Infrared radiative properties of vegetation involved in forest fires

An experimental study has been carried out on the radiative properties of six vegetal species. Measurements have been performed in the infrared range on the directional-hemispherical transmissivity and reflectivity. The spectral absorptivity of the various species has been obtained after a pre-processing step, taking into account the porosity of the samples and therefore yielding the characteristics of the plant matter itself. A near constant absorption has been obtained between 2.8 and 10μm (3500 and 1000cm-1, respectively), showing negligible discrepancies between the species. Some variations are observed in the near infrared range, however. Complementary measurements have been conducted after 40 days on dry species, indicating variations in the characteristics linked to a loss in water content. In parallel, numerical investigations have been also conducted applying a ray-tracing method on the basis of these measured characteristics in order to evaluate the radiative properties of a medium featuring a set of leaves. The absorption of an ensemble of individual vegetation particles has been sought, as a preliminary step before describing a realistic vegetation medium. Comparisons with a classical approximate formula for the extinction of an equivalent medium are discussed.