近红外波段气溶胶的衰减特性研究

在已知大气气溶胶折射率和气溶胶谱分布的基础上, 对近红外波段的气溶胶衰减特性进行了研究。利用Mie散射理论计算并讨论了气溶胶的消光、散射、吸收效率因子随尺度参数的变化和消光系数随半径和波长的变化, 并且在MATLAB中对各种变化情况进行了仿真。结果表明, 三种气溶胶粒子的消光和散射能力依次为沙尘性粒子, 水溶性粒子, 烟煤。消光系数在粒子半径和入射波长相近时达到最大, 并且粒子半径对消光、散射、吸收系数的影响比入射波长更明显。这些结论可以为红外辐射在大气中的衰减计算和分析提供依据。     

基于MODTRAN模型使用被动傅里叶变换红外光谱技术对生物气溶胶的探测研究

利用MODTRAN模型在水平低仰角探测模式下,对生物气溶胶探测的问题进行了分析讨论.用傅里叶变换红外(FTIR)光谱技术对生物气溶胶进行探测研究.首先介绍了MODTRAN模型的大气模式和廓线,根据FTIR光谱技术对生物气溶胶的被动探测要求,利用辐射传输理论和最简单的三层模型,仿真计算得到大气背景和目标生物气溶胶之间的辐射亮度差?L,然后对?L进一步差值得到信号?2Lt,同时再结合光谱仪自身的噪声等效辐射亮度值,得到实际情况下的信号值?2Lt;最后根据探测条件和MODTRAN提供的大气模式,使用被动遥测红外光谱方法预测每种大气模式下生物气溶胶的探测限浓度.每种大气模式下探测限浓度的不同,是因为边界层温度、透过率和背景辐射亮度的不同所导致,同时还与生物气溶胶的吸收系数有关.研究表明,FTIR光谱被动遥测技术能够探测到生物气溶胶的存在,进一步说明探测生物气溶胶的可行性,也为生物气溶胶实际探测提供了一种方法.     

空间垂直方向气溶胶消光特性的研究

在已知大气气溶胶折射率和气溶胶谱分布的基础上,对空间垂直方向的气溶胶消光特性进行了研究。文章利用Mie散射理论计算讨论了气溶胶的消光、散射、吸收系数随各参数的变化,并在MATLAB中对变化情况进行了仿真。结果表明,粒子半径与入射波长大小相近时气溶胶的消光最强,并且随着能见度的增大,气溶胶衰减系数减小。这些结论可以为红外辐射的大气衰减计算和分析提供依据。     

地基消光测量确定大气气溶胶模型

 分别取大陆型、海洋型、城市型和Junge谱分布气溶胶模型，用6S辐射传输算法计算出对应于太阳光度计测量时的各波段大气气溶胶光学厚度。将模式计算值与测量值进行比较，确定测量地区的大气气溶胶模型。将该方法用于2004年在北京地区测量的太阳光度计数据，结果显示该地区当日实际大气在几种气溶胶模型中较为符合城市型气溶胶模型。     

大气模式与气溶胶模型对辐射传输计算的影响

大气模式与气溶胶模型选择是影响定量遥感应用的辐射传输计算的重要因素。人们一般凭感性认识去进行选择,有一定随意性,对其带来的影响程度关注甚少。以太阳辐射计测量为依据,对大气模式与气溶胶模型选择方法作了研究,并利用经辐射定标过的光谱辐射计地面测量对辐射传输计算精度进行了验证。在昆明进行的试验表明,在0.50~0.68μm范围内,选择中纬度冬季大气模式和大陆型气溶胶,经辐射传输计算后得到的光谱辐射亮度与光谱辐射计测量结果一致性很好,差别在3.3%以内;变换大气模式对辐射传输计算产生明显影响,差别达10%左右;选择不同的气溶胶模型对辐射传输计算影响也很大,差别达11%左右。基于辐射计测量的大气模式与气溶胶模型选择避免了主观选择的不可靠性,有益于减少辐射传输计算或卫星遥感大气订正的误差。     

太赫兹波大气传输衰减模型

基于修正的Van-Vleck Weisskopf线型、辐射传输色散理论和水汽连续体吸收模型,结合HITRAN数据库,建立了太赫兹波大气传输衰减模型——VVWH,形成了对宽频太赫兹波在真实大气中水平传输衰减的数值模拟能力。同时对太赫兹时域光谱技术(THz-TDS)获取的透射光谱实测数据进行了对比分析。计算结果与实验结果总体变化趋势一致,在吸收谱线处两者吻合良好,但在低频的大气窗口区,实验结果相比计算呈现出更强的传输衰减。考察了相对湿度对太赫兹波大气传输衰减特性的影响变化。     

