Research On Ocean Water Environment Parameter Detection Based On Laser Induced Fluorescence

With the rapid development of national economy, an important aspect of the ecological environment protection and sustainable development is the reasonable protection and utilization of large-scale water. The requirement of quantitative remote sensing of water parameters has become increasingly urgent. Meanwhile, new demands are made on the measurement and inversion of water parameters. The main focus of our country is coastal water which is closely related to coastal social and economic development. Because that these waters are much more complex optically than open ocean waters, there is a big uncertainty on the measurements using conventional optical instruments; a wide range of measurement can be realized using satellite ocean color remote sensing, this method is often with low accuracy and the accuracy is to tested and verified. Compared to the on-site biochemical measurements and ocean color remote sensing, laser remote sensing, which utilize laser induced fluorescence (LIF) and light backscattering to analyze bodies of water remotely and are installed as a payload on airborne, shipboard or stationary platforms, is an efficient, proven tool capable of providing quantitative, spatially-resolved, real-time data for over large water surface areas with high spatial resolution.At present, ocean color remote sensing is often used in marine environmental monitoring. Compared to passive remote sensing:selective excitation by monochromatic laser emission; directionality; controlled light source; the use of an inner spectroscopic bench mark; and the possibility of time-(distance)-resolved measurement by time-gating the echo-signal are all advantages of the LIF LIDAR technique. This paper studied the aspects of LIF detection mechanism, algorithms, instrument design and application, as follows:1) Development of portable LIF detection experimental system. Firstly the principle of laser induced fluorescence was discussed. The general optical configuration and optical component of LIF detectors were investigated. In addition, a laser fluoromter with confocal optical configuration and another with orthogonal Czerny-Turner optical configuration were developed. Subsequently, some parameters of the performance, such as baseline noise, baseline drift, sensitivity, limit of detection and dynamic range, were tested systematically. Then research on Raman normalization and temperature correction on lidar data were studied, and the comparison between by measurements by LIF system and those by RF-5301 was carried out.2) In situ measurements of ocean color parameters in Case 2 waters using the LIF systems. Two empirical models based on peak intensity and function fitting were used. In addtion, the formula of semi-empirical model based on radiative transfer were derived. Experiments for the two empirical algorithms with the same data were carried out, and the results were also compared and analyzed. Measuring in situ CDOM absorption coefficient is difficult because it requires prefiltration of water samples. A field-portable laser fluorometer for fast acquisition of in situ CDOM fluorescence data without sample filtering was used, artificial work resulting from sample filtration or storage can be avoided. Subsequently, simultaneous estimates of three important water quality parameters, namely, chlorophyll a (chl-a), colored dissolved organic matter (CDOM), and total suspended matter (TSM) measured by the laser fluorometer were observed to agree well with those measured by traditional methods (chl-a, R2= 0.88; CDOM absorption, R2= 0.90; and TSM, R2= 0.86) in Hangzhou Bay water. In addition, fluorescence optical characteristics in estuarine area were analysed and CDOM appear conservative in mid-salinity waters, suggesting that mixing dominates production and removal processes over time scales. The LIF technique achieved good effect.3) A portable laser-induced fluorescence system for discriminating phytoplankton species has been used. The measured fluorescent spectra were overlapped by various fluorescent components, and were then decomposed by a bi-Gaussian mixture model. A new spectral shape description index was designed to characterize fluorescent spectral shapes for discriminating the phytoplankton species cultured in our laboratory. The phytoplankton species were successfully distinguished from each other at genus level.Finally, we summarized our major work and innovations, and discussed our future research orientations on laser induced fluorescence detection technology.