Variations of the optical constants and size spectra of smoke aerosols produced during the thermal decomposition of different types of wooden materials

Abstract

The directed light scattering coefficient β(ϑ j , λ j ) of smoke mixtures was measured at five scattering angles ϑ j = 15°, 45°, 110°, 135°, and 165° and at nine wavelengths in an interval of 440–690 nm within the framework of the smoke experiment conducted in the 1800-m3-volume Big Aerosol Chamber at the Institute of Atmospheric Optics using an automated polarized spectral nephelometer. The duration of the measurements was about 100 h. Based on the results of the spectral nephelometric measurements, the parameters of the microstructure and the complex refractive index were determined at three ranges of dispersity with the help of the improved method of the inversion of the aerosol light scattering characteristics. The specificity of the formation of the microstructure of a complex multicomponent dispersion mixture produced by the sequential mixing of pyrolysis products of different types of wooden materials is considered. The obtained estimates of the aerosol optical constants show that variations of the real n and imaginary χ parts of the complex refractive index (CRI) for coarse particles with r > 300 nm in the composition of the sequentially mixed smokes of pyrolysis are not as significant as for the fine fraction. In particular, for these particles, n varies in an interval of 1.58–1.62 and χ = 0.008–0.03, which characterizes them as weakly and moderately absorbing particles. The absorption of the particle’s finest fraction is sufficiently strong, varying in a wider interval of values: n = 1.78–2.20 and χ = 0.1–0.8. The moderate-sized particles (100 < r < 300 nm) have the real part of CRI primarily close to n ∼ 1.64 and χ ∼ 0.045 for coarse particles. Only for smoke generated during the pyrolysis of birch and aspen bark are these characteristics noticeably higher, approaching the estimates obtained for the fine fraction: n ∼ 1.80 and χ ∼ 0.20.