海洋气溶胶多光谱偏振特性研究

为提高海洋气溶胶的反演精度,定量研究了气溶胶在可见近红外波段的多光谱偏振辐射特性。首先建立了合理的海洋气溶胶及海面模型,基于逐次散射法准确模拟了光在气溶胶中及大气—海洋交界面处的矢量辐射传输过程,接着从光谱角度定量分析了典型波段处的反射率和偏振反射率,在此基础上提出了海洋气溶胶偏振辐射的光谱分布模型,并用卫星数据对模型进行验证,最后研究了气溶胶光学厚度、观测角度、海水叶绿素a浓度和海面风速对气溶胶多光谱偏振辐射的影响。研究表明,在可见近红外波段不考虑大气吸收和耀斑效应时,气溶胶偏振辐射的光谱变化符合幂函数模型;在不同波段处,海洋气溶胶的偏振辐射受海水叶绿素a浓度和海面风速的影响不同;海洋气溶胶的多光谱偏振辐射信息有效体现了气溶胶的自身特性,且与反射率相比,偏振反射率随各因素的变化存在更明显的谱段差异性。因此,可以运用气溶胶的多光谱偏振信息反演海洋气溶胶参数,多光谱探测的加入对提高气溶胶反演精度具有重要意义。     

激光在不同类型气溶胶中传输特性研究

激光在大气中的传输衰减特性是激光工程应用中需要考虑的一个重要问题.本文针对常用的1.06 μm和10.6μm激光,基于Mie散射理论计算了气溶胶粒子的单次散射参量;对于激光在气溶胶中多次散射传输衰减,建立了蒙特卡罗模拟计算模型,利用Matlab语言编制了相应的计算程序,计算分析了两种波长的激光分别在沙尘性、水溶性、海洋性和煤烟性四种不同类型气溶胶中透过率与传播距离、能见度的关系,并将蒙特卡罗方法和单次散射的计算结果进行了比较.结果表明,当能见度较低、气溶胶粒子反照率较高时,单次散射计算存在很大的误差,用蒙特卡罗方法更能揭示多重散射现象;煤烟性气溶胶对1.06 μm激光的传输衰减影响最大,沙尘性气溶胶对10.6 μm激光的传输衰减影响最大.     

气溶胶对大气CO_2短波红外遥感探测影响的模拟分析

气溶胶引起的光学路径长度改变是影响高分辨率近红外光谱反演大气CO_2浓度的重要误差源.本文利用高精度大气辐射传输模式模拟中国碳卫星观测,结合CALIPSO(Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations)卫星的气溶胶廓线产品研究了不同特性的气溶胶对卫星观测光谱的影响.模拟结果显示:气溶胶散射引起的光学路径长度改变与气溶胶类型、模态以及垂直分布密切相关;城市型和海洋型气溶胶对观测光谱影响很大;多层分布的积聚模态大陆型和海洋型气溶胶在光学厚度小于0.3时,会引起5%以内的负辐射变化,随光学厚度不断增加会引起正的辐射变化;主要以粗粒子模态存在的气溶胶在不同的垂直分布情况下均会引起辐射的负变化,从而造成CO_2浓度的高估;另外,随气溶胶分布高度变高,负的辐射变化程度会逐渐减小.     

太赫兹电磁波大气吸收衰减逐线积分计算

大气对太赫兹辐射传输存在一定的非协作性,即存在吸收衰减。为了实现太赫兹辐射的有效应用必须细致地了解太赫兹辐射大气传输的窗口位置、宽度及大气透过率。选取了处理大气非均匀路径、吸收带重叠等大气辐射传输问题的最精确方法逐线积分法,发展计算程序,并基于HITRAN分子谱线数据,对水汽、氧气、臭氧、氮气、二氧化碳等单组分气体分子对太赫兹辐射传输的吸收衰减情况进行了计算与分析,并给出了在太赫兹电磁波大气传输衰减中占主要因素的水汽和氧气的衰减峰位置。     

Transmission of radiation through an aerosol medium

The properties of radiation through an aerosol medium have been achieved. This has been done by employing Mie scattering theory to calculate the radiation transfer scattering parameters in the form of extinction, absorption and scattering efficiencies. The equation of radiative transfer for the heat flux through a plane parallel atmosphere of aerosol has been solved. The aerosol size distributions are found in practical systems. Average efficiencies over size distribution for spherical particles of complex refractive index are calculated. Therefore, the radiative properties of stratospheric aerosols have been done. The obtained results found to be in a good agreement with the previous work.     

