Aerosol optical depth retrieval by HJ-1/CCD supported by MODIS surface reflectance data

The high spatial resolution and temporal observation frequency of HJ-1/CCD make it suitable for aerosol monitoring. However, because of the lack of a shortwave infrared band, it is difficult to use HJ-1/CCD imagery to retrieve aerosol optical depth (AOD). We developed a new algorithm for HJ-1/CCD AOD retrieval by introducing MODIS surface reflectance outputs (MOD09) as support. In this algorithm HJ-1/CCD blue band surface reflectance was retrieved through MOD09 blue band surface reflectance by band matching of the two sensors. AOD at 550 nm was then generated through a pre-calculated look-up table for HJ-1/CCD. Eighteen HJ-1/CCD images covering the Jing-Jin-Tang (Beijing-Tianjin-Tangshan) region were used to retrieve AOD using the new algorithm, and the AODs were then validated using AERONET ground measurements in Beijing and Xianghe. The validation shows that compared with AERONET ground measurements, 27/29 AODs have error less than 0.1 in absolute value.

     

华北典型污染地区多轴差分吸收光谱技术的气溶胶反演研究

基于华北典型污染地区的地基多轴差分吸收光谱仪（MAX-DOAS）近4年的观测数据,利用最优估计算法和LIDORT辐射传输模式反演了该区域气溶胶的消光系数垂直廓线和光学厚度（AOD）.MAX-DOAS观测反演的AOD与全球气溶胶观测网络同波段在华北地区的AOD间具有很好的一致性,相关系数都在0.9以上,证明MAX-DOAS具备对华北污染区域气溶胶光学厚度和消光系数垂直廓线的反演能力.AOD的反演误差表现为冬季最大,春夏最小,早晚大于正午,这是因为冬季以及早晚太阳天顶角较大导致信噪比偏小,所以AOD反演误差偏大.反演廓线表明该区域气溶胶主要集中在1 km以下的边界层,浓度随高度呈指数递减,部分情况下峰值出现在300 m处;气溶胶光学厚度夏季最大,冬季最小,正午较大,早晚较小.在东风条件下浓度最高,表明东边（即重工业城市唐山方向）的输送对香河和周边区域的气溶胶积聚有重要贡献.     

Aerosol retrieval over land by exploiting the synergy of TERRA and AQUA MODIS DATA

Aerosol particles over land mainly come from man- made source such as biomass burning, industrial de-bris and natural source such as soil dust, sea salt parti-cles, etc. More and more research results show that, aerosols impact global and regional energy radiative budget; aerosol particles also modify the cloud mi-crophysics, as a result, aerosol particles may change the cloud radiative properties. Aerosol particles also play an important role in many biogeochemical cycles. All the above-menti…     

Monitoring temporal–spatial variations of AOD over the Yangtze River Delta,China

Aerosol can induce visibility reduction, affect radiation balance and modify cloud property on the environmental effect, and show the harmful effects on human health. Insight of aerosol becomes an integral task in the process of control measures for environmental pollution. The present study provided an analysis of temporal–spatial variations of aerosol optical depth (AOD) using the MOD04 level-2 in collection 6 (C6) with the deep blue retrieval algorithm from January 2005 to December 2015 over Yangtze River Delta (YRD) in China. The AOD validations between MODIS and Aerosol Robotic Network (AERONET) were estimated by the methods of regression, correlation. Then, the periodic features and trends of AOD and angstrom exponent (AE) were explored with the wavelet transformation (WT) procedure. Further, the variations of AOD and AE spatial distribution on multi-time scales (annual, monthly and season) were demonstrated. Meantime, the sources of AOD are discussed. It was found that the daily AOD from MODIS has a strong correlation relationship (slope?=?0.9838, r?=?0.84) with AERONET over YRD. The variations of both AOD and AE on time series have been distinct temporal periodic (12, 6 and 4 months) characteristics, and show the decreasing trends on annual and semi-annual periods. On annual, the AOD on spatial distribution is slowly declining from the northwest towards the southeast, and the AE on spatial distribution is gradually decreasing from the northwest to the southeast and from the land to the coast. The variations both inter-annual AOD and AE on spatial distribution show the inverse trends, respectively. On monthly, the means of AOD range from minimum 0.46 in January to maximum 0.90 in July, and the variations of spatial distribution mainly occur in the north parts of Yangtze River and some scattered areas with high terrain and south coast. The means of AE range from minimum 1.13 in October to maximum 1.58 in April, and the variations of spatial distribution are mainly found in the south of Henan, the north of Jiangsu, the coast belt and the riverside of Yangtze River and the high terrain regions. On seasonality, the means of AOD reaches its maximum 0.68 in summer and minimum 0.50 in winter, and the variations of spatial distribution mainly occur in the coast belt, the north parts of Hongze Lake and the south parts with high terrain. The means of AE reaches its maximum 1.48 in spring and minimum 1.25 in autumn, and the variations of spatial distribution were shown the similarity with that of monthly.     

