Effective lidar ratios of dense dust layers over North Africa derived from the CALIOP measurements

Lidar ratio (i.e., extinction-to-backscatter ratio) is a key parameter required for retrieving extinction profiles and optical depths from elastic backscatter lidar measurements, and the quality of any extinction retrieval depends critically on the accuracy of the assumed or measured lidar ratio. In this study, we analyze the first two and a half years of the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) data acquired during nighttime. Distributions of the effective lidar ratio (ELR), which is the product of the lidar ratio and an instrument-dependent multiple scattering factor, are derived for opaque dust layers observed by CALIOP over the North Africa. The median and mean ELR values are, respectively, 36.4 and 38.5 sr at 532 nm and 47.7 and 50.3 sr at 1064 nm. For these opaque dust layers, the derived ELR decreases as the volume depolarization ratio (VDR) increases, reflecting the impact of multiple scattering within the dense layers. The particulate depolarization ratio is typically ∼0.3 at 532 nm for African dust observed by CALIOP. This ratio can increase to ∼0.4 in the presence of significant multiple scattering. Correspondingly, the calculated ELR will decrease to ∼20 sr at 532 nm and to ∼30 sr at 1064 nm. The median and mean effective lidar ratio values approach, respectively, to 38 and 40 sr at 532 nm and 52 and 55 sr at 1064 nm for smaller VDR values measured in less dense layers where the multiple scattering is relatively insignificant. These values are very close to those derived in previous case studies for moderately dense dust. Case studies are also performed to examine the impacts of multiple scattering. The results obtained are generally consistent with Monte-Carlo simulations.     

Algorithms to retrieve optical properties of three component aerosols from two-wavelength backscatter and one-wavelength polarization lidar measurements considering nonsphericity of dust

We developed backward and forward types of algorithms for estimating the vertical profiles of extinction coefficients at 532 nm for three component aerosols (water-soluble, dust, and sea salt) using three-channel Mie-scattering lidar data of the backscatter (β) at 532 and 1064 nm and the depolarization ratio (δ) at 532 nm. While the water-soluble and sea-salt particles were reasonably assumed to be spherical, the dust particles were treated as randomly oriented spheroids to account for their nonsphericity. The introduction of spheroid models enabled us to more effectively use the three-channel data (i.e., 2β+1δ data) and to reduce the uncertainties caused by the assumption of spherical dust particles in our previously developed algorithms. We also performed an extensive sensitivity study to estimate retrieval errors, which showed that the errors in the extinction coefficient for each aerosol component were smaller than 30% (60%) for the backward (forward) algorithm when the measurement errors were ±5%. We demonstrated the ability of the algorithms to partition aerosol layers consisting of three aerosol components by applying them to shipborne lidar data. Comparisons with sky radiometer measurements revealed that the retrieved optical thickness and angstrom exponent of aerosols using the algorithms developed in this paper agreed well with the sky radiometer measurements (within 6%).     

Comparison of CALIPSO and ground-based lidar profiles over Chung-Li, Taiwan

We report the first inter-comparison of vertical profiles of aerosols and clouds derived from space (CALIPSO) and ground based lidar over Chung-Li, Taiwan. Results show that inter-comparison is closer in case of aerosols than clouds. The strength/shortcoming of the comparison has been also discussed. An iterative calculation to retrieve extinction-to-backscatter ratio (lidar ratio) by using sun-photometer and CALIPSO data is also documented. By using the mentioned method, a mean lidar ratio of 23.5±8.2 sr was found. The derived lidar ratios are lower than former studies. The possible reasons for the difference have been discussed in this paper. The discussed methodology will be helpful to reduce the uncertainty of optical parameters derived from lidar data especially near the surface where the atmosphere is inhomogeneous.     

Two-wavelength lidar inversion algorithm for determination of aerosol extinction-to-backscatter ratio and its application to CALIPSO lidar measurements

A modified two-wavelength lidar inversion algorithm is proposed to aid in the retrieval of aerosol extinction-to-backscatter ratios (lidar ratio) as well as backscatter coefficients and extinction color ratios from simultaneous two-wavelength elastic backscatter lidar measurements. To demonstrate the feasibility of the algorithm, both the Raman method and the two-wavelength method have been applied to the ground-based measurements at 355 and 532 nm; moreover, it has been applied to the data acquired by the Cloud Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) lidar, and to the simultaneous ground-based lidar measurements carried out at Napoli (southern Italy, 40.838 °N, 14.183 °E, 118 m above sea level). Three cases of Saharan dust transport towards Europe have been considered. From the comparison, it can be found that the values of lidar ratio and backscatter coefficient retrieved by the modified two-wavelength algorithm are in good agreement with those obtained by the Raman method. Moreover the retrieved mean values of the lidar ratios and color ratios are in agreement with those reported by other authors.     

