基于MODIS的大气因素对蓝藻水华信息提取的影响

本文利用MODIS提供的两种影像产品和代表大气条件的气溶胶产品,运用单波段(NIR)和比值植被指数(NIR/G)两种方法分别提取太湖蓝藻信息.在阐述气溶胶的光学厚度对遥感影像影响的基础上,定量分析了气溶胶光学厚度对两种方法提取蓝藻水华面积的净变化值和选取阈值的影响程度.研究结果表明,运用单波段和比值植被指数两种方法提取太湖蓝藻水华面积差异与气溶胶光学厚度的相关性,固定阈值情况下的相关性高于自选阈值中的对应值.两种指数中大气校正前的阈值与气溶胶光学厚度的相关性都要高于大气校正后两者的相关性.这说明大气会对阈值的设定产生一定的影响,进而影响蓝藻信息提取精度.因此,定量分析气溶胶光学厚度对监测蓝藻是至关重要的.     

基于分区暗像元和Spline插值方法估算太湖气溶胶光学厚度

传统暗像元大气校正算法认为研究区域上空的气溶胶光学厚度呈均匀分布状态。对于Ⅱ类水体,尤其是气溶胶类型复杂的内陆湖区,暗像元算法的均匀性假设将不再适用。针对传统暗像元算法的不合理性,本研究将太湖湖区划分为9个子区域,每个子区域利用传统暗像元算法估算其气溶胶光学厚度,然后结合Spline插值算法获取整个太湖的气溶胶光学厚度信息,并以传统暗像元大气校正算法作为参照,探讨与分析分区暗像元算法的精度状况。通过本文的研究可知：气溶胶光学厚度是遥感大气校正的关键参数;在2003年10月28日,受西北风的影响,太湖上空的气溶胶光学厚度呈湖南低,湖北高的分布模式;分区暗像元大气校正算法获取的气溶胶光学厚度平均值为0.79,标准偏差为0.099,标准偏差与平均值的比值为12.58%,与传统暗像元算法相比,分区暗像元算法综合考虑了水体上空气溶胶光学厚度空间分布的不均匀性,进而有利于改善大气校正的精度。     

太湖水体GF-1/WFV影像的6S逐像元大气校正

水体光谱信息微弱,常用的基于辐射传输模型的大气校正方法在水体中校正精度较差。基于覆盖太湖水体的2016年4月29日的高分一号宽幅相机影像（GF-1/WFV）和同步的实测光谱数据,对6S辐射传输模型的输入参数进行敏感性分析,逐像元计算观测几何,使用分区气溶胶类型、分区暗像元和Spline插值确定的气溶胶光学厚度（Aerosol Optical Depth,AOD）进行6S逐像元大气校正。实验结果表明:气溶胶模式对6S大气校正结果的影响最大,与FLAASH方法相比,逐像元计算观测几何和气溶胶参数的校正方法对大气校正精度有改进作用,4个波段的平均相对误差分别降低了1.84%、7.78%、4.79%和17%。结合精确大气参数输入的6S逐像元大气校正方法可以改进水体表面遥感反射率的大气校正精度。     

基于高程或气溶胶厚度与6S模型校正参数回归方程的遥感图像大气校正模型

为了消除影像中的大气影响,反演地表真实反射率,需要进行大气校正。6S是通用的大气校正模型之一,但是对于覆盖面积较广、下垫面较复杂的影像,若用平均的气溶胶厚度和高程参数来对影像进行校正,将出现较大误差。提出了建立一种基于6S模型,以高程或气溶胶厚度为参数的大气校正模型与方法,并以广州Landsat ETM+为例,对校正前后的反射率直方图、NDVI灰度直方图以及各类地物的光谱特征进行分析。结果证明了该方法简单实用,校正精度高,可直接用于该地区相似气候条件下影像的大气校正,对类似的研究有借鉴意义。&     

