Radiative Effects of Soot and Sulfate Aerosols: Internal vs. External Mixtures

Aerosols are extremely complex in both their chemical and physical structure, and highly variable in their spatial and temporal distribution. It is therefore important to have an understanding of these, as they determine their optical properties and ultimately their climatic effects. The other key factor is how the aerosol components are combined. We have compared the optical and radiative properties of external and internal mixtures of sulfate and soot aerosols. The internal mixtures have been modelled as both coated spheres and homogeneous mixtures, from which the optical properties of extinction, absorption, asymmetry factor and single scattering albedo are computed. These optical properties are used to perform broadband radiative transfer calculations of the radiative effects of these aerosol models. The radiative forcing has been examined, however the main focus is on flux divergence, as soot is the major (aerosol) contributor to the absorption of solar radiation. The results show that there is a distinguishable difference in absorption between the three internal mixtures, but the most significant outcome is that internal mixtures absorb up to twice as much solar energy as do the external mixtures for the same mass of soot. This result clearly has significant implications for the absorption anomaly problem. Because aerosol particles come in a range of sizes the range of `parameter space' is potentially very large, especially for the internal mixtures. For this reason, two studies were conducted, with the results of the first helping to fix the parameters of the second. The first phase was restricted to monodisperse particles, which indicated that the precise nature of an internal mixture - coated sphere or homogeneous - has less of an impact than the fact that the particles are internally mixed. Therefore, in the second phase polydisperse aerosols were examined, for a constant soot mass and a constant soot mixing ratio per particle.