利用多角度POLDER偏振资料实现陆地上空大气气溶胶光学厚度和地表反照率的同时反演Ⅰ.理论与模拟

陆地上空标量辐射对地表反射率和大气气溶胶散射都具有很强的敏感性, 而偏振反射只对大气气溶胶敏感, 对地表不敏感.根据这个原理并结合POLDER (POLarization and Directionality of Earth Reflectance) 资料的特点, 作者提出综合利用标量辐射和偏振反射信息来实现陆地上空大气气溶胶和地表反照率的同时反演.首先, 利用多角度偏振辐射观测提取大气气溶胶光学参数, 再利用标量辐射测量对偏振反演结果作进一步筛选和订正, 同时获得地表反射率.数值模拟试验结果证明, 仅利用偏振信息只能获取大气气溶胶信息, 而且其结果误差较大, 特别是对于散射作用较强的短波长通道如670 nm误差更大, 但经过标量辐射订正后的结果得到明显改善, 气溶胶光学厚度和地表反射率与真实值之间相关系数都达到0.99以上.为提高查找表的计算效率, 提出并建立了反演方案所需要的半参数化数值表, 利用内插方法寻求气溶胶光学厚度和地表反射率的数值解的反演方法.     

利用多角度POLDER偏振资料实现陆地上空大气气溶胶光学厚度和地表反照率的同时反演 II. 实例分析

由于陆地地表反照率的复杂性,陆地上空气溶胶的反演一直是卫星对地观测的一个难点,针对这个难点,作者提出联合利用偏振反射率和总反射率提取陆地上空大气气溶胶光学厚度和地表反照率及其区域分布的反演方案,提出了利用NCEP资料订正由海拔高度引起的Rayleigh散射变化的具体方法,并利用POLDER（POLarization and Directionality of Earth's Reflectance）的LEVEL-1B资料进行实际反演计算,给出中国华北地区气溶胶光学厚度和地表反照率的区域分布。反演结果与地基观测进行了对比验证分析,结果表明,综合利用标量辐射和偏振信息的可以实现区域乃至全球尺度的大气气溶胶和地表反照率的定量反演。     

利用多通道MODIS遥感资料反演沙尘气溶胶尺度分布的个例试验

为了利用水面进行大气气溶胶粒子尺度分布的反演,需要水体光谱反射率资料;本文利用水库上空MODIS观测的表观反射率资料来订正出实际的水体光谱反射率。假设在干旱的春季水库水面反射率变化不大,用订正的水库水面光谱反射率平均值进行大气气溶胶尺度分布的反演。从两天个例研究来看,利用多通道MODIS资料可以反演出大气气溶胶粒子的尺度变化。结合天气分析2002年4月11日水库上空气溶胶光学厚度增加、粒子变粗,是由于上游区域的沙尘在高空强西北气流引导下到达本地区。     

GMS5卫星遥感气溶胶光学厚度的试验研究

通过模拟 GMS5卫星可见光通道的表观反射率 R对不同大气气溶胶模型、不同下垫面反射率以及不同气溶胶光学厚度的敏感性 ,对利用 GMS5卫星资料反演湖面上空气溶胶的可行性进行了分析 ,并结合地面多波段太阳光度计观测数据 ,反演了巢湖上空大气气溶胶光学厚度。结果表明 ,反演所得 0 .5336μm气溶胶光学厚度强烈依赖于湖面反射率的选取 ,通过选取合适的湖面反射率 ,卫星反演的气溶胶光学厚度和地面光度计遥感的月均值相对误差不超过 30 %。     

大气气溶胶光学厚度的卫星双通道遥感方法

提出了一种在晴空条件下,均匀下垫面上利用ＮＯＡＡ－１４极轨卫星甚高分辨率辐射计（ＡＶＨＲＲ）可见光和近红外两个通道观测的反射率资料,遥感整层大气气溶胶光学厚度的双通道方法,该方法把气溶胶散射的波长关系加以推广,把近红外通道的光学厚度均值引入了可见通道,使单个通道中地表反射率和大气因子参数化的误差得以抵消,从而极大地提高了反演精度（〉９０％）。     

单星多角度法同时反演气溶胶光学厚度和地表反射率

尝试以单星多角度卫星观测数据同时反演晴空陆地的气溶胶光学厚度和地表反射率,并选取2009年5月的MODIS(Moderate Resolution Imaging Spectroradiometer)1B资料进行了反演试验.结果表明:单星多角度法反演得到的气溶胶光学厚度结果与MODIS气溶胶产品(MOD04)平均值的相关系数为0.7914;反演的地表反射率结果与MODIS地表反射率产品(MOD09)也具有较好的一致性.对直接利用单星多角度观测数据反演获得一段时间内平均的气溶胶光学厚度进行了有益的尝试.     

