Optical remote sensing of marine constituents in coastal waters: a feasibility study

Optical remote sensing of ocean color is a well-established technique for inferring ocean properties. However, most retrieval algorithms are based on the assumption that the radiance received by satellite instruments is affected only by the phytoplankton pigment concentration and correlated substances. This assumption works well for open ocean water but becomes questionable for coastal waters. To reduce uncertainties associated with this assumption, we developed a new algorithm for the retrieval of marine constituents in a coastal environment. We assumed that ocean color can be adequately described by a three-component model made up of chlorophyll a, suspended matter, and yellow substance. The simultaneous retrieval of these three marine constituents and of the atmospheric aerosol content was accomplished through an inverse-modeling scheme in which the difference between simulated radiances exiting the atmosphere and radiances measured with a satellite sensor was minimized. Simulated radiances were generated with a comprehensive radiative transfer model that is applicable to the coupled atmosphere-ocean system. The method of simulated annealing was used to minimize the difference between measured and simulated radiances. To evaluate the retrieval algorithm, we used simulated (instead of measured) satellite-received radiances that were generated for specified concentrations of aerosols and marine constituents, and we tested the ability of the algorithm to retrieve assumed concentrations. Our results require experimental validation but show that the retrieval of marine constituents in coastal waters is possible.     

悬浮颗粒物海水及其声吸收

通过对悬浮颗粒物粘滞性声吸收的分析,指出了在悬浮颗粒物海水声传播研究中所需要的悬浮颗粒物参数;经海水中声吸收计算分析,给出了能够对声传播产生影响的悬浮颗粒物参数的范围。为应用于声学研究的悬浮颗粒物海水海洋调查、数据积累及海区划分提供依据。     

Effect of suspended particulate-size distribution on the backscattering ratio in the remote sensing of seawater

Mie theory is used to study the influence of the particle-size distribution (PSD) on the backscattering ratio for case 1 and 2 waters. Several in situ measured PSDs from coastal water and the open ocean, representing typical case 2 and 1 waters, were used in this investigation. Calculation of the backscattering ratio requires integration of the PSD over a much broader size range than is usually measured. Consequently extrapolation from fitted data is necessary. To that purpose the measured data are fitted with hyperbolic (Junge) and the two-component model of the PSD. It is shown that the result of extrapolation, hence the backscattering ratio, critically depends on the chosen PSD model. For a particular PSD model the role of submicrometer particles and the applied integration limits on the backscattering ratio is discussed. The use of the hyperbolic PSD model largely overestimates the number of small (submicrometer) particles that significantly contribute to backscattering and consequently leads to an erroneously high backscattering ratio. The two-component model proves to be an adequate PSD model for use in backscattering/scattering calculations providing satisfactory results complying with experimental data. The results are relevant for the inversion of remotely sensed data and the prediction of optical properties and the concentration of phytoplankton pigments, suspended sediment, and yellow substance.     

Optical remote sensing of waters with vertical structure

Optical remote sensing of ocean color is a well-established technique that is used to produce maps of marine constituents on a routine basis. Retrieval algorithms used to infer pigment concentrations from measurements of ocean color are usually based on the assumption that the upper ocean column is vertically homogeneous. However, stable stratification of the water column is often encountered in coastal waters and in fjords. This stratification is decisive for the initiation, maintainance, and species composition of phytoplankton blooms. Here we present an optical remote-sensing algorithm with the ability to resolve such a vertical structure of oceanic waters. The vertical structure is assumed to consist of two homogeneous layers with different concentrations of chlorophyll a. The algorithm is designed to determine the chlorophyll-a concentrations of the two layers as well as the thickness of the upper layer. These three parameters influence the ocean color and are simultaneously retrieved through an inverse-modeling technique. This technique consists of using radiative-transfer computations for a coupled atmosphere-ocean system to simulate radiances received in various bands of the satellite sensor and to compare these simulated results with measured radiances. The sum of absolute values of differences between simulated and measured radiances is minimized by use of an optimization algorithm, and the retrieved parameters are those that yield the minimum sum of differences between measured and simulated data. The optimization algorithm that we used in our study is the simulated annealing method, which is an extension of the downhill simplex algorithm. In this study the algorithm was tested on synthetic data generated by the forward model. The results indicate that it should be possible to retrieve vertical variations in the pigment concentration. The synthetic data were generated for spectral bands that coincide with those of the Medium Resolution Imaging Spectrometer sensor, which will be a part of the instrument package of the upcoming Environmental Satellite.     

