Simultaneous retrieval of the complex refractive indices of the core and shell of coated aerosol particles from extinction measurements using simulated annealing

Retrieving the complex refractive index of homogeneous aerosol particles given the measured extinction efficiency as a function of particle size can be achieved to good precision with almost any minimization algorithm. This is because, given the proper range of sizes and number of sizes in the aerosol size distribution, the measured extinction minus the calculated extinction has an easily distinguished minimum. Simultaneously retrieving the complex refractive index of the core and shell of coated aerosol particles given the measured extinction efficiency as a function of particle size is much more difficult, however. Not only must the minimization be performed over a four-parameter space, requiring more computational resources, but in addition, the measured extinction minus the calculated extinction does not have an easily distinguished global minimum. Rather, there are numerous local minima (other physical solutions leading to approximately the same extinction) to which almost all conventional retrieval algorithms rapidly converge. In this work, preliminary results using two new retrieval algorithms, one that employs a simple random search technique rather than a usual iterative technique, and second that employs the simulated annealing technique, will be shown. Both of these techniques are especially designed to converge on the global minimum when local minima are present. The results of these new retrieval algorithms will be discussed.