Spectral aerosol optical depth characterization of desert dust during SAMUM 2006

The aerosol optical depth (AOD) in the range 340–1550 nm was monitored at Ouarzazate (Morocco) during the Saharan Mineral Dust Experiment (SAMUM) experiment in May–June 2006. Two different sun photometers were used for this purpose. The mean AOD at 500 nm was 0.28, with a maximum of 0.83, and the mean Ångström exponent (AE) was 0.35. The aerosol content over the site changed alternatively from very low turbidity, associated to Atlantic air masses, to moderate dust load, associated to air masses arriving in the site from Algeria, Tunisia and Libya. The dusty conditions were predominant in the measurement period (78% of data), with AOD (500 nm) above 0.15 and AE below 0.4. The spectral features of the AOD under dusty conditions are discussed. Air mass back trajectory analysis is carried out to investigate the origin and height patterns of the dust loaded air masses. The advection of dust occurred mainly at atmospheric heights below 3000 m, where east flow is the predominant. At the 5000 m level, the air masses originate mainly over the Atlantic Ocean. Finally the Optical Properties of Aerosols and Clouds (OPAC) model is used to perform a set of simulations with different aerosol mixtures to illustrate the measured AOD and AE values under varying dust concentrations, and a brief comparison with other measurement sites is presented.     

Measurements of desert dust optical characteristics at Porte au Sahara during SAMUM

Main optical characteristics of desert dust, such as phase function and single scattering albedo, have been derived from combinations of sun-/sky-radiometer and satellite measurements during the SAMUM experiment (10 May–10 June 2006) at the site Porte au Sahara (30.237°N, 5.607°W) in South Morocco. Scattering phase functions have been retrieved using combined data of spectral aerosol optical thickness (AOT) and spectral sky brightness in the almucantar, considering non-spherical light scattering. Intercomparisons of modelled top-of-atmosphere (TOA) reflectance with satellite observations of the Medium Resolution Imaging Spectrometer (MERIS) and Scanning Imaging Absorption Spectrometer for Atmospheric Chartography () instrument have been used for the estimation of spectral single scattering albedo. For the radiative transfer calculations scattering phase functions and AOT from ground-based observations have been used. The spectral single scattering albedo ranges from 0.93 in the blue to 0.98 at 753 nm.     

Regional Saharan dust modelling during the SAMUM 2006 campaign

The regional dust model system LM-MUSCAT-DES was developed in the framework of the SAMUM project. Using the unique comprehensive data set of near-source dust properties during the 2006 SAMUM field campaign, the performance of the model system is evaluated for two time periods in May and June 2006. Dust optical thicknesses, number size distributions and the position of the maximum dust extinction in the vertical profiles agree well with the observations. However, the spatio-temporal evolution of the dust plumes is not always reproduced due to inaccuracies in the dust source placement by the model. While simulated winds and dust distributions are well matched for dust events caused by dry synoptic-scale dynamics, they are often misrepresented when dust emissions are caused by moist convection or influenced by small-scale topography that is not resolved by the model. In contrast to long-range dust transport, in the vicinity of source regions the model performance strongly depends on the correct prediction of the exact location of sources. Insufficiently resolved vertical grid spacing causes the absence of inversions in the model vertical profiles and likely explains the absence of the observed sharply defined dust layers.     

Retrieval of aerosol optical depth for Chongqing using the HJ-1 satellite data

Aerosol optical depth (AOD) is a common indicator applied in monitoring aerosols in the atmosphere. The hilly landscape and rapid economic growth of the megacity Chongqing have facilitated increased aerosol concentration, and it is meaningful to accurately retrieve AOD over Chongqing. The HJ-1A/B satellite of China carries a sensor/camera called the Charge Coupled Device (CCD), the spatial resolution of which meets the requirement for retrieving high resolution AOD. In this paper, analysis of the AOD retrievals from different methods using the HJ-1 satellite data revealed the most suitable algorithm. Through comparison with the AOD product of Moderate Resolution Imaging Spectroradiometer (MODIS), the AOD retrieval results using enhanced vegetation index (EVI) to estimate dark pixels showed the highest correlation. The continental aerosol model was used to build a lookup table that was able to facilitate a good AOD retrieval for both city and rural areas. Finally, the algorithm that combined dark pixels, buffer areas, and the deep blue algorithm was found to be most suitable for AOD retrieval. The AOD retrieval results based on the HJ-1 data were consistent with MODIS products, and our algorithm yields reasonable results in most cases. The results were also compared with ground-based PM₁₀ measurements synchronized with the overpass time of the HJ-1 satellite, and high correlation was found. The findings are relevant to other Chinese satellite data used for retrieving AOD on the same channels.     