Effect of bromide ion on isolated fractions of dissolved organic matter in secondary effluent during chlorination

The role of bromide ion in the trihalomethane (THM) formation and structure of dissolved organic matter (DOM) during chlorination of the secondary effluent taken from the Wenchang Wastewater Treatment Plant (Harbin, China) was investigated. DOM was fractionated using XAD resins into five fractions: hydrophobic acid (HPO-A), hydrophobic neutral (HPO-N), transphilic acid (TPI-A), transphilic neutral (TPI-N) and hydrophilic fraction (HPI). The patterns of individual THM species with increased bromide concentrations were similar for all DOM fractions. The THM speciation as well as halogen fraction for these five fractions followed similar trends with the Br(-)/Cl(2) ratio. Chlorination resulted in decreased ultraviolet (UV) absorbance across wavelengths from 250 to 280 nm for DOM fractions whether bromide ions existed or not, and bromide addition led to lower differential UV absorbance values. Fourier-transform infrared (FT-IR) results indicated that chlorination, whether bromide ions existed or not, resulted in the near elimination of aromatic CH and amide peaks, increased CO absorption intensity and occurrence of CO and CCl peaks for HPO-A, HPO-N, TPI-A and TPI-N. Furthermore, bromide addition in chlorination led to the occurrence of CBr peak for all four fractions.     

Partitioning spectral absorption in case 2 waters: discrimination of dissolved and particulate components

A series of three mathematical procedures is derived to discriminate the light absorption by phytoplankton, colored dissolved organic matter, and nonpigmented particulates in waters in which absorption is dominated by factors other than phytoplankton (i.e., case 2 waters). The procedures utilize normalized absorption cross-sectional spectra of the absorption components and matrix inversion to solve for the coefficients that scale the normalized spectra. The procedures differ in the amount of ancillary measurements incorporated to reduce the variability of the estimates. The procedure that incorporates no ancillary information is expected to be unbiased only over long time periods. Application of the procedures to a 15-day time series of continuously monitored data from the Rhode River, Maryland, revealed the presence of large (approximately twofold) changes in absorption at 440 nm over periods of a few hours. Hourly sampling over a 24-h period confirmed that the changes in measured optical coefficients corresponded to changes in water quality. Errors in estimates of absorption components were of a magnitude consistent with those observed in development of the procedures and confirmed the progressive improvement achieved by incorporation of additional information. Over the time period observed, changes in optical properties appeared to be driven by advective processes.     

Source characterization of total suspended particulate matter near a riverbed in Central Taiwan

Samples of total suspended particulate (TSP) matter were collected by using TSP samplers from certain areas representing the estuary of the Jhuoshuei River, Taiwan during monsoon and non-monsoon seasons in 2005. A total of 12 elements and nine inorganic ions were identified by inductively coupled plasma atomic emission spectrometry (ICP-AES) and ion chromatography (IC), respectively. Enrichment factors explaining a preponderance of the variance in the data were applied to the data sets. The results show that wind direction significantly affected the concentration of TSP during the monsoon season; moreover, these concentrations were also markedly higher during the monsoon than during the non-monsoon season. Low enrichment factor (EF) values (1.18-2.88) were observed during the monsoon for Ca, Fe, Na, Ba, Cd, Co, Li, Mn, and Sr, reflecting the importance of dust contribution by natural processes. Conversely, the EF values calculated for Ca, K, Ba, Co, Li, and Zn were relatively high (7.03-20.26) when observed during non-monsoon season, a phenomenon suggesting that they are mainly contributed from non-crustal sources. The observations of relatively enhanced EF values during monsoon indicate that the changes in wind direction between monsoon and non-monsoon seasons are associated with the different contribution sources. The high concentration of TSPs observed in the ambient air is believed to be primarily due to surface soil particle emission from the riverbeds.     