Effects of polarization calibration on aerosol optical depth retrieval: An ocean case sensitivity analysis

Reflectance measurements of both the visible and infrared bands of passive remote sensing sensors are widely used to retrieve aerosol optical depth(AOD) information. This is performed commonly for data obtained over both ocean and land, and these measurements allow for the off line development of a lookup table using radiative transfer models. Owing to molecular and aerosol effects, the reflected light received by the sensor is usually highly polarized. The linear polarization effect may be up to 100%, and the polarization factor of a sensor optical system will change the total intensity as well as the polarization status of the signal reaching the detector. The detector response will be different when the incident light polarization status changes, even if the total intensity remains constant. However, if the polarization calibration is neglected, it will cause obvious errors in the aerosol data retrieval. This is especially true for aerosol optical depth retrieval over an ocean. This measurement relies directly on the reflectance output of the sensor. Cases involving land surfaces are not discussed herein because the inhomogeneous properties conceal the error due to polarization. Taking the 550 and 860 nm bands as examples, the difference between the real top-of-atmosphere(TOA) reflectance and the reflectance reaching the detector is calculated using three different sensor polarization standards according to the Sea-viewing Wide Field-of-view Sensor(Sea Wi FS) and Moderate Resolution Imaging Spectroradiometer(MODIS) standards. The differences in AOD retrieval are also demonstrated using the lookup table developed previously from a vector radiative transfer code. The results reveal that under a normal situation in which the AOD is 0.15, the maximum AOD retrieval error could reach 0.04 in 550 nm but only 0.02 in 860 nm for the dust aerosol model. For the soot aerosol model, the maximum AOD retrieval error is 0.1 in 550 nm and 0.12 in 860 nm, indicating that the lack of polarization calibration will lead to large errors in aerosol retrieval over an ocean.     

Cases of aerosol optical depth estimation in the Athens area,Greece

This work estimates the Ångström turbidity coefficients and investigates the variation of the aerosol optical depth (AOD) in the Athens area, during different atmospheric conditions. The AOD is estimated in the wavelength band of 400–670 nm from direct-beam spectral irradiance measurements using ground-based instrumentation, during an experimental campaign performed in the period 22 September–1 October 2002. All data were collected under clear-sky conditions near the city center of Athens; the AODs were estimated relative to the local zenith to avoid the influence of the optical air mass. The study shows that the AOD is influenced by wind regime and traffic in the Athens area. The Angstrom's turbidity coefficients and the AOD values were found to be higher under the influence of South-sector winds compared to those from the North-sector. Under South-sector low winds, the pollutants are accumulated in the Athens basin. On the contrary, the North-sector winds clean the atmosphere.     

基于太湖气溶胶类型分区的环境一号卫星CCD大气校正

由两颗卫星组成的环境卫星星座系统所提供的CCD数据具有较高的时间分辨率,使其在内陆湖泊水环境遥感监测中具有较大的应用潜力,对其有效的大气校正方法的研究则是其定量化参数反演的前提.基于准同步的MODIS数据辅助,根据气溶胶的差异性,将太湖划分为北部湖区、其他湖区两块区域,利用辐射传输模型,研究太湖环境一号CCD数据大气校正的方法,并对2009年4月17、21、25日数据进行大气校正.研究结果表明,该大气校正方法直接使用较为成熟的MODIS各类产品,克服了传统大气校正中依赖于现场同步测量大气参数的缺陷,能够快速、有效地完成环境一号CCD数据的大气校正.基于气溶胶类型对太湖进行分区后,所求算的遥感反射率精度高于6S模型和暗像元等大气校正方法得到的结果.     