利用MODIS卫星资料对比反演兰州地区气溶胶光学厚度

Kaufman的暗像元方法是目前利用MODIS卫星资料反演气溶胶光学厚度的方法之一,但在获取可见光通道地表反射率时存在局限性。我们用一种对比方法进行了反演试验,研究了反演粒径较大的气溶胶光学厚度的可行性。用6S辐射传输模式模拟了两天的表观反射率差异对气溶胶光学厚度的敏感性;利用两天(“清洁日”和“污染日”)MODIS的红、蓝和近红外通道表观反射率资料,通过查算表反演了水面上空的气溶胶光学厚度和几何平均质量粒径;在此基础上反演了兰州地区气溶胶光学厚度的分布情况。反演结果与地面光度计观测作了比较,两者比较接近,说明反演方法是可行的。     

北京晴天紫外波段气溶胶光学厚度反演与分析

利用太阳一大气紫外光谱辐射计(SAUVS)，测量到达北京地表的太阳直接和散射紫外光谱辐射，给出反演大气气溶胶光学厚度的一种方法。结果表明：在紫外波段，大气气溶胶的光学厚度随波长的增加而单调减小，用指数函数可以很好地拟合反演结果。统计得到了3个水平能见度状况下拟合函数的系数值，与全球气溶胶监测网络(AERONET)北京站的资料对比，表明反演结果基本合理。     

气溶胶辐射特性的观测研究

利用一个简化的辐射平衡模式,讨论了气溶胶直接辐射强迫和气溶胶辐射特性以及与地面反射率之间的关系。模式分析表明,增加气溶胶层以后,地气系统对太阳辐射的反射率可能增大,也可能减少,取决于气溶胶的单散射反射率~ω0、不对称度因子g与地面反射率αg之间的配置,但与气溶胶光学厚度δ没有直接的关系。气溶胶光学厚度值仅和反射率变化的幅度成正比。从辐射平衡模式的结果可知,为了定量地研究气溶胶直接辐射强迫作用,需要有系统的有关气溶胶辐射特性的观测资料。从地面和空间对中国地区气溶胶的辐射特性开展了观测研究,包括从地面用太阳光度计测量气溶胶的光学厚度、用浊度计测量气溶胶的散射系数、黑碳仪测量气溶胶的吸收系数,并且利用MODIS资料反演气溶胶的光学厚度。地面太阳光度计观测的气溶胶光学厚度用于检验卫星遥感的气溶胶光学厚度值,对中国东部地区,遥感的结果是可以接受的,主要是由于这一地区存在较低的地表反射率;但对中国北方植被条件不很好的地区,在遥感反演时对地面反射率的估计可能偏低,如做适当的修改还有可能提高遥感反演的精度。从地面直接测量气溶胶的散射系数和吸收系数算出的气溶胶单散射反射率在0.8左右,需要有更多的观测,以便进一步查清这一问题。     

宽带太阳漫射辐射法反演辐射加权平均的气溶胶一次散射反照率研究

引入了一个辐射加权平均的宽带气溶胶一次散射反照率(SSA)的定义,提出了一个从宽带的漫射信息反演该SSA的方法.数值模拟结果表明,在Junge气溶胶谱分布情形下,对气溶胶光学厚度、(A)ngstr(o)m 指数与气溶胶虚部的通常变化范围,应用该SSA所计算的2160组宽带太阳辐射反射率、漫射透过率、总透过率和吸收率的相对标准误差都在1.107%以内; 绝对标准差在0.00287以内.对非Junge的大陆性和都市工业污染气溶胶模式(由水溶性、沙尘和碳粒子组成),在72组反射率、漫射透过率、总透过率和吸收率计算中,相对标准差都在2.047%以内,绝对标准差在0.0075以内.在Junge气溶胶假设下,作者提出了一个综合应用宽带太阳直射和漫射信息同时反演气溶胶光学厚度与辐射加权平均SSA的方法,并通过模拟反演分析了SSA反演的3个主要误差因子.从反演结果可以看出:(1)如果(A)ngstr(o)m指数误差在±0.2以内,对0.55 μm气溶胶光学厚度大于0.312大陆性气溶胶,SSA误差在±0.0418以内;(2)波长无关的宽带地表反照率适用于SSA反演;(3)气溶胶光学厚度越大,辐射资料误差所引起的SSA解误差越小.当辐射误差在±2%以内以及0.55 μm气溶胶光学厚度大于0.312时,SSA解的误差在±0.0149以内; 辐射误差在±4%以内时,SSA解的误差在±0.0317以内.     