Radiative transfer model for the computation of radiance and polarization in an ocean-atmosphere system: polarization properties of suspended matter for remote sensing

A radiative transfer code termed OSOA for the ocean-atmosphere system that is able to predict the total and the polarized signals has been developed. The successive-orders-of-scattering method is used. The air-water interface is modeled as a planar mirror. Four components grouped by their optical properties, pure seawater, phytoplankton, nonchlorophyllose matter, and yellow substances, are included in the water column. Models are validated through comparisons with standard models. The numerical accuracy of the method is better than 2%; high computational efficiency is maintained. The model is used to study the influence of polarization on the detection of suspended matter. Polarizing properties of hydrosols are discussed: phytoplankton cells exhibit weak polarization and small inorganic particles, which are strong backscatterers, contribute appreciably to the polarized signal. Therefore the use of the polarized signal to extract the sediment signature promises good results. Also, polarized radiance could improve characterization of aerosols when open ocean waters are treated.     

Impact of exotic and inherent dissolved organic matter on sorption of phenanthrene by soils

The impacts of exotic and inherent dissolved organic matter (DOM) on phenanthrene sorption by six zonal soils of China, chosen so as to have different soil organic carbon (SOC) contents, were investigated using a batch technique. The exotic DOM was extracted from straw waste. In all cases, the sorption of phenanthrene by soils could be well described by the linear equation. The presence of inherent DOM in soils was found to impede phenanthrene sorption, since the apparent distribution coefficients (K(d)(*)) for phenanthrene sorption by deionized water-eluted soils were 3.13-21.5% larger than the distribution coefficients (K(d)) by control soils. Moreover, the enhanced sorption of phenanthrene by eluted versus control soils was in positive correlation with SOC contents. On the other hand, it was observed that the influence of exotic DOM on phenanthrene sorption was related to DOM concentrations. The K(d)(*) values for sorption of phenanthrene in the presence of exotic DOM increased first and decreased thereafter with increasing the added DOM concentrations (0-106mgDOC/L). The K(d)(*) values at a low exotic DOM concentration (< or =28mgDOC/L) were 14.7-48.4% larger than their control K(d) values. In contrast, higher concentrations (> or =52mgDOC/L) of added exotic DOM clearly impeded the distribution of phenanthrene between soil and water. The effects of exotic and inherent DOM on phenanthrene sorption by soils may primarily be described as 'cumulative sorption', association of phenanthene with DOM in solution, and modified surface nature of soil solids due to DOM binding.     

Use of sky brightness measurements from ground for remote sensing of particulate polydispersions

The software code SKYEAD.pack for retrieval of aerosol size distribution and optical thickness from data of direct and diffuse solar radiation is described; measurements are carried out with sky radiometers in the wavelength range 0.369-1.048 μm. The treatment of the radiative transfer problem concerning the optical quantities is mainly based on the IMS (improved multiple and single scattering) method, which uses the delta-M approximation for the truncation of the aerosol phase function and corrects the solution for the first- and second-order scattering. Both linear and nonlinear inversion methods can be used for retrieving the size distribution. Improved calibration methods for both direct and diffuse radiation, the data-analysis procedure, the results from the proposed code, and several connected problems are discussed. The results can be summarized as follows: (a) the SKYRAD.pack code can retrieve the columnar aerosol features with accuracy and efficiency in several environmental situations, provided the input parameters are correctly given; (b) when data of both direct and diffuse solar radiation are used, the detectable radius interval for aerosol particles is approximately from 0.03 to 10 μm; (c) besides the retrieval of the aerosol features, the data-analysis procedure also permits the determination of average values for three input parameters (real and imaginary aerosol refractive index, ground albedo) from the optical data; (d) absolute calibrations for the sky radiometer are not needed, and calibrations for direct and diffuse radiation can be carried out with field data; (e) the nonlinear inversion gives satisfactory results in a larger radius interval, without the unrealistic humps that occur with the linear inversion, but the results strongly depend on the first-guess spectrum; (f) aerosol features retrieved from simulated data showed a better agreement with the given data for the linear inversion than for the nonlinear inversion.     