基于VIIRS数据的中国东部气溶胶光学厚度反演算法改进

VIIRS(Visible Infrared Imaging Radiometer Suite)作为MODIS(The MODerate resolution Imaging Spectroradiometer)的后继传感器,可在全球范围内实现对气溶胶的连续时空监测.卫星反演的气溶胶光学厚度(Aerosol Optical Depth,AOD)是研究地球能量收支平衡、气候效应和空气质量的重要大气参数.但在中国重污染天气情况下,现有的VIIRS陆地气溶胶产品存在一定不足.因此,本研究改进云识别方法,优化像元筛选,约束气溶胶类型选择,实现重污染情况下AOD的反演.基于地基AERONET(AErosol RObotic NETwork)的验证结果表明,相比NOAA(National Oceanic and Atmospheric Administration)产品,改进后的反演结果克服了反演值偏低的问题,且表现出更好的相关性,RMSE从0.236下降到0.219.为验证在重污染条件下改进算法的适用性和准确性,本文对比了两种污染条件下的反演结果(0.61).统计结果表明,在较重污染天气条件下(AODAERONET>1),相比NOAA的AOD产品,本文结果的反演率从32.3%提升为68.8%,回归分析的斜率提高为0.80,相关系数达到0.76,均方根误差为0.307,在增加反演量的同时保证了反演的精度.     

珠江三角洲对流层气溶胶时空变化特征分析

利用AERONET资料对珠三角地区气溶胶物理性质特征进行分析,建立珠三角地区的气溶胶模型,在此基础上,根据RT3 辐射传输模型构建矢量查找表,采用多角度偏振方法从PARASOL L1B数据反演得到细模态气溶胶光学厚度(AOD),最后采用2007-2009年MODIS总的AOD产品和本文的细模态AOD三年的反演结果分析了珠三角地区气溶胶的时间变化和空间分布特征,为深入研究珠三角地区污染物的局地排放和输送提供了条件.结果表明:(1)珠三角地区对流层气溶胶呈双峰型对数正态分布,其中细粒子平均半径主要集中在0.05~0.1,标准方差以0.5、0.6为主,粗粒子平均半径以0.9、1.0为主,标准方差为0.6、0.7,复折射指数实部以1.4、1.5 居多,虚部以0、0.01为主,细粒子所占比例大于70%,珠三角气溶胶呈现出粗颗粒物和细颗粒物并存的特征;(2)PARASOL业务算法中的气溶胶模型在珠三角地区有较大的局限性,引入当地气溶胶模型使细模态AOD的反演精度较卫星产品有了很大提高,细模态AOD主要反映了珠三角地区二次污染的强度;(3)珠三角地区总AOD值春季较大,秋夏季次之,冬季较小,并呈现逐年较小的趋势;(4)珠三角地区细模态AOD也在逐年降低,2009年细模态AOD年均值比2007年低了0.02,在空间分布上,高值地区主要集中在广州、佛山、中山等城市.     

Land cover mapping using time series HJ-1/CCD data

It is very difficult to have remote sensing data with both high spatial resolution and high temporal frequency; thus, two categories of land-use mapping methodology have been developed separately for coarser resolution and finer resolution data. The first category uses time series of data to retrieve the variation of land surface for classification, which are usually used for coarser resolution data with high temporal frequency. The second category uses fine spatial resolution data to classify different land surface. With the launch of Chinese satellite constellation HJ-1in 2008, four 30 m spatial resolution CCDs with about 360 km coverage for each one onboard two satellites made a revisit period of two days, which brought a new type of data with both high spatial resolution and high temporal frequency. Therefore, by taking the spatiotemporal advantage of HJ-1/CCD data we propose a new method for finer resolution land cover mapping using the time series HJ-1/CCD data, which can greatly improve the land cover mapping accuracy. In our two study areas, the very high resolution remote sensing data within Google Earth are used to validate the land cover mapping results, which shows a very high mapping accuracy of 95.76% and 83.78% and a high Kappa coefficient of 0.9423 and 0.8165 in the Dahuofang area of Liaoning Province and the Heiquan area of Gansu Province respectively.     

