Optical Properties of Organic-based Aerosols Produced by Burning Vegetation

Abstract

The real and imaginary refractive indexes of organic-based nonvolatile aerosols produced by burning vegetation are reported for the infrared spectral region. The data were obtained by an iterative Kramers-Kronig analysis of the transmission spectra (2–25 μm) obtained from thin-film samples deposited on a KBr substrate immersed directly in the smoke plumes of small-scale test fires. The results include samples from fires fueled by lawn grass, alfalfa, mesquite, tumble-weed, pine needles, and mixed twigs, leaves, and weeds. The most significant characteristics of all spectra are as follows: (a) strong absorptions in the 3–5-μm region owing to condensed water and CH₃/CH₂ groups of aliphatic hydrocarbons; (b) characteristic peaks in the 6-, 8-, and 10-μm regions owing to skeletal modes of aromatic and terpenic groups; and (c) relatively little absorption in the 10–12-μm region. The imaginary refractive index of all samples is comparable (∼ 0.10–0.30) in the 3–5- and 8–10-μm regions and is 3–10 times lower in the 10–12-μm region. In the 3–5-μm region, most of the absorption takes place in the interval between 3 and 4 μm as opposed to the 4–5-μm interval which shows significantly less absorption. It is also noteworthy that there is insignificant absorption in the 2.0–2.5-μm window region. Results are significantly different from values for elemental carbon which are known to be relatively wavelength independent in these spectral regions and of magnitude between ∼ 0.50 and 1.0 for the imaginary index. Theoretical calculations, assuming particle sizes in the Rayleigh scattering regime, yields a mass extinction coefficient (m²/g) for the organic smokes on the order of ∼ 0.20 for the 8–10-μm region and as high as 0.40 at 3 μm to near zero at 5 μm in the 3–5-μm window.