Statistical modelling of laser induced fluorescence in plant cover

Abstract

A system of mutually coupled radiative transfer equations is proposed as a formal basis for the numerical analysis of a broad range of spectroscopic effects associated with the propagation of laser radiation in the environment. In particular, this concerns the reabsorption of laser induced fluorescence which inevitably occurs in dense disperse media containing two or more fluorophors. One practical application is for the pressing problem of lidar monitoring the state of plant cover, in particular the concentration of chlorophyll, which regulates the vital activity of plants. A new concept for an optical model of plant cover has been developed in which leaves are not treated as separate scattering elements, but as local volumes of a multiphase medium with a complex polydisperse structure. A modified Monte-Carlo algorithm is created for imitating the fluorescence and reabsorption processes. Test calculations confirm the adequacy of this approach.Introduction. Current advances in the techniques for laser probing of the atmosphere, ocean, and underlying surface involve the use of the informational properties of the transspectral linear and nonlinear processes which accompany the propagation of optical pulses [1-4]. Among the linear process, to which we limit our discussion for now, the best known are spontaneous Raman scattering and laser induced fluorescence (LIF). The current interest in this problem arises from the fact that LIF, as an optical phenomenon, serves as a basis for the creation of highly sensitive means for detecting and monitoring a wide class of molecular compounds, including some which are harmful to the environment. However, an adequate quantitative interpretation of the LIF spectra of chlorophyll a (Chl a ), as well as of other possible pigments, depends on a correct accounting for a number of distorting factors, in particular noise in the atmospheric channel. These have been examined in some detail in the literature [4][5][6][7]. Optical distortions of the signal spectrum at the level of the primary object, i.e., a leaf, are just as important. The fluorescence spectrum of the leafy cover is characterized by two bands [8,9], the first of which, I R (λ), lies in the red region of the spectrum and is centered within 670-690 nm, while the second, I IR (λ), lies in the near IR at λ = 730-740 nm. The LIF intensity ratio (I R /I IR ) is widely used as an indicator of the amount of chlorophyll in a leaf and of its correlations with the surrounding environment (temperature, season, manmade stress, etc.) [9,10]. However, in every specific case, besides these external factors, the ratio (I R /I IR ) depends on internal photochemical processes taking place in the volume of a leaf. Of these, the most important is reabsorption [11] and intermolecular energy transfer [10,12]. The phenomenon of reabsorption, which is the focus of attention in this paper, originates in the same physical circumstances through which the fluorescence produced in molecules of chlorophyll b (Chl b ) in a chloroplast has a chance of un...