Estimating turbidity and total suspended matter in the Adour River plume (South Bay of Biscay) using MODIS 250-m imagery

The Basque coastal waters (South Bay of Biscay) are directly influenced by the Adour River freshwater plume. The Adour outflow leads to important variations of suspended matter concentrations and turbidity, which in turn may affect biological productivity and water quality. This study aims at both developing specific algorithms and testing the efficiency of atmospherically corrected MODIS-Aqua 250-m surface reflectance product (MYD09) to map total suspended matter concentrations and turbidity within the Adour coastal region. First, regional empirical algorithms based on in-situ data were tested to retrieve the concentration of total suspended matter and turbidity from the remote sensing reflectance. Then, the respective sensitivity of MODIS surface reflectance bands 1 and 2 for water quality application was investigated as well as the quality of atmospheric corrections. Finally, selected algorithms were applied to the MYD09 product. The resulting 250-m resolution maps were then compared to 1000-m maps produced by IFREMER and comparisons between satellite measurements and in-situ sampling points were performed. Results show that MODIS-Aqua band 1 (620–670 nm) is appropriate for predicting turbidity and total suspended matter concentrations using polynomial regression models, whilst band 2 is unadapted. Comparison between total suspended matter concentration 250-m resolution maps and mineral suspended matter 1000-m maps (generated by IFREMER) produced consistent results. A high correlation was obtained between turbidity measured in-situ and turbidity retrieved from MODIS-Aqua satellite data.     

基于HJ-CCD和MODIS的吉林省中西部湖泊透明度反演对比

水体透明度能够反映光在水体中的穿透程度,影响水生植被及以光为依赖条件的水生生物的分布,获取透明度的传统方法是采用透明度盘进行测量,但也可以通过遥感方法获得.环境减灾卫星是专门用于环境与灾害监测预报的小卫星星座,影像覆盖范围广,空间、时相分辨率较高,可以为水环境遥感提供较好的数据源.MODIS数据在近岸水体和内陆大型湖泊水环境监测中也有广泛应用,它的时相分辨率也很高,但空间分辨率低.利用HJ-1A卫星CCD数据和MODIS日反射率产品(MOD09GA),以2012年9月吉林省石头口门水库、二龙湖、查干湖、月亮泡等地的实测透明度为基础(实测点数74个,最小值为0.134 m,最大值为1.410 m,平均值为0.488 m),根据灰色关联度选取构建模型的波段组合,建立水体透明度反演模型.HJ1A-CCD数据与MOD09GA数据建立的模型R2分别为0.639和0.894,均方根误差(RMSE)分别为0.248和0.135,模型验证的平均相对误差(MRE)分别为17.1%和9.5%,RMSE分别为0.207和0.089.MODIS数据以其较高的辐射分辨率使模型精度较高,但是HJ数据在应用于透明度小于1 m的水体时精度也较高(MRE=13.5%,RMSE=0.066).HJ-CCD数据在空间分辨率上的优势使其能够获得透明度空间分布的细节信息.比较两者反演得到的湖泊平均透明度,结果较为一致.     

Aerosol microphysics over a tropical coastal station inferred from the spectral dependence of Angstrom wavelength exponent and inversion of spectral aerosol optical depths

Six years of spectral aerosol optical depth (AOD) measurements have been analyzed from a tropical coastal site, Trivandrum (8.55°N, 76.9°E, 3 m msl) to infer on the seasonal changes in the microphysical properties of columnar aerosols, by examining the derivatives of the Angstrom wavelength exponent (α) in the wavelength domain (α′_λ) as well as in AOD domain (α′_τ) and by retrieving the columnar size distribution by numerical inversion of the AODs. The inference of the changes in the aerosol microphysics drawn from the features of the derivatives α′_λ and α′_τ is consistent with the pattern revealed by the aerosol properties obtained from the columnar size distributions retrieved from the AOD spectra as well as from the surface measurements of mass-size distributions, which are supported by the back-trajectory cluster analysis and the results of chemical species analysis.     

三峡水库水体漫衰减系数光学特性及其遥感反演

根据2009年8月22-24 日实测的三峡水库水体的光谱数据以及水质参数,对三峡水库漫衰减系数的光学特性及影响因子进行分析研究,构建了490nm处漫衰减系数的Kd(490)反演模型,并利用环境一号卫星CCD数据对2009年7月21日的三峡水库Kd(490)进行反演.结果表明:由于水体中非色素颗粒物的含量较高,使得研究区...     