利用MODIS数据反演多层云光学厚度和有效粒子半径

利用卫星资料反演云微物理参数不仅有助于对天气变化的监测和预报,而且对人工影响天气的研究十分有益.目前卫星反演云微物理参数的算法一般是假设视场中只有一层云,但是实际环境中多层云出现很频繁.文中研究了多层云的光学厚度和有效粒子半径微物理参数的反演算法,主要针对薄的冰云覆盖在低层水云的多层云情形.算法利用中分辨率成像光谱仪(MODIS)吸收通道和非吸收通道同时进行反演,在此基础上利用SBDART辐射传输模式模拟冰云覆盖在低层水云上的多层云对云微物理参数反演的影响,模拟表明反演时将多层云作为单层云处理会使反演结果产生较大误差.为此,文中提出了云光学厚度和有效粒子半径反演算法中要考虑多层云的因素,并设计了一套云光学厚度和有效粒子半径反演方案.该方案使用SBDART辐射传输模式建立不同观测几何条件、下垫面类型、大气环境等条件下以光学厚度和有效粒子半径为函数变量的多层云、水云和冰云辐射查找表.经过云检测、云相态识别和多层云检测后,在该查找表的基础上,对MODIS通道1和通道7的数据采用最小方差拟合法反演光学厚度、有效粒子半径.利用该方案对2006年7月12日TERRA卫星MODIS数据进行反演试验,反演结果与NASA发布的MOD06产品中云的光学厚度和有效粒子半径的结果较一致,表明方案具有合理性.     

Retrieval of aerosol optical properties over the Beijing area using POLDER/PARASOL satellite polarization measurements

Aerosol optical properties over Beijing and Xianghe under several typical weather conditions (clear sky, light haze, heavy pollution and dust storm) are derived from POLDER (POLarization and Directionality of the Earths Reflectances)/PARASOL (Polarization and Anisotropy of Reflectances for Atmospheric Sciences coupled with Observations from a Lidar) multi-directional, multi-spectral polarized signals using a more reliable retrieval algorithm as proposed in this paper. The results are compared with those of the operational retrieval algorithm of POLDER/PARASOL group and the ground-based AERONET (AErosol RObotic NETwork)/PHOTONS (PHOtometrie pour le Traitement Operational de Normalisation Satellitaire) measurements. It is shown that the aerosol optical parameters derived from the improved algorithm agree well with AERONET/PHOTONS measurement. The retrieval accuracies of aerosol optical thickness (AOT) and effective radius are 0.06 and 0.05 mu m respectively, which are close to or better than the required accuracies (0.04 for AOT and 0.1 mu m for effective radius) for estimating aerosol direct forcing.     

塔克拉玛干沙漠春季大气气溶胶光学特性研究

本文利用多波段光度计在新疆塔克拉玛干沙漠探测的1986年4月11日至5月10日的太阳直接辐射强度和天空亮度资料,分析了塔克拉玛干沙漠晴空大气和沙暴期间大气气溶胶的光学厚度、谱分布以及折射率的变化特性。     

Aircraft observations of the microphysical and optical properties of major aerosol species

We summarise the microphysical and optical parameters of some principal aerosol species obtained by instrumentation on the UK Met Office C-130 aircraft during international field campaigns since 1996. The aerosol species include Saharan dust, biomass burning aerosol, European continental pollution, eastern seaboard USA pollution, and clean maritime aerosol. The typical structure of the aerosol in the vertical from each airmass type is described. Microphysical parameters are described that comprise the mode radius and geometric standard deviation associated with 2–3 lognormal fits to the mean observed aerosol size distributions spanning the accumulation and coarse modes. Optical parameters comprising the aerosol single scattering albedo (which was both measured and derived from Mie theory), specific extinction coefficient, and asymmetry factor (which were derived from Mie theory) are also presented. Where available, evolution of the physical and optical properties of the aerosol has been highlighted. Comparisons with long-term ground-based AERONET aerosol retrievals show reasonable agreement. Our observations provide useful data for validating and improving global circulation models (GCMs) that use physically based aerosol representation and for validating satellite retrievals of the physical and optical properties of aerosols.     

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.     