Impact of low molecular weight organic acids and dissolved organic matter on sorption and mobility of isoproturon in two soils

Isoproturon is a selective herbicide belonging to the phenylurea family and widely used for pre- and post-emergence control of annual weeds. Soil amendments (e.g. organic compounds or dissolved organic matter) may affect environmental behavior and bioavailability of pesticides. However, whether the physiochemical process of isoproturon in soils is affected by organic amendments and how it is affected in different soil types are unknown. To evaluate the impact of low molecular weight organic acids (LMWOA) and dissolved organic matter (DOM) on sorption/desorption and mobility of isoproturon in soils, comprehensive analyses were performed using two distinct soil types (Eutric gleysols and Hap udic cambisols). Our analysis revealed that adsorption of isoproturon in Eutric gleysols was depressed, and desorption and mobility of isoproturon were promoted in the presence of DOM and LMWOA. However, the opposite result was observed with Hap udic cambisols, suggesting that the soil type affected predominantly the physiochemical process. We also characterized differential components of the soils using three-dimensional excitation-emission matrix (EEM) fluorescence spectroscopy and Fourier transform infrared (FT-IR) spectroscopy and show that the two soils displayed different intensity of absorption bands for several functional groups.     

基于神经网络方法的高光谱遥感浅海水深反演

利用人工神经网络方法进行了高光谱遥感反演浅海水深的初步研究.在产生模拟数据时,为保证模拟数据的合理性,引入了根据水体和海底特性来划分光学浅水和光学深水的方法,并初步研究了利用光谱徽分技术进行光学浅水和光学深水区分的有效性.在人工神经网络建模过程中,采用主成分分析的方法对网络的输入数据进行预处理,显著提高了网络的学习速度.建立的人工神经网络模型和基于非线性最优化方法的反演算法与实测数据的反演结果相比较,人工神经网络模型的反演精度明显高于非线性最优化反演算法.     

南黄海有色可溶有机物日变化分析

用高时间分辨率的韩国静止海洋水色仪(GOCI)影像,分析了2015年10月2日及2016年9月22日南黄海有色可溶有机物(CDOM)的时空分布特征及影响因素。结果表明,南黄海近岸CDOM浓度高于中央海域,江苏沿岸含量最高;近岸水体以陆源输入为主,远岸水体以微生物现场生产为主。结合叶绿素(chl-a)浓度采样数据分析发现,CDOM日变化时间、空间特征与chl-a日变化一致,回归分析的R2高于0.88,验证了二者时空变化的正相关关系。在深水区,CDOM随悬浮物浓度增加而增加,且线性趋势明显。与提取的潮汐、潮流数据的对比分析发现,潮流是近岸水体(山东半岛南岸与江苏沿岸)表层CDOM变化的主要动力因素。     

Effect of soil moisture on microwave scattering for remote sensing

The main objective of remote sensing is to design space borne microwave sensors to sense a target and derive useful geophysical parameters. For this purpose, the knowledge of the target characteristics must be obtained through ground based remote sensing. In this paper, soil has been taken as the target. Various percentages of gravimetric soil moisture (m _g) have been taken for establishing its relation with the scattering coefficient (σ ₀) for both like polarizations at X-band frequencies. A linear model has been developed for correlating these two variables (i.e.m _g andσ ₀). Using this model, regression analysis has been done for obtaining different regression parameters and predicted values. Treating measurements from bare smooth soil fields with different soil moistures, the data were analysed to examine the effect of soil moisture on scattering co-efficient (σ ₀) at 9.50 GHz frequency in X-band. The scattering coefficient increases with increase in soil moisture content. Different regression parameters have been obtained, which show that the best look-angle is at 25° for HH-pol and 60° for VV-pol for observingσ ₀ from bare moist soil. Data analysis indicated that the basic cause and effect relationship between the sensor measurements and soil moisture can be extrapolated from theory and small-scale tests to larger resolution elements observed by the sensing aircraft. These results indirectly provide reference data for a satellite-borne remote sensor. From the results, the look angles suitable for operation with radar antennas can be suggested.     