A parameterization of the diffuse transmittance and reflectance for aerosol remote sensing problems

To account for surface reflection is a major and most complex issue of any satellite aerosol retrieval algorithm. The surface contribution to the satellite signal is usually calculated assuming Lambertian reflection. Then the problem is reduced to the calculation of total atmospheric transmittance and spherical albedo. Usually, these parameters are stored in so-called look-up tables (LUTs), which can be quite large. The main aim of this paper is the parameterization of these look-up tables in terms of simple approximate equations, which can be subsequently used in aerosol remote sensing techniques over reflecting land surfaces.     

Retrieval of aerosol optical properties over a vegetation surface using multi-angular,multi-spectral,and polarized data

An algorithm to retrieve aerosol optical properties using multi-angular,multi-spectral,and polarized data without a priori knowledge of the land surface was developed.In the algorithm,the surface polarized reflectance was estimated by eliminating the atmospheric scattering from measured polarized reflectance at 1640 nm.A lookup table （LUT） and an iterative method were adopted in the algorithm to retrieve the aerosol optical thickness （AOT,at 665 nm） and the （A）ngstr（o）m exponent （computed between the AOTs at 665 and 865 nm）.Experiments were performed in Tianjin to verify the algorithm.Data were provided by a newly developed airborne instrument,the Advanced Atmosphere Multi-angle Polarization Radiometer （AMPR）.The AMPR measurements over the target field agreed well with the nearby ground-based sun photometer.An algorithm based on Research Scanning Polarimeter （RSP） measurements was introduced to validate the observational measurements along a flight path over Tianjin.The retrievals were consistent between the two algorithms.The AMPR algorithm shows potential in retrieving aerosol optical properties over a vegetation surface.     

Aerosol polarized phase function and single-scattering albedo retrieved from ground-based measurements

Aerosol optical parameters, polarized phase function and single-scattering albdeo, have been retrieved from ground-based sun photometer measurements in Beijing 2003. The measured aerosol optical thickness varies from 0.12 to 0.77 with an average value of 0.39. The measured Ångström coefficient ranges from 0.75 to 1.47 with an average value of 1.21. The retrieved single-scattering albedo at 870 nm is within the 0.76–0.94 range and the average value is 0.85, suggests there are considerable aerosol absorptions in Beijing. The maximum value of retrieved polarized phase function at 870 nm ranges from 0.068 to 0.225 with an average value of 0.16, and it illustrates good correlations with the Ångström coefficient, i.e. the relative size of aerosol particles. Analyses of measurements and theoretical calculations show the polarized phase function is sensitive to aerosol size distribution and complex refractive index, especially the imaginary part of the refractive index which denotes aerosol light absorbing effects. These results suggest that the polarized phase function is an effective and unique aerosol optical parameter and is able to improve the retrieval of aerosol physical properties.     

用MODIS反演北京城市地区地表反照率精度以及算法改进

MODIS(MODerate-resolution Imaging Spectroradiometer)地表反照率的精度在乡村地区已经得到了检验,但是至今没有在城市地区的有关研究。地表反照率的精度在很大程度上取决于大气订正的精度,作者利用2002年以来的北京AERONET(国际气溶胶检测网络)站点Cimel气溶胶观测资料对反射率进行大气订正,通过对比来评价MODIS地表反照率算法中大气订正的精度。结果发现,MODIS大气订正在蓝光波段具有明显的过度订正现象,MODIS大气订正后地表反射率平均偏低0.03。MODIS地表反照率在冬季有约75%的缺测,这是因为冬季严重的空气污染使得MODIS云检测得到晴空观测较少。MODIS使用三参数双向反射率函数（BRDF）要求16天以内至少有3次以上的晴空观测（MODIS算法中要求7次）。通过分析MODIS反演得到的三参数,发现虽然它们的绝对值具有明显的季节变化,但是它们的比值是十分稳定的,这样使BRDF函数降低到只需要一个参数,有效降低了对晴空观测次数的要求,这一思想可以应用到热带等晴空日数较少的地区。     

Nephelometer derived and directly measured aerosol optical depth of the atmospheric boundary layer

The aerosol optical depth of the atmospheric boundary layer was determined both from direct solar irradiance measurements and from vertical extrapolation of ground-based nephelometry, during a period with cloudless skies and high aerosol mass loadings in the Netherlands. The vertical profile of the aerosol was obtained from lidar measurements. From humidity controlled nephelometry at the ground and humidity profiles from soundings, the scattering aerosol extinction as a function of height was assessed. Integration of the extinction over the aerosol layer gave the aerosol optical depth of the atmospheric boundary layer. This optical depth at the narrow band of the nephelometer was translated to a spectrally integrated value, assuming an Angstrom wavelength exponent of 1.5, a typical value for The Netherlands.It was found that scattering by the boundary layer aerosol contributed on average 80% to the total atmospheric aerosol optical depth. The uncertainty in this value is estimated to be of the order of 13%. Ammonium nitrate dominated the light scattering. This is an anthropogenic aerosol component.The radiative forcing caused by the light scattering of the anthropogenic aerosol was calculated assuming an upward scattered fraction of 0.3. An average value of − 12 W m ⁻² was found (with an estimated uncertainty of 20%). This corresponds to an absolute increase in the planetary albedo of 0.03, which is equivalent to a 15% increase in the local planetary albedo of 0.2.     