基于高程或气溶胶厚度与6S模型校正参数回归方程的遥感图像大气校正模型

为了消除影像中的大气影响,反演地表真实反射率,需要进行大气校正。6S是通用的大气校正模型之一,但是对于覆盖面积较广、下垫面较复杂的影像,若用平均的气溶胶厚度和高程参数来对影像进行校正,将出现较大误差。提出了建立一种基于6S模型,以高程或气溶胶厚度为参数的大气校正模型与方法,并以广州LandsatETM＋为例,对校正前后的反射率直方图、NDVI灰度直方图以及各类地物的光谱特征进行分析。结果证明了该方法简单实用,校正精度高,可直接用于该地区相似气候条件下影像的大气校正,对类似的研究有借鉴意义。     

大气光谱光学厚度测量方法研究

为满足辐射校正场同步观测的需要,寻求一种能以简便的操作、较高的精度测量大气光谱光学厚度的观测方法是必要的。在合肥用地物光谱辐射计VF921-256和太阳辐射计DTF-1进行了为期几天的同步测量,经分析比较,发现VF921-256与DTF-1的测量结果之间存在1%~4%的相对偏差。这一偏差不但取决于光谱辐射计的测量误差,而且与太阳辐射计的标定误差也有一定关系。从VF921-256数据的Langley Plot拟合结果来看,其标准偏差小于0.03。因此,应用光谱辐射计测量大气光谱光学厚度的辐照度基法,可以用于辐射校正中对大气光谱光学厚度的同步测量。此外,我们利用光学厚度信息反演了气溶胶谱分布、臭氧和水汽的柱总量。     

利用MODIS资料反演杭州市500米分辨率气溶胶光学厚度

反演城市/区域范围内高空间分辨率的气溶胶光学厚度时,如果气溶胶类型选取的不合理造成的反演误差会很大,甚至超过地表反射率确定误差导致的反演误差。针对这一问题,本文提出了一种结合MODIS L1B资料和AERONET(Aerosol Robotic Network)的气溶胶光学厚度产品,基于6S大气辐射传输模型的计算,确定杭州市在2008年12月16日的气溶胶类型的方法。利用得到的气溶胶类型,结合改进的暗像元法,反演了杭州市500m空间分辨率的气溶胶光学厚度。将气溶胶光学厚度反演结果与采用标准气溶胶类型时的反演结果进行比较,结果表明,本文确定的气溶胶类型更符合杭州市当天的情况,应用到气溶胶光学厚度反演中,精度也最好,相对误差的绝对值在20%以内。     

中国东南地区及近海海域气溶胶反演遥感研究

大气气溶胶在很多生物地球化学循环中具有重要作用,但是由于它的来源广泛并且具有很大的时空变化性,难以在全球范围内精确、实时确定气溶胶的性质、组成及时空分布,因而对大气气溶胶的研究依赖于监测手段的发展。地基试验能获取点源的大气气溶胶光学厚度(AOD)的地面测量数据,得到的气溶胶光学厚度用于卫星数据的预处理以及气溶胶光学厚度反演的精度验证。而经过地基校验后的卫星遥感数据,可以反映大范围内实时动态的气溶胶信息。利用MODIS资料和地基探测的太阳光度计资料,对中国东南地区及近海海域的大气气溶胶光学特性进行了分析,讨论了适用于中国东南地区的大气气溶胶模型;利用连续的太阳光度计数据对MODIS资料的反演结果进行校验,结果表明：改进气溶胶模型和采用连续波段太阳光度计探测数据,可以提高MODIS AOD的校验结果。     

高分六号卫星遥感估算大气细颗粒物方法研究北大核心CSCD

针对目前利用高分六号卫星开展地区高空间分辨率大气细颗粒物监测研究较少的问题,提出了基于高分六号卫星宽幅相机数据的气溶胶光学厚度及大气细颗粒物遥感反演的技术方法,并在京津冀及周边地区开展了应用实验和对比分析。首先,基于改进暗像元法反演了高分六号卫星数据的气溶胶光学厚度;然后,结合地面大气细颗粒物监测数据与多种气象辅助数据,基于随机森林算法,构建了多参量综合的大气细颗粒物估算模型,对京津冀地区的大气细颗粒物浓度进行了估算。研究表明:高分六号气溶胶光学厚度反演结果与地基站点监测结果的相关系数为0.94,反演精度较高;大气细颗粒物估算结果与地基站点监测结果的决定系数达到0.79以上,较好地反映了京津冀地区的大气细颗粒物空间分布。     

