气溶胶光学厚度、类型及廓线对CO2反演的敏感性分析

位于1.6和2.06 μm附近的短波红外波段可用于探测低层大气CO²信息,然而除大气分子(主要是CO²)吸收外,大气分子散射、气溶胶和云散射也是限制这些波段的卫星测量信号的主要因素,因此气溶胶光学参数及其垂直分布也必然制约着GOSAT(Greenhouse Gases Observing Satellite)CO²的反演精度。利用正演模式DISORT分析气溶胶光学厚度(AOD)、类型以及廓线对CO²反演精度的敏感性,旨在提高CO²反演精度。结果表明:①不同类型气溶胶AOD增大对CO²柱总量的影响各不相同,且CO²柱总量变化量的变化趋势也不相同,这取决于气溶胶的散射相函数与单次散射反照率的大小;②气溶胶类型变化会导致CO²柱总量变化,且这种变化随着AOD增大而增大;③对于满足指数分布的沙尘气溶胶,标高的低估会导致反演的CO²柱总量偏大,而气溶胶的集中分布导致反演的CO²柱总量偏小,且气溶胶层越高,反演的CO²柱总量越小。     

两种氧气A吸收带云顶高度反演算法的理论比较与实测结果验证

近年来,氧气A吸收带作为云高反演通道重新引起学者们的关注,其中用于欧空局ENVISat/SCIAMACHY仪器的SACURA和FRESCO+两种云高算法是目前比较成熟的氧气A带云高反演算法,并已有相关产品发布。为充分了解SACURA和FRESCO+两种算法的适用性及反演效果,对两种算法特点及其模拟反演的结果进行了深入比较,在敏感性分析的基础上评估这两种算法的特点及产品精度。结果表明,FRESCO+算法对地表反照率、云量等参数的依赖性较低,得到的云高结果偏低达20～750 m,而SACURA算法对太阳天顶角、云光学厚度、地表反照率和云量等参数较敏感,得到的云高值波动性较大。不包括光学厚度小于5的云且满云时,平均云顶误差小于300 m。在亮地表上的薄云结果最差,误差最大可达10 km以上。为验证上述结论,我们对2008年度覆盖我国及周边地区两种算法的相应产品进行了统计分析,并与寿县ARM计划的地基雷达和探空资料进行了对比,表明模拟反演得到的结论可信。     

氧气A吸收带大气遥感应用研究进展

位于可见光波谱的氧气A吸收带具有独特的谱线结构,随波长或仪器分辨率的变化表现出不同的光谱特性。因而从20世纪60年代到现在,氧气A吸收带在遥感中的应用一直是国外学者研究的热点。总结了业已取得的成果和未来应用的发展方向并分析了其潜在探测能力,希望能够引起国内相关学者的重视,为我国氧气A带探测技术的应用与发展提供理论基础。     

Validation of MODIS derived aerosol optical depth over the Yangtze River Delta in China

MODIS derived aerosol optical depths (AODs) at 550 nm are compared with sunphotometer CE318 measurements at 7 sites located at Yangtze River Delta (YRD) in China from July to October, 2007. The evaluation result indicates that MODIS AODs (Collection 5, C005) are in good agreement with those from CE318 in dense vegetation regions, but show more differences in those regions with complex underlying surface (such as at lake water and urban surface sites). Reasons for these differences are discussed after removing cases with significant errors caused by validation scheme. The final validation result shows that MODIS AODs are in good agreement with CE318 with a correlation coefficient of 0.85 and RMS of 0.15. 90% of MODIS cases fall in the range of Δτ = ± 0.05 ± 0.20τ, indicating MODIS aerosol retrieval algorithm, aerosol models and surface reflectance estimate are generally suitably reasonable for aerosol retrieval in YRD. However, MODIS AODs show a systemic errors with fitted line of y = 0.75x + 0.13, indicating underestimation of AOD when aerosol loadings are high. Aerosol models and surface reflectance estimations are dominant sources of MODIS aerosol retrieval errors.     

