OPTICAL-PROPERTIES OF ORGANIC-BASED AEROSOLS PRODUCED BY BURNING VEGETATION

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-mu-m) 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, tumbleweed, 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-mu-m region owing to condensed water and CH3/CH2 groups of aliphatic hydrocarbons; (b) characteristic peaks in the 6-, 8-, and 10-mu-m regions owing to skeletal modes of aromatic and terpenic groups; and (c) relatively little absorption in the 10-12-mu-m region. The imaginary refractive index of all samples is comparable (approximately 0.10-0.30) in the 3-5- and 8-10-mu-m regions and is 3-10 times lower in the 10-12-mu-m region. In the 3-5-mu-m region, most of the absorption takes place in the interval between 3 and 4-mu-m as opposed to the 4-5-mu-m interval which shows significantly less absorption. It is also noteworthy that there is insignificant absorption in the 2.0-2.5-mu-m 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 approximately 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 approximately 0.20 for the 8-10-mu-m region and as high as 0.40 at 3-mu-m to near zero at 5-mu-m in the 3-5-mu-m window.