Measurements of aerosol distribution by an elastic-backscatter lidar in summer 2008 in Beijing

Elastic lidar observations of profiles of the aerosol extinction, backscattering coefficients, and the lidar ratio have been performed in Beijing. The elastic lidar transmitts wavelengths of 532 and 355 nm. The measurement altitude can reach up to 6 km. The similarity of the extinction and baekscattering profiles suggests a close relation between the mean transmission and reflection properties. The lidar ratio on July 22, 2008 varied from 10 to 30 sr with the mean value of 20 sr. The profiles of the aerosol properties indicate the cirrus at 6 km altitude and a well-mixed boundary layer from July 22 to 24, 2008. The detected boundary layer also agrees well with the high and stable ozone concentration obtained from the differential optical absorption spectroscopy （DOAS） system.     

Retrieving the aerosol lidar ratio profile by combining ground- and space-based elastic lidars

The aerosol lidar ratio is a key parameter for the retrieval of aerosol optical properties from elastic lidar, which changes largely for aerosols with different chemical and physical properties. We proposed a method for retrieving the aerosol lidar ratio profile by combining simultaneous ground- and space-based elastic lidars. The method was tested by a simulated case and a real case at 532 nm wavelength. The results demonstrated that our method is robust and can obtain accurate lidar ratio and extinction coefficient profiles. Our method can be useful for determining the local and global lidar ratio and validating space-based lidar datasets.     

双视场米散射激光雷达探测对流层气溶胶

介绍了自行研制的用来探测对流层大气气溶胶消光特性的双视场米散射激光雷达.该激光雷达采用两个具有独立接收视场的探测通道分别接收高低层532 nm的大气回波信号,可以兼顾低层大视场角低探测盲区和高层小视场角高探测高度的要求.叙述了该雷达系统的总体结构和技术参量以及数据处理方法,给出了合肥地区(东经117.16°,北纬31.90°)大气气溶胶消光系数廓线和对流层光学厚度的探测结果.测量结果表明,该雷达具备昼夜连续观测对流层大气气溶胶的能力,可以很好地反映气溶胶粒子的时间和空间分布特征.      

Derivation of component aerosol direct radiative forcing at the top of atmosphere for clear-sky oceans

A two-step approach is proposed to derive component aerosol direct radiative forcing (ADRF) at the top of atmosphere (TOA) over global oceans from 60°S to 60°N for clear-sky condition by combining Terra CERES/MODIS-SSF shortwave (SW) flux and aerosol optical thickness (AOT) observations with the fractions of component AOTs from the GSFC/GOCART model. The derived global annual mean component ADRF is +0.08±0.17 W/m² for black carbon, −0.52±0.24 W/m² for organic carbon, −1.10±0.42 W/m² for sulfate, −0.99±0.37 W/m² for dust, −2.44±0.84 W/m² for sea salt, and −4.98±1.67 W/m² for total aerosols. The total ADRF has also been partitioned into anthropogenic and natural components with a value of −1.25±0.43 and −3.73±1.27 W/m², respectively. The major sources of error in the estimates have also been discussed. The analysis adds an alternative technique to narrow the large difference between current model-based and observation-based global estimates of component ADRF by combining the satellite measurement with the model simulation.     

车载式1 064 nm和532 nm双波长米散射激光雷达

 新近研制的车载式双波长米散射激光雷达可用于1 064 nm 和532 nm两个波长对白天与夜晚对流层气溶胶消光系数垂直分布进行的探测。该激光雷达由激光发射单元、接收光学和后继光学单元、信号探测和采集单元以及系统运行控制单元组成,后继光路之间采用光纤导光、高低层分层探测等关键技术。该激光雷达使用1 064 nm和532 nm的两个波长,其单发脉冲能量分别为400和300 mJ,重复频率都为20 Hz,光束发散角小于0.5 mrad ;望远镜接收视场为1～3 mrad,滤光片的中心波长为1 064 nm和532 nm,带宽1 nm。分别使用R3236及H7680的PMT和VT120及Phillips777的放大器对两个波长的信号进行探测;对532 nm波长用3 A/D采集卡、1 064 nm波长用了光子计数卡。给出了双波长测量对流层气溶胶消光系数垂直分布的结果,该激光雷达可以探测10 ^-5～1之间的消光系数,探测高度可达10 km以上。     

Impact of extinction ratio of single arm sin LiNbO3 Mach-Zehnder modulator on the performance of 10 and 20 Gb/s NRZ optical communication system