中国中东部MODIS与MISR气溶胶光学厚度的对比

Terra/MODIS前一版本C4和最新版本C5的气溶胶光学厚度(AOT Aerosol OpticalThickness)数据,以及搭载于同一卫星上的Terra/MISR气溶胶光学厚度数据,在中国中东部地区存在差异。本文利用AERONET气溶胶光学厚度数据对以上三种资料验证的结果表明:MODIS气溶胶算法改进之后得到的C5 AOT数据较C4精度确有很大提高,且优于MISR的AOT数据。     

贵州省2009年冬季大气气溶胶光学厚度的特征分析

利用FY-3A陆上气溶胶日产品,结合ENVI遥感影像软件和ArcGIS地理信息软件的相关模块,处理和分析了贵州省2009年冬季大气气溶胶光学厚度,并由此计算出Angstrom浑浊度系数和波长指数。分析表明:贵州省2009年冬季整层大气气溶胶光学厚度在0.55μm波段内的旬变化幅度为0.28~0.53μm,并呈现出波动性的发展趋势,特别是2010年1月下旬—2月下旬有显著增加,较大值在整个冬季都相对集中于贵州省的中部、西南部以及北部地区;在污染物浓度变化中,细粒子浓度变化因素占主导地位;与冬季的前期相比,后期气溶胶光学厚度较大,细粒子比重有所增加,浑浊度稍微偏高,造成空气轻微污染。     

In situ measurements of optical properties at Tinfou (Morocco) during the Saharan Mineral Dust Experiment SAMUM 2006

In situ measurements of optical and physical properties of mineral dust were performed at the outskirts of the Saharan Desert in the framework of the Saharan Mineral Dust Experiment part 1 (SAMUM-1). Goals of the field study were to achieve information on the extent and composition of the dust particle size distribution and the optical properties of dust at the ground. For the particle number size distribution, measured with a DMPS/APS, a size dependent dynamic shape factor was considered. The mean refractive index of the particles in this field study is  1.53–4.1 × 10⁻³ i   at 537 nm wavelength and  1.53–3.1 × 10⁻³ i   at 637 nm wavelength derived from measurements of scattering and absorption coefficients, as well as the particle size distribution. Whereas the real part of the refractive index is rather constant, the imaginary part varies depending on the mineral dust concentrations. For high dust concentration the single scattering albedo is primarily influenced by iron oxide and is  0.96 ± 0.02  and  0.98 ± 0.01  at 537 nm and 637 nm wavelength, respectively. During low dust concentration the single scattering albedo is more influenced by a soot-type absorber and is  0.89 ± 0.02  and  0.93 ± 0.01  for the same wavelengths.     

Variations of aerosol optical thickness of the atmosphere in Russia in 1976–2003

Space-time variations of aerosol optical thickness of the atmospheric column are analyzed on the basis of observational data of the Russian actinometric network in 1976–2003. From the measured direct solar radiation flux and air humidity for 44 stations in European and Asian Russia, spectral aerosol optical thickness (AOT) of the atmosphere is estimated for a wavelength λ ?0.55 μm. Interannual, seasonal, and space variations of AOT are estimated. Effects of eruptions of the volcanoes El Chichon (April 1982) and Pinatubo (June 1991) are considered. For most stations, a negative tendency is found in the AOT interannual changes, which is more pronounced in spring and summer. It is shown that the period after the Pinatubo eruption is characterized by “purification” of the atmosphere from aerosol.     

AIRS晴空大气温度廓线反演试验

2002年5月发射的Aqua是美国地球观测系统中的第2颗卫星EOS-2（Earth Observing System）,它携带的大气红外探测仪AIRS（Atmospheric Infrared Sounder）是目前国际上最先进的超高光谱分辨率红外大气垂直探测仪。1200的光谱分辨率和全球覆盖能力使其可以观测全球大气状态及其变化。对其超高光谱分辨率资料的处理和应用的研究既有科学意义,又有实用价值。利用AIRS实际观测资料反演大气温度,本文使用的是便于处理大数据量的统计反演方法中的特征向量法,计算回归系数使用的是AIRS观测辐射值和T213数值预报分析场。本文从通道个数、地形及地表发射率影响、增加预报因子、国外同类产品比较等4个方面进行了大气温度反演试验研究。研究结果表明:（1）在利用超高光谱分辨率仪器进行反演时,合理选择通道是非常必要的;（2）AIRS反演的大气温度在对流层顶和中上层具有较高的精度,在对流层低层,尤其是近地面反演精度不够理想;（3）增加地表温度,地表发射率和地形3个预报因子,可以提高近地面的反演精度;（4）文中反演的大气温度精度与国外同类产品精度基本一致。     