卫星影像大气校正中气溶胶模型的影响分析-以天津地区为例

基于大气校正中常用的大气辐射传输模拟软件6S模型,探讨了不同气溶胶模型对卫星影像大气校正的影响及其适用性问题。选取我国环境小卫星星座HJ\|1的CCD传感器数据,以天津地区大气污染和较清洁条件下的卫星影像为例,基于6S进行大气校正,定量估算了气溶胶模型选取对大气校正精度的影响。与此同时,通过地面太阳-天空辐射计实测,获得研究区域整层大气气溶胶参数进行大气校正,并与基于气溶胶模型的校正结果进行对比,得到以下结论:①6S气溶胶模型中,大陆型与海洋型的校正结果比较接近,而城市型气溶胶模型由于含有较高的煤烟成分比例,其校正结果与前两种相差较大;②天津地区在大气污染情况下,适合应用大陆型气溶胶模型进行大气校正,而在大气较清洁的情况下可选择海洋型气溶胶模型。
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Toward aerosol optical depth retrievals over land from GOES visible radiances: determining surface reflectance

Frequent observations of aerosol over land are desirable for aviation, air pollution and health applications. Thus, a method is proposed here to correct surface effects and retrieve aerosol optical depth using visible reflectance measurements from the Geostationary Operational Environmental Satellite (GOES). The surface contribution is determined from temporal compositing of visible imagery, where darker pixels correspond to less atmospheric attenuation and surface reflectance is deduced from the composite using radiative transfer. The method is applied to GOES‐8 imagery over the eastern US. Retrieved surface reflectance is compared with separate retrievals using a priori ground‐based observations of aerosol optical depth. The results suggest that surface reflectances can be determined to within ±0.04. The composite‐derived surface reflectance is further analysed by retrieving aerosol optical depth and validating retrievals with Aerosol Robotic Network (AERONET) observations. This analysis indicates that the retrieved optical depth is least biased, hence the surface reflectance is most accurate, when the composite time period varies seasonally. Aerosol optical depth retrievals from this validation are within ±0.13 of AERONET observations and have a correlation coefficient of 0.72. While aerosol optical depth retrieval noise at low optical depths may be limiting, the retrieval accuracy is adequate for monitoring large outbreaks of aerosol events.     

The effects of aerosol scattering on remote pressure measurement via oxygen A-band absorption

Abstract

The technique recently proposed by Barton and Scott to monitor surface pressure using absorption of reflected sunlight in the oxygen A-band is subject to errors due to aerosol scattering. Errors arising from specified uncertainties in the aerosol optical depth are evaluated. It is shown that, for a sea surface over which the optical depth is 0.1 ± 20 per cent, measurements accurate to better than a few millibars can be obtained only by observing towards the maximum of the sunglint region. The method is not feasible under hazy conditions where the optical depth is 0.3 + 20 per cent. Zones of feasibility are presented for typical surface conditions and aerosol parameters. This shows that the region within which an accuracy of 2 mbar is obtained is so small as to render the technique operationally unviable, even if monitoring of the aerosol optical depth to ±0.02 was available. Measurements over land would be feasible where surface albedo is above ～ 10 per cent provided that the aerosol optical depth uncertainty is below ±0.02. However, obtaining this accuracy over land may be problematic.     

