On time dynamics of the complex refractive index and particle microstructure according to data of spectronephelometer measurements in mixed-composition smokes

Abstract

The specific features in the dynamics of the complex refractive index (CRI) are studied for three particle fractions during aging of mixed-composition wood smokes, generated due to simultaneous particle emission from two sources in the regimes of pyrolysis and flaming combustion, for two days. The initial stage is characterized by the generation of smoke in which the imaginary part of the particle refractive index χ decreases, on average, by a factor of 600 with the growth of particle size. When smoke is aged for a long time, χ decreases (by a factor of 1.2) for ultrafine black carbon particles, simultaneously increasing for moderately (by a factor of 1.5) and coarsely (by a factor of 4.7) dispersed particles. This suggests that the dynamics of microphysical smoke composition, at all stages of its existence, is mainly determined by the coagulationdriven penetration of black carbon from the nanometer-size range toward the region of larger particle sizes. Numerical experiment showed that the inverse problem during aging of mixed-composition smokes can be solved correctly only using CRI values strictly corresponding to a given time instant. Attempts to solve the inverse problem using some time invariable CRI values lead to a several-fold increase in the discrepancy between the measured and retrieved optical characteristics versus the 6–10% discrepancy for the actual CRI values. Testing of the three-fraction method for solving the inverse problem with the help of modeled aerosol with a known CRI (ethylene glycol) showed that our approach ensured high-accuracy retrieval of the true CRI for the optically active particles in a moderately dispersed size range. Using the three-fraction method decreases the discrepancy, on average, by a factor of two versus the traditional one-fraction method (assuming that particles are homogeneous in the nature).