双光路DOAS系统获取大气颗粒物粒谱分布方法研究

当前大气复合污染日趋严重,造成大气氧化性增强,气体向颗粒物的转化加快。大气颗粒物粒径大小及谱分布决定其在大气中的行为,以差分吸收光谱法（DOAS）为基础,结合双光路设计技术,开展实时、在线、获取近地面大气气溶胶颗粒物的粒谱分布的光谱方法研究。首先构建低噪声性能稳定的宽带氙弧灯为光源的双光路差分吸收光谱系统,基于干净天气条件下大气的能见度数据对系统进行校准,通过两个不同光路获得的光谱信号强度之比获取近地面紫外-近红外波段的大气总宽波段消光系数。基于宽波段消光系数,在去除瑞利散射以及气体吸收对消光系数的影响后,解析出气溶胶颗粒物的消光系数。基于核函数准则,利用均匀球型粒子的电磁场Mie理论来反演气溶胶物理特性,获得气溶胶粒子在该测量谱段的体积谱分布,利用体积谱与数密度谱的关系,反演出气溶胶粒子的数密度谱分布。开展利用直方图方法来表现颗粒物的粒谱分布方法研究,首先将DOAS测量波段近似等分为若干谱段,利用谱段处平均值,获取气溶胶粒谱直方分布图。最后把该系统和方法应用于外场实验,获得了气溶胶颗粒物在300～650 nm范围内的消光系数,将测量波段等分为11个谱段,反演了颗粒物的在0.1～1.25 μm粒径范围的数密度谱分布。该研究为整治我国灰霾天气,研究大气气相/粒子非均相化学反应提供科学依据。同时将推动DOAS技术的进一步发展和应用。     

随机取向气溶胶凝聚粒子光学特性研究

利用Cluster-cluster aggregation (CCA)模型,模拟了由64个球形原始微粒凝聚而成的四种随机取向气溶胶凝聚粒子。结合离散偶极子近似方法,分别得到了不同入射角和不同尺寸参数情况下气溶胶凝聚粒子的吸收、散射、消光效率因子和不对称因子的数值结果,并比较分析了四种形状气溶胶凝聚粒子吸收、散射、消光效率因子和不对称因子的差异。结果显示：对于相同数目原始微粒的气溶胶凝聚粒子,其光学特性明显依赖于入射光的入射角度和气溶胶凝聚粒子的形状;对于不同尺寸参数的气溶胶凝聚粒子,当入射光波长给定时,吸收、散射和消光效率因子最初随尺寸参数增大而快速增大,随后又随尺寸参数增大缓慢变小,但在尺寸参数变化过程中存在一个极大值;不对称因子则随尺寸参数的增大一直呈增大趋势,且随尺寸参数的增大趋近于1。     

Telephotometer measurements and retrieved mass increase coefficients of the aerosol size distribution

Summary A non-linear iterative procedure is applied to the spectral sets of aerosol extinction coefficients, which were measured with the University of Vienna telephotometer during the Southern California Air Quality Study in Los Angeles (SCAQS), to obtain the mass size distribution of at least the accumulation mode aerosol at ambient relative humidity. The retrieved distributions show the expected shift of their mean diameters with changing relative humidity. The effect of advection of the Los Angeles plume to an upwind receptor region can also be seen very well from the inversions. Successive size distributions inverted from extinction measurements at different humidities are used to estimate the mass increase coefficient μ of the H?nel humidity model. The shift of the retrieved size distributions with humidity is used to trace the humidity behaviour of particles of a certain dry radius and convert the radius change to the μ. The results indicate that it is indeed possible to obtain information on μ if the aerosol stays in a steady state during the measurement period, if the dilution effect caused by the rise of the mixing height with increasing insolation is known with sufficient accuracy to be included quantitatively in the retrieval process for μ, and if the humidity and extinction measurements have the same temporal resolution.     

Extinction coefficients of surface atmospheric aerosol above LHAASO

We investigate the extinction coefficients of the surface atmospheric aerosol over the Large High Altitude Air Shower Observatory(LHAASO),located at the Haizi Mountain,Daocheng County,China.To this end,we utilize the Longtin model,Mie scattering theory,and experimental data obtained by the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation(CALIPSO).Our theoretical calculations show that the total extinction coefficients of the atmospheric aerosol at the wavelength of 200-500 nm are inversely proportional to the laser wavelength,and