海洋浮游植物生长过程中溶解有机物质的三维荧光光谱研究

利用三维激发-发射矩阵荧光光谱技术,监测了赤潮异弯藻和中肋骨条藻培养过程中产生的溶解有机物,分析了三维荧光光谱图中的荧光峰位置、数量及荧光强度的变化情况.结果表明,微藻生长过程中会产生类蛋白和类腐殖质两类荧光有机物质,这两类有机物的荧光峰的位置及荧光强度有较大差异.在不同生长期,两类荧光有机物的产生机制不同.在指数生长期,两类有机物的荧光强度与藻密度成正相关,说明浮游植物释放了一定量的荧光物质;在平稳期和衰亡期,两类有机物的荧光强度迅速增加,这可能由于衰老、死亡藻细胞的破碎释放出大量的荧光有机物质所致,此外细菌对非荧光有机物进一步降解,也可能是产生该现象的一个原因.     

Application of empirical neural networks to chlorophyll-a estimation in coastal waters using remote optosensors

This paper presents chlorophyll-a estimation in coastal waters off the Gulf of Finland using remote optosensors. Concurrent remote optosensor data and in situ measurements of water quality were obtained in the study area. Significant correlations were observed between digital values and chlorophyll-a measurements. The results as a case study show that the estimated accuracy of chlorophyll-a retrieval using neural networks is higher than the accuracy of chlorophyll-a estimation using regression analyzes in the area. The study also shows one example why remote optosensors are critical to monitor water quality in coastal areas such as the Gulf of Finland.     

海洋微藻产生的不同粒级溶解有机物质的三维荧光光谱

通过过滤(0.7μm)和切向超滤(100kDa、10kDa、1kDa)技术将实验室内培养的赤潮异弯藻和中肋骨条藻藻液分离成100kDa-0.7μm、10～100kDa、1～10kDa的胶体浓缩液和＜1kDa的超滤液,利用激发-发射矩阵荧光光谱技术测定了不同粒级溶液中有机物的荧光性质,分析了三维荧光光谱图中的荧光峰位置、数量及荧光强度的变化情况.结果表明,两种微藻产生的类腐殖质荧光物质主要是＜1kDa的小分子物质.赤潮异湾藻产生的类蛋白荧光物质中,15%是100kDa-0.7μm的大胶体,8%是10～100kDa的中胶体,而大部分是＜1kDa的小分子物质.赤潮异弯藻各级超滤截留液的类蛋白荧光峰的发射波长与过滤液相比,发生蓝移,而超滤液的则发生红移,表明赤潮异湾藻产生的类蛋白荧光物质随着粒径的增大极性减弱.     

Optical scattering and backscattering by organic and inorganic particulates in U.S. coastal waters

We present the results of a study of optical scattering and backscattering of particulates for three coastal sites that represent a wide range of optical properties that are found in U.S. near-shore waters. The 6000 scattering and backscattering spectra collected for this study can be well approximated by a power-law function of wavelength. The power-law exponent for particulate scattering changes dramatically from site to site (and within each site) compared with particulate backscattering where all the spectra, except possibly the very clearest waters, cluster around a single wavelength power-law exponent of -0.94. The particulate backscattering-to-scattering ratio (the backscattering ratio) displays a wide range in wavelength dependence. This result is not consistent with scattering models that describe the bulk composition of water as a uniform mix of homogeneous spherical particles with a Junge-like power-law distribution over all particle sizes. Simultaneous particulate organic matter (POM) and particulate inorganic matter (PIM) measurements are available for some of our optical measurements, and site-averaged POM and PIM mass-specific cross sections for scattering and backscattering can be derived. Cross sections for organic and inorganic material differ at each site, and the relative contribution of organic and inorganic material to scattering and backscattering depends differently at each site on the relative amount of material that is present.