Characteristics of aerosol optical depth and Ångström parameters over Mohal in the Kullu Valley of Northwest Himalayan Region,India

The measurements using a ground based multi wavelength radiometer (MWR) at Mohal (31°54′N, 77°07′E, 1154 m AMSL) in the Kullu valley of Northwestern Himalayan region show that the spectral aerosol optical depth (AOD) and turbidity coefficient, β, are high in summer, moderate in monsoon season, low in winter and lowest in autumn, while wavelength exponent, α, has an opposite trend. Average annual value of AOD at 500 nm is 0.24±0.01, 0.43±0.02, and 0.28±0.02; that of β is 0.14±0.01, 0.22±0.02, and 0.17±0.03; and that of α is 1.06±0.09, 1.16±0.10, and 0.86±0.13, respectively, for clear, hazy and partially clear sky days. The considerably greater value of β on hazy days indicates more coarse particles in mountain haze. The fractional asymmetry factor (AF) is more negative in summer and autumn months. The AOD and β have significantly positive correlation with temperature and wind speed, suggesting high AODs and turbidity on hot and windy days.     

Photosynthetically active radiation retrieval based on HJ-1A/B satellite data

Photosynthetically active radiation (PAR) is essential for plant photosynthesis and carbon cycle, and is also important for meteorological and environmental monitoring. To advance China’s disaster and environmental monitoring capabilities, the HJ-1A/B satellites have been placed in Earth orbit. One of their environmental monitoring objectives is the study of PAR. We simulated direct solar, scattered and environment radiation between 400 and 700 nm under different atmospheric parameters (solar zenith angle, atmospheric water vapor, atmospheric ozone, aerosol optical thickness, surface elevation and surface albedo), and then established a look-up table between these input parameters and PAR. Based on the look-up table, we used HJ-1A/B aerosol and surface albedo outputs to derive the corresponding PAR. Validation of inversed instantaneous and observed PAR values using HJ-1 Heihe experimental data had a root mean square error of 25.2 W m^?2, with a relative error of 5.9%. The root mean square error for accumulated daily PAR and observed values was 0.49 MJ m^?2, with a relative error of 3.5%. Our approach improved significantly the computational efficiency, compared with using directly radiation transfer equations. We also studied the sensitivity of various input parameters to photosynthetically active radiation, and found that solar zenith angle and atmospheric aerosols were sensitive PAR parameters. Surface albedo had some effect on PAR, but water vapor and ozone had minimal impact on PAR.     

Quantitative estimation of the shrub canopy LAI from atmosphere-corrected HJ-1 CCD data in Mu Us Sandland

The leaf area index (LAI) is an important ecological parameter that characterizes the interface between vegetation canopy and the atmosphere. In addition, it is used by most process-oriented ecosystem models. This paper investigates the potential of HJ-1 CCD data combined with linear spectral unmixing and an inverted geometric-optical model for the retrieval of the shrub LAI in Wushen Banner of Inner Mongolia in the Mu Us Sandland. MODTRAN (Moderate Resolution Atmospheric Radiance and Transmittance Model) was used for atmospheric correction. Shrubland was extracted using the threshold of the normalized difference vegetation index, with which water bodies and farmland were separated, in combination with a vegetation map of the People’s Republic of China (1:1000000). Using the geometric-optical model, we derive the per-pixel reflectance as a simple linear combination of two components, namely sunlit background and other. The fraction of sunlit background is related to the shrub LAI. With the support of HJ-1 CCD data, we employ linear spectral unmixing to obtain the fraction of sunlit background in an atmospherically corrected HJ image. In addition, we use the measured shrub canopy structural parameters for shrub communities to invert the geometric-optical model and retrieve the pixel-based shrub LAI. In total, 18 sample plots collected in Wushen Banner of Inner Mongolia are used for validation. The results of the shrub LAI show good agreement with R ² of 0.817 and a root-mean-squared error of 0.173.     