风云二号卫星的冰云光学厚度反演偏差分析

冰云的微物理特性参数反演是云参数反演的难点和热点问题,目前风云二号(FY-2)卫星还没有相关的业务产品。考虑薄卷云覆盖在中低云上的两层云情况,采用六棱柱形状的冰云,在云相态识别基础上,利用FY-2 卫星观测数据,采用双通道算法反演冰云光学厚度。选取2013年8月的EOS/Terra和EOS/Aqua云参数产品对反演的FY-2云光学厚度精度进行比对分析。研究结果表明,联合FY-2的可见光通道和中波红外通道可反演冰云光学厚度。基于匹配得到的34个分析个例,FY-2反演的云光学厚度分布态势与EOS/ MODIS云产品相同,但FY-2云光学厚度反演值小于EOS/MODIS 云光学厚度产品值。FY-2 反演云光学厚度与EOS/MODIS云光学厚度产品的平均偏差为6.41,相关系数平均为0.92,线性拟合平均斜率为0.74。FY-2 与EOS/MODIS云光学厚度值偏差出现原因除了反演算法存在差异外,与反演所用数据的不同存在密切关系,基础观测数据越相近,FY-2 与EOS/MODIS云光学厚度反演结果的偏差越小。     

Measurements of the spectral aerosol optical depth using a sun photometer

Summary A sun photometer fitted with 9 narrowband interference filters from 368 nm to 1024 nm was used to determine the aerosol optical depth at 8 measuring stations in Europe. Of the 9 filters, 4 were not included in the standard equipment of the sun photometer. Given the temporal instabilities of the calibration factors of these 4 filters, meaningful results for the aerosol optical depth can be achieved only if calibration and measurements follow each other closely. Calibration was performed using the Langley plot method. For wavelengths < 500 nm, the measured aerosol optical depths were compared with the results of a high resolution spectrometer. Broadband aerosol optical depths, centred at 427 nm, were determined with an actinometer and were also used for comparison. Generally, there was good agreement of the results. To characterize the aerosol optical depths, the Ångström parameters and were used. The individual measuring stations showed a clustered distribution of and values.With 6 Figures     

Spatial distribution and optical properties of Saharan dust observed by airborne high spectral resolution lidar during SAMUM 2006

Airborne measurements of pure Saharan dust extinction and backscatter coefficients, the corresponding lidar ratio and the aerosol optical thickness (AOT) have been performed during the Saharan Mineral Dust Experiment 2006, with a high spectral resolution lidar. Dust layers were found to range from ground up to 4–6 km above sea level (asl). Maximum AOT values at 532 nm, encountered within these layers during the DLR Falcon research flights were 0.50–0.55. A significant horizontal variability of the AOT south of the High Atlas mountain range was observed even in cases of a well-mixed dust layer. High vertical variations of the dust lidar ratio of 38–50 sr were observed in cases of stratified dust layers. The variability of the lidar ratio was attributed to dust advection from different source regions. The aerosol depolarization ratio was about 30% at 532 nm during all measurements and showed only marginal vertical variations.     

Extinction versus backscatter relationships for lidar applications at 351 nm: maritime and desert aerosol simulations and comparison with observations

Functional relationships linking at λ₀=351 nm aerosol extinction α_λ0^aer and backscatter coefficient β_λ0^aer of maritime and desert type aerosols are determined to allow for inversion of the single-wavelength lidar signals. Such relationships are derived as mean behavior of 20,000 extinction versus backscatter computations, performed for aerosol size distributions and compositions whose describing parameters are randomly chosen within the naturally observed variability. For desert-type aerosols, the effect of the particle non-sphericity is considered and it is shown that the extinction to backscatter ratio of non-spherical dust particles can be up to 60% larger than the values obtained for spherical particles. Aerosol extinction and backscatter coefficient profiles obtained inverting the single-wavelength lidar signal with the modeled relationships are then compared to the same profiles measured by a combined elastic-Raman lidar operating at 351 nm. Analytical back trajectories and satellite images are used to characterize advection patterns during lidar measurements and to properly choose the modeled functional relationship. A good accordance between the two techniques is found for advection patterns over the lidar site typical of maritime and dust conditions. Maximum differences between the model-based α_λ0^aer and β_λ0^aer vertical profiles and the corresponding ones measured by the combined elastic-Raman lidar technique are of 30% and 40% in maritime and desert dust conditions, respectively. The comparison of elastic-Raman lidar measurements and model-based results also reveals that particle non-sphericity must be taken into account when mineral dust-type aerosols are directly advected over the measurement site.