Aerosol retrieval with satellite image and correlation analyses between aerosol distribution and urban underlaying surface

Human activity is one of the most important aerosol sources. Because the underlaying surface feature records most human activities, it is important to recognize the correlation between aerosol distribution and the underlaying surface. In this research, the dark object algorithm and a second-generation operational algorithm of Moderate-Resolution Imaging Spectroradiometer (MODIS) aerosol retrieval are used to estimate aerosol optical depth from Enhanced Thematic Mapper Plus (ETM+) images acquired by the Landsat 7 satellite system in urban regions, and the correlations between aerosol distribution and urban underlaying surface features (including landform, land cover and urbanization level) is analysed. Results show that (1) it is feasible to apply a second-generation algorithm to retrieve aerosol optical depth with ETM+?images. When a validation is performed with the ground observation meteorological range converted into aerosol optical depth with the correlation model acquired by a Moderate-Resolution Atmospheric Transmission (MODTRAN) simulation, the retrieval error is about 0.0094. For higher spatial resolution of an ETM+?image, it is better to study the aerosol distribution features in the urban regions. Additionally, (2) there are obvious variations in spatial distribution of aerosol over the different features of the underlaying surface. For the landform, aerosol optical depth is mountain < hill < plain; for the land cover, aerosol optical depth is dense vegetation < sparse vegetation < water < bare soil < residential area; for the different urbanization-level regions, there is bigger and bigger aerosol optical depth with increasing of the urbanization level. On the whole, as human activities increase, so too does the aerosol optical depth.     

Variation of atmospheric optical depth for remote sensing radiance calculations

An analysis of the radiative transfer processes in the atmosphere suggests that the ratio between the diffuse radiance flux and the directly transmitted flux may be a useful parameter for investigating the variation of atmospheric optical depth. The radiative parameter is derived for an observer at the top of the atmosphere in the position of a satellite. One of the important variables required in the computations of the radiative parameter is the aerosol phase function which can be satisfactorily fitted by the two-term Henyey-Greenstein function. When the surface reflectance equals zero, representing the atmospheric radiance alone without any influence from an oceanic surface, the study shows that the variation of the optical depth is associated with the changing of aerosol size distribution. The optical depth increases with the presence of large aerosol particles. The results indicate that inhomogeneities in aerosol optical properties, such as the variation of the scattering phase function due to the changes in aerosol size distribution, can affect the magnitude of the optical depth.     

The atmospheric optical depth spectrum determined from CZCS radiance

Calculations of diffusely reflected radiance emerging at the top of the atmosphere for a known aerosol size distribution have been made using the radiative transfer equation. The computed radiance was compared with the CZCS radiance collected within 1-2 hours after the aerosol measurements from the same area. An optical depth of the Earth's atmosphere is inferred from the comparison. The procedure was carried out for wavelengths corresponding to the CZCS channels (443, 520, 550, 670 and 750nm). Values of optical depth are plotted against the wavelengthsto provide an optical depth spectrum. The results indicate that there isa strong dependenceof optical depth on the wavelength. Thus, the ratio of optical depths at different wavelengths throughout the whole spectrum is not constant. Also, at various aerosol concentrations of large particles, the ratio is found to be variable. This suggests that a precise value of the optical depth ratio cannot be easily estimated although the use of the ratio in an atmospheric correction algorithm can provide satisfactory accuracy. The computed radiance is found to be linearly related to the optical depth and, therefore, a simple approximation for the aerosol radiance based on a lineardependenceon the aerosol optical depth should be accurate. The study also shows that the effect due to misclassification between the contaminated and uncontaminated pixels by thin haze or cloud, is to cause a significant reduction in the optical depth ratio.(443)/.(550). The resultingvaluesof the optical depth ratio cannot beestimated by the approximation suggestedin Gordon and Clark (1981)and Gordon et al. (1983).     