We investigate the impact of extinction ratio of single arm sin² LiNbO₃ Mach-Zehnder (MZ) amplitude modulator on the performance of 10 and 20 Gb/s single-channel optical communication system. For different fiber lengths, the system performance has been analyzed with the increase in the extinction ratio. The effect of variation in dispersion parameter has also been illustrated. The impact of extinction ratio (ζ), dispersion parameter and length of the fiber has been further optimized with minimum bit error rate (BER) at optimal decision threshold (10⁻⁹) for 10 and 20 Gb/s bit rate. It is found that the system gives optimum performance at extinction ratio (ζ) value 20 dB. The increase in the transmission distance from 468 km for 10 Gb/s to 532 km for 20 Gb/s has been reported, and 8 dB improvement in the Q value has been observed as the value of ζ is increased from 10 to 20 dB. At 20 Gb/s, the system gives optimum performance for dispersion parameter value only up to 4 ps/nm km; however, at 10 Gb/s the system can operate for dispersion values up to 14.3 ps/nm km. Further we investigate the self-phase modulation (SPM) effect for the increase in the input power. It is observed that the SPM effect is negligible below 3 dB m input power and it increases at higher power levels.     

CALIPSO validation using ground-based lidar in Hefei (31.9°N, 117.2°E), China

Validation measurement in collaboration with existing lidar sites is a very important part of CALIPSO validation program, lidar site in Hefei is invited to collaborate in the CALIPSO validation program. In this paper, ground-based lidar measurements in Hefei performed in coincidence with CALIPSO overpass are presented, attenuated backscatter profiles at 532 nm and 1064 nm, as well as volume depolarization ratio profile at 532 nm measured by CALIPSO are compared with the ones measured by ground-based lidar. The comparisons indicate that CALIPSO measurements are consistent with the ground-based lidar measurements. However, due to the fact that horizontal distributions of aerosols in the lower troposphere and clouds are in most cases inhomogeneous, there are some differences between two lidar measurements in the boundary layer and clouds. The aerosol layer below the semi-transparent thick cloud can be detected by the 532 nm channel of CALIPSO in daytime.     

Direct measurement of the Rayleigh scattering cross section in various gases

Using the laser-based technique of cavity ring-down spectroscopy extinction measurements have been performed in various gases straightforwardly resulting in cross sections for Rayleigh scattering. For Ar and N₂ measurements are performed in the range 470-490 nm, while for CO₂ cross sections are determined in the wider range 470-570 nm. In addition to these gases also for N₂O, CH₄, CO, and SF₆ the scattering cross section is determined at 532 nm, a wavelength of importance for lidar applications and combustion laser diagnostics. In O₂ the cross section at 532 nm is found to depend on pressure due to collision-induced light absorption. The obtained cross sections validate the cross sections for Rayleigh scattering as derived from refractive indices and depolarization ratios through Rayleigh's theory at the few %-level, although somewhat larger discrepancies are found for CO, N₂O and CH₄.     

偏振-米散射激光雷达的研制

研制的偏振-米散射激光雷达（PML）,可用于探测卷云和沙尘气溶胶的后向散射光退偏振比以及研究流层大气气溶胶的消光特性。采用窄带滤光片和光阑,将接收到的激光大气回波信号谱线（米散射和瑞利散射光谱）从天空太阳背景噪声中分离出来,以提高系统的白天探测能力。介绍了偏振-米散射激光雷达的结构、技术参数、测量方法和数据处理方法。对偏振-米散射激光雷达的性能参数进行了测定,并对测定结果进行了分析与讨论,给出了偏振-米散射激光雷达对合肥市地区(117.16°E, 31.90°N)上空大气气溶胶的消光特性和卷云的结构、退偏振比垂直廓线以及光学厚度的典型探测结果,对这些结果进行了分析和讨论。结果表明：研制的偏振-米散射激光雷达性能可靠,能对大气气溶胶和卷云的物理和光学特性进行有效的探测。     

A review of optical measurements at the aerosol and cloud chamber AIDA

This paper provides a survey of recent studies on the optical properties of aerosol and cloud particles that have been conducted at the AIDA facility of Forschungszentrum Karlsruhe (Aerosol Interactions and Dynamics in the Atmosphere). Reflecting the broad accessible temperature range of the AIDA chamber which extends from ambient temperature down to 183 K, the investigations feature a broad diversity of research topics, such as the wavelength-dependence of the specific absorption cross sections of soot and mineral dust aerosols at room temperature, depolarization and infrared extinction measurements of ice crystal clouds generated at temperatures below 235 K, and the optical properties of polar stratospheric cloud constituents whose formation was studied in chamber experiments at temperatures well below 200 K. After reviewing the AIDA research activity of the past decade and introducing the optical instrumentation of the AIDA facility, this paper presents illustrative examples of ongoing and already published work on optical measurements of soot aerosols, mineral dust particles, and ice crystal clouds.     