Algorithm for retrieval of aerosol optical properties over the ocean from the Geostationary Ocean Color Imager

An aerosol retrieval algorithm for the first Geostationary Ocean Color Imager (GOCI) to be launched in March 2010 onboard the Communication, Ocean, and Meteorological Satellite (COMS) is presented. The algorithm retrieves aerosol optical depth (AOD), fine-mode fraction (FMF), and aerosol type in 500 m × 500 m resolution. All the products are retrieved over clear water which is defined by surface reflectance ratio between 640 nm and 860 nm (SRR) less or equal to 2.5, while only AOD is retrieved over turbid water (SRR > 2.5) due to high surface reflectance. To develop optimized algorithm for the target area of GOCI, optical properties of aerosol are analyzed from extensive observation of AERONET sunphotometers to generate lookup table. Surface reflectance of turbid water is determined from 30-day composite of Rayleigh- and gas corrected reflectance. By applying the present algorithm to MODIS top-of-the atmosphere reflectance, three different aerosol cases dominated by anthropogenic aerosol contains black carbon (BC), dust, and non-absorbing aerosol are analyzed to test the algorithm. The algorithm retrieves AOD, and size information together with aerosol type which are consistent with results inferred by RGB image in a qualitative way. The comparison of the retrieved AOD with those of MODIS collection 5 and AERONET sunphotometer observations shows reliable results. Especially, the application of turbid water algorithm significantly increases the accuracy in retrieving AOD at Anmyon station. The sensitivity study between MODIS and GOCI instruments in terms of relative sensitivity and scattering angle shows promising applicability of the present algorithm to future GOCI measurements.     

Aerosol retrieval over ocean from SEVIRI for the use in GERB Earth's radiation budget analyses

To study the effect of aerosols on the Earth's radiation budget (ERB), the Royal Meteorological Institute of Belgium (RMIB) has integrated spectral aerosol optical depth (AOD) measurements over the ocean from the Spinning Enhanced Visible and Infra-Red Scanner (SEVIRI) into its Geostationary Earth's Radiation Budget, or GERB, processing system referred to as the RGP. Aerosols affect the ERB both directly (when radiation interacts with an aerosol particle) and indirectly (when aerosols act as cloud condensation nuclei). Quantifying the indirect effect is challenging as it requires accurate aerosol retrievals in the close proximity to clouds, where aerosol retrievals may be biased due to leakages from the cloud mask (CM). The initial focus of the RGP project was on the direct effect using confidently clear scenes.

A single channel CM exploiting the SEVIRI temporal sampling was developed at the RMIB for the use in the RGP project. In this study, that single channel mask was evaluated against two multi-channel CMs, one from the Meteorological Products Extraction Facility (MPEF) at the European Organization for the Exploitation of Meteorological Satellites (EUMETSAT), and the other from the Satellite Application Facility for Supporting NoWCasting and Very Short Range Forecasting (SAFNWC), respectively. The NOAA/NESDIS Advanced Very High Resolution Radiometer (AVHRR) single channel aerosol algorithm was adjusted to SEVIRI spectral bands and consistently applied to the pixels identified as cloud-free. The aerosol products corresponding to the three CMs were compared, and the RMIB CM was found to be sufficiently accurate and conservative, for RGP applications.

Comparisons with independent AODs derived from the MODerate resolution Imaging Spectroradiometer (MODIS) onboard Terra and Aqua satellites show that the RMIB CM-based SEVIRI aerosol product compares well with its MODIS counterpart. However, a small fraction of cloud-contaminated pixels may still remain in the SEVIRI AOD imagery, chiefly within one to two SEVIRI pixels of the cloud boundary, thus limiting its use for indirect forcing studies. Also, the RMIB CM may screen high AOD non-dust aerosol events (e.g., smoke from biomass burning) as cloud. The potential of the new SEVIRI aerosol product is illustrated by generating 9 km-resolution seasonal maps of AODs and ´ÅǺngström Exponents, and by using the GERB radiative flux measurements for a preliminary quick assessment of the direct aerosol forcing.     

Comparison of atmospheric aerosol climatologies over southwestern Spain derived from AERONET and MODIS