Satellite remote sensing of atmospheric optical depth spectrum†

An analysis of the atmospheric radiative transfer processes suggests that the diffuse radiance emerging at the top of the Earth's atmosphere in the position of a satellite can be approximated by a linear relationship with the optical depth. It is found that the variation of the optical depth is associated with the changing of aerosol size distribution. The optical depth increases with the presence of large aerosol particles. Thus, inhomogeneities in aerosol size distribution can affect the high-altitude atmospheric radiance in the visible wavelengths (400–800 nm). To demonstrate an application of ocean colour remote sensing for atmospheric studies, computations of diffusely reflected radiance at the top of the atmosphere, using the radiative transfer equation, have been made for known aerosol size distribution and oceanic surface albedo. The condition of a clear sky over clear water has been considered in the study. The computed radiance was compared with the radiance detected by the Coastal Zone Colour Scanner (CZCS) aboard the NIMBUS-7 satellite. The CZCS radiance was collected within 1–2 hours after the aerosol measurements from the same area (Monterey Bay, California). An optical depth of the Earth's atmosphere is inferred from the comparison. The procedure was carried out for wavelengths corresponding to the CZCS channels (443, 520, 550,670 and 750 nm). Values of optical depth are plotted against the wavelengths to provide an optical depth spectrum. The results indicate that there is a strong dependence of optical depth on the wavelength. The dependence, however, does not agree well with the relationship proposed by Angstrom (1964) for the aerosol optical depth in the atmosphere. Furthermore, the resulting optical depth ratio cannot be estimated by the approximation suggested by Gordon and Clark (1981) and Gordon et al. (1983).     

Applying SPOT data to estimate the aerosol optical depth and air quality

The improvement in the structure function method for retrieving aerosol optical depth (AOD) with SPOT HRV data and its application in air quality monitoring are highlighted in this paper. Generally speaking, estimation of the aerosol optical depth will be affected by the temporal change of surface canopy, observation geometry and terrain effect when applying the contrast reduction method to the multi-temporal satellite image set. In order to reduce the errors induced by such effects, the single-directional structure function is replaced by the multi-directional mode, which can describe the real characteristics of the surface structure more completely. Comparison of the results with in-site observations show a significant improvement in the accuracy of the retrieved AOD. Furthermore, due to the linear relationship between aerosol optical depth and turbidity coefficient, satellite images can be employed for monitoring air quality. Application of the method is demonstrated with a case study situated around the northern Taiwan area.     

Aerosol mapping over land with imaging spectroscopy using spectral autocorrelation

A new method for aerosol retrieval over land is proposed that makes explicit use of the contiguous, high-resolution spectral coverage of imaging spectrometers. The method is labelled Aerosol Retrieval by Interrelated Abundances (ARIA) and is based on unmixing of the short-wave infrared sensor signal by region-specific endmembers, assuming low aerosol radiative influence in this spectral region. Derived endmember abundances are transferred to the visible part of the spectrum in order to approximate surface reflectance where aerosol influence is generally strongest. Spectral autocorrelation of surface spectra is a precondition for ARIA and demonstrated using a reference spectrum database. The re-mixed surface reflectance is used as input quantity for the inversion of aerosol optical depth τ_a at 0.55 µm wavelength on a pixel basis. Except for the choice of endmembers and the atmospheric vertical profile, no a priori assumptions on the image scene are required. The potential of the presented method for aerosol retrieval is demonstrated for an Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) scene, collected in California in 2000. Comparisons with existing aerosol retrieval methods showed encouraging results in terms of achieved spatial smoothness and degree of uncertainty of aerosol optical depth across the scene.     

双通道算法与MODIS算法反演中国近海气溶胶光学特性的对比研究

利用双通道和IMAPP气溶胶反演算法处理TERRA/MODIS L1B数据得出中国近海气溶胶的光学厚度,与AERONET太阳光度计的反演结果作对比分析,验证了反演方法的可行性。同时,对各海域的反演结果及表征粒子谱宽度的Angstrom指数(α)的变化情况进行了分析,结果表明：在东海和日本以南等广阔海域,两种反演算法的结果同AERONET太阳光度计的观测结果基本一致,相关性较好;在渤海和黄海近海岸一带两者气溶胶光学厚度的反演值均偏高,其原因主要是由这些海域的二类水体的影响导致的。探讨分析了这些海域的水域特征及光学特性,为研究发展适合中国近海气溶胶特性的反演算法提供了依据。