorption from layers of varying thickness adsorbed on the primary particles to more realistically predict the effects of soot chemistry and structure in atmospheric irradiative forcing. Allowance for the effect of the condensed layers on soot and the fractal structure of soot is important in predicting the impact of soot on the irradiative forcing function in global climate models. These results predict that: 1) black and brown soot have very different aerosol forcing effects, with black soot having a larger positive aerosol forcing effect; 2) the strong absorption properties of black soot are effectively enhanced by fractal structure and adsorbed layers, which increase the aerosol forcing, and 3) the low fractal dimension of soot enhances light scattering and limits light absorption, which lowers aerosol forcing.