Photosynthetically active radiation retrieval based on HJ-1A/B satellite data

Photosynthetically active radiation (PAR) is essential for plant photosynthesis and carbon cycle, and is also important for meteorological and environmental monitoring. To advance China’s disaster and environmental monitoring capabilities, the HJ-1A/B satellites have been placed in Earth orbit. One of their environmental monitoring objectives is the study of PAR. We simulated direct solar, scattered and environment radiation between 400 and 700 nm under different atmospheric parameters (solar zenith angle, atmospheric water vapor, atmospheric ozone, aerosol optical thickness, surface elevation and surface albedo), and then established a look-up table between these input parameters and PAR. Based on the look-up table, we used HJ-1A/B aerosol and surface albedo outputs to derive the corresponding PAR. Validation of inversed instantaneous and observed PAR values using HJ-1 Heihe experimental data had a root mean square error of 25.2 W m⁻², with a relative error of 5.9%. The root mean square error for accumulated daily PAR and observed values was 0.49 MJ m⁻², with a relative error of 3.5%. Our approach improved significantly the computational efficiency, compared with using directly radiation transfer equations. We also studied the sensitivity of various input parameters to photosynthetically active radiation, and found that solar zenith angle and atmospheric aerosols were sensitive PAR parameters. Surface albedo had some effect on PAR, but water vapor and ozone had minimal impact on PAR.

     

Scale effects of leaf area index inversion based on environmental and disaster monitoring satellite data

The spatial distribution of sub-pixel components has an impact on retrieval accuracy, and should be accounted for when inverting a three-dimensional adiative transfer model to retrieve leaf area index (LAI). To investigate this effect, we constructed three realistic scenarios with the same LAI values and other properties, except that the simulated plants had different distributions. We implemented the radiosity method to subsequently produce synthetic bidirectional reflectance factor (BRF) datasets based upon these simulated scenes. The inversion was conducted using these data, which showed that spatial distribution affects retrieval accuracy. The inversion was also conducted for LAI based on charge-coupled device (CCD) data from the Environment and Disaster Monitor Satellite (HJ-1), which depicted both forest and drought-resistant crop land cover. This showed that heterogeneity in coarse-resolution remote sensing data is the main error source in LAI inversion. The spatial distribution of global fractal dimension index, which can be used to describe the area of sub-pixel components and their spatial distribution modes, shows good consistency with the coarse resolution LAI inversion error.     

Estimating forest aboveground biomass using HJ-1 Satellite CCD and ICESat GLAS waveform data

The ecosystem in northeastern China and the Russian Far East is a hotspot of scientific research into the global carbon balance. Forest aboveground biomass (AGB) is an important component in the land surface carbon cycle. In this study, using forest inventory data and forest distribution data, the AGB was estimated for forest in Daxinganlin in northeastern China by combining charge-coupled device (CCD) data from the Small Satellite for Disaster and Environment Monitoring and Forecast (HJ-1) and Geoscience Laser Altimeter System (GLAS) waveform data from the Ice, Cloud and land Elevation Satellite (ICESat). The forest AGB prediction models were separately developed for different forest types in the research area at GLAS footprint level from GLAS waveform parameters and field survey plot biomass in the Changqing (CQ) Forest Center, which was calculated from forest inventory data. The resulted statistical regression models have a R ²=0.68 for conifer and R ²=0.71 for broadleaf forests. These models were used to estimate biomass for all GLAS footprints of forest located in the study area. All GLAS footprint biomass coupled with various spectral reflectivity parameters and vegetation indices derived from HJ-1 satellite CCD data were used in multiple regression analyses to establish biomass prediction models (R ²=0.55 and R ²=0.52 for needle and broadleaf respectively). Then the models were used to produce a forest AGB map for the whole study area using the HJ-1 data. Biomass data obtained from forest inventory data of the Zhuanglin (ZL) Forest Center were used as independent field measurements to validate the AGB estimated from HJ-1 CCD data (R ²=0.71). About 80% of biomass samples had an error less than 20 t ha⁻¹, and the mean error of all validation samples is 5.74 t ha⁻¹. The pixel-level biomass map was then stratified into different biomass levels to illustrate the AGB spatial distribution pattern in this area. It was found that HJ-1 wide-swath data and GLAS waveform data can be combined to estimate forest biomass with good precision, and the biomass data can be used as input data for future carbon budget analysis.