Structural characterization and optical properties of UO2 thin films by magnetron sputtering

Uranium dioxide films were deposited on Si (1 1 1) substrates by dc magnetron sputtering method at different sputtering parameters. The structure, morphology and chemical state of the films were studied by field emission scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and atomic force microscopy. Influences of film thickness on the microstructure and optical properties were investigated. Experimental results show that the film crystallites are preferentially oriented with the (1 1 1) planes. The average grain size increases with increasing film thickness. AFM images show that the root mean square roughness of the films is between 1.2 nm and 2.1 nm. Optical constants (refractive index, extinction coefficient) of the films in the wavelength range of 350-1000 nm are obtained by ellipsometric spectroscopy. The result shows that the refractive index decreases with the increasing film thickness, while extinction coefficient increases with the film thickness.     

Raman激光雷达探测气溶胶光学特性

介绍了武汉大学自行研制的Raman多通道激光雷达系统,给出了整个系统的设计原理及主要技术参量.详细描述了利用Raman激光雷达原理反演大气气溶胶消光系数、后向散射系数和激光雷达比等光学特性的方法,并对求解消光系数过程中的关键部分做了讨论分析.同时对武汉上空对流层低空大气气溶胶、云以及边界层等光学特性进行了实时探测反演.实验结果表明:该Raman多通道激光雷达系统在夜晚对低空气溶胶的垂直分布特性具有较好的探测能力,工作性能可靠.     

机载激光雷达沙尘探测能量优化配置的统计研究

依据美国ANSI标准,结合1999~2004年间地基激光雷达探测的合肥地区沙尘暴消光廓线,统计分析了机载大气探测激光雷达探测沙尘时偏振532 nm垂直通道和1064 nm通道所需的最小激光能量,模拟计算了机载大气探测激光雷达探测沙尘暴时在0~12km高度范围内不同激光束发散角和激光脉冲能量比例的532 nm和1064 nm激光脉冲眼睛安全最大阈值能量。以合肥地区沙尘暴消光特性的统计结果、地面人眼安全标准和两个"瓶颈"通道的探测能力为依据提出两种激光脉冲能量配置方案。     

Wavelength dependent resonant nonlinearities in a standard saturable absorber IR26 on picosecond time scale

Nonlinear optical properties of a standard dye IR26 have been studied by using the Z-scan technique to decipher the difference in the mechanism of nonlinear absorption on picosecond time scale at two wavelengths i.e. at 1064 nm and 532 nm. A prominent contribution of nonlinear absorption is observed in the Z-scan profiles at 1064 nm. The dye exhibits the mechanism of self-defocusing at 1064 nm in contrast to that of self-focusing at 532 nm. While the two photon absorption has been found to be the dominant mechanism of reverse saturable absorption at 1064 nm, the mechanism of excited state absorption is operating at 532 nm. Additionally, the optical phase conjugate geometry of degenerate four wave mixing (DFWM) technique has been used to measure the third order nonlinear susceptibility values at 532 nm to compare with those obtained from the Z-scan profiles.     

大气消光特性与机载激光雷达眼睛安全关系的研究

依据美国ANSI标准，定量分析了气溶胶、卷云和沙尘的垂直消光特性对机载激光雷达激光脉冲眼睛安全最大阈值能量的影响；以地基激光雷达探测得到的合肥上空沙尘暴为例，定量给出了大气中各成分消光特性对飞机眼睛安全最低高度的影响；模拟计算了各种大气成分的消光特性对激光脉冲眼睛安全系数的影响随高度的变化规律。计算结果表明：沙尘层对激光脉冲眼睛安全最大阈值能量的影响幅度可达几十毫焦。该结果为我国第一台机载大气探测激光雷达激光脉冲能量设置提供了基本依据。     

Nonlinear optical properties of an azo-based dye irradiated by picosecond and nanosecond laser pulses

The nonlinear optical properties of an azo-based dye were investigated using Z-scan technique employing 38 ps pulses at 532 and 1064 nm, and 6 ns laser pulses at 532 nm. Large nonlinear absorption and nonlinear refraction were observed at both ps and ns 532 nm in the azoic dye. When excited at ps 1064 nm, this dye displayed a large two-photon absorption cross-section (σ₂=1810 GM). Meanwhile, the optical nonlinearity mechanism was discussed in terms of molecular structure, excitation wavelength, and pulse width.