Simultaneous determination of aerosol size distribution and refractive index and surface albedo from radiance- Part III: Parametrization and application

Based on the radiative transfer calculation results, three approximate expressions of the sky radiance in almucantar and its increment caused by surface albedo are presented. They are simple, but accurate enough. The dependence of the fitted aerosol scattering phase functions on refractive index is also studied, and its reasonable form is given. For Junge size distribution, the approximate equations of the phase functions with some special scattering angles are obtained. These approximate equations significantly simplify the retrieval algorithm of simultaneous determination of aerosol size distribution and its refractive index and surface albedo. This method can be realized with a microcomputer, and has been used to process and analyse the experimental data measured in Hefei of Anhui Province.     

A polarized microwave radiative transfer model for passive remote sensing

The objective of the present work is to develop a general purpose, polarized, microwave radiative transfer forward model, including calculation of interaction parameters for fast, simultaneous and accurate generation of radiances, for use in a wide variety of atmospheric retrieval applications. This complete polarized model includes the generation of atmospheric profiles, calculation of the interaction parameters and the solution of the vector radiative transfer equations. In the present work, the doubling and adding method is used to calculate radiances. Mie theory is used for spherical particles to evaluate the polarized scattering matrix. Two types of sea surface models applicable for specular and diffuse surfaces respectively are implemented. The model has been validated against standard benchmark cases and experimental data available in literature. While the model is itself generic, parametric studies are performed to study the influence of the vertical structure of the atmosphere, for two polarizations and a set of frequencies to be used on the microwave imager aboard the proposed Indo-French climate research satellite MEGHA-TROPIQUES.     

SIMULTANEOUS DETERMINATION OF AEROSOL SIZE DISTRIBUTION AND REFRACTIVE INDEX AND SURFACE ALBEDO FROM RADIANCE-PART III: PARAMETRIZATION AND APPLICATION

Based on the radiative transfer calculation results, three approximate expressions of the sky radiance in almucantar and its increment caused by surface albedo are presented. They are simple, but accurate enough. The dependence of the fitted aerosol scattering phase functions on refractive index is also studied, and its reasonable form is given. For Junge size distribution, the approximate equations of the phase functions with some special scattering angles are obtained. These approximate equations significantly simplify the retrieval algorithm of simultaneous determination of aerosol size distribution and its refractive index and surface albedo. This method can be realized with a microcomputer, and has been used to process and analyse the experimental data measured in Hefei of Anhui Province.     

The effect of ozone and aerosols on the surface erythemal UV radiation estimated from OMI measurements

Surface erythemal UV radiation is mainly affected by total column ozone, aerosols, clouds, and solar zenith angle. The effect of ozone on the surface UV radiation has been explored many times in the previous studies due to the decrease of ozone layer. In this study, we calculated the effect of aerosols on the surface UV radiation as well as that of ozone using data acquired from Ozone Monitoring Instrument (OMI). First, ozone, aerosol optical depth (AOD), and surface erythemal UVB radiation measured from satellite are compared with those from ground measurements. The results showed that the comparison for ozone was good with r ² of 0.92. For aerosol, there was difference between satellite measurements and surface measurements due to the insufficient information on aerosol in the retrieval algorithm. The r ² for surface erythemal UV radiation was high (～0.94) but satellite measurements showed about 30% larger values than surface measurements on average by not considering the effect of absorbing aerosols in the retrieval process from satellite measurements. Radiative amplification factor (RAF) is used to access the effect of ozone and aerosol quantitatively. RAF for ozone was 0.97～1.49 with solar zenith angle. To evaluate the effect of aerosol on the surface UV radiation, only clear-sky pixel data were used and solar zenith angle and total column amount of ozone were fixed. Also, RAF for aerosol was assessed according to the single scattering albedo (SSA) of aerosols. The results showed that RAF for aerosol with smaller SSA (< 0.90) was larger than that for with larger SSA (> 0.90). The RAF for aerosol was 0.09～0.22 for the given conditions which was relatively small compared to that for ozone. However, considering the fact that aerosol optical depth can change largely in time and space while the total column amount of ozone does not change very much, it needs to include the effect of aerosol to predict the variations of surface UV radiation more correctly.