The southwestern area of Spain, by its geographical and climatological conditions, is a key location for the characterization of atmospheric aerosol properties. The present study is aimed at evaluating the reliability of satellite-based aerosol climatologies, as inferred from level 2 standard aerosol products such as the Terra-MODIS (Moderate Resolution Imaging Radiometer) MOD04 aerosol product, with an application over this region during the period 2000-2008.This evaluation is carried out by means of comparison with ground-based data from the AERONET station of El Arenosillo (Spain, 37.1N, 6.7W), which has been providing continuous data since 2000. The focus of this paper is the climatology of two aerosol optical parameters: the aerosol optical depth (AOD) and the Ångström exponent.AERONET ground-based measurements give an annual mean value of 0.16 ± 0.12 and a median of 0.12 for the AOD, and a mean value of 1.20 ± 0.47 for the Ångström exponent. The seasonal pattern is characterized by two maxima, the most important maximum occurs in summer months, and the other one in late-winter/early-spring. Lowest values appear in fall and winter, however, a local minimum is observed in July which is only detected with the long-term data series.The mean climatological AOD based on AERONET exhibits complex seasonal patterns (i.e. with multiple local extrema), which are not always captured by MODIS-based climatology. MODIS only reproduces low values of the AOD in winter and high values in summer, as well as the local minimum of July which is sharper when using over-land retrievals. The time series of the AOD retrieved from MODIS both over land and ocean are in relatively good agreement with the ground-based measurements, with a monthly overestimation of about 30% on average, and higher differences in spring. Seasonal patterns from MODIS are better reproduced over land than over ocean. The agreement between daily AERONET and MODIS, as assessed by linear regression, gives correlation coefficients above 80% and an intercept bias below 0.03.     

Retrieval of aerosol optical and physical properties from ground-based spectral, multi-angular, and polarized sun-photometer measurements

An inversion algorithm to retrieve a complete set of aerosol optical and physical properties has been developed. The algorithm is based on sun-photometer measurements with emphasis on polarimetric observations. At present, these polarized sun-photometer observations are mainly provided by PHOTONS network included in AERONET worldwide network. With ground-based measurement of direct sun irradiance, as well as total and polarized sky radiance, most of key characteristics of atmospheric aerosols are retrieved, including spectral single-scattering albedo from UV to NIR wavelength, elements of scattering matrix F₁₁ and − F₁₂ (i.e. phase function and polarized phase function), complex refractive index, and size distribution. These parameters are fundamentally valuable in atmospheric physical and chemical studies. The theoretical accuracy is assessed based on three distinct bimodal log-normal aerosol models. The sensitivity studies to measurement uncertainties and to observing geometrical conditions are implemented to estimate the retrieval accuracy. Examples of typical retrievals when applying to real measurements are illustrated and compared with AERONET operational inversions. The particle shape-independent advantage of the retrieved single-scattering albedo, phase function, and polarized phase function is validated by considering a non-spherical aerosol model, which consisted of spheroid particles.     

Himawari-8气溶胶产品的验证及应用

为准确分析中国地区气溶胶空间分布与时间变化特征规律,首先利用中国地区9个AERONET（Aerosol Robotic Network）地基站点观测资料对新一代静止气象卫星Himawari-8气溶胶光学厚度（Aerosol Optical Depth, AOD）产品数据进行一致性验证,并在此基础上选取2015年7月至2018年4月Himawari-8逐小时AOD数据分析了中国地区气溶胶光学厚度时空变化特征。结果表明：①Himawari-8 AOD与AERONET AOD之间相关性很高,9个站点的相关系数R在0.64 ~ 0.91之间,拟合曲线斜率k的范围为0.57 ~ 0.68。②Himawari AOD产品与AERONET AOD的相关性在中午时段较其他时段相对较低;北方地区Himawari-8 AOD冬季反演效果与夏季相比较差,南方地区则相反。③中国地区年平均AOD呈东高西低分布,春、夏两季AOD明显高于秋、冬两季,其中夏季最高,春季次之;地区间AOD月变化差异也较大;大部分地区AOD日变化呈现先下降后上升再下降的趋势,AOD最高值出现在午后14 ~ 16时,最低值出现在18时。研究结果为了解中国地区大气气溶胶的时空变化规律和全天时的大气污染监测方法提供新的参考。     

基于HJ-1卫星数据反演长江三角洲地区气溶胶光学厚度

探索利用我国HJ-1卫星CCD数据,运用深蓝算法开展长江三角洲地区气溶胶光学厚度反演的可行性,并将结果与其他气溶胶光学厚度产品进行比较。针对HJ-1A和HJ-1B数据,反演结果分别与MODIS气溶胶光学厚度产品以及AERONET地基观测数据进行对比验证。结果表明:深蓝算法得到A星、B星的反演结果与MODIS气溶胶产品呈显著相关,但在数值上普遍高于MODIS产品;反演结果与AERONET站点数据之间的误差介于0.008~0.364之间,研究时段内站点数据缺乏,未对误差进行严格的统计分析。基于深蓝算法的HJ-1卫星数据反演结果虽然在数值上与MODIS气溶胶光学厚度产品存在系统性偏差,但在空间上能够较好地反映长江三角洲地区大气气溶胶分布状况,且具有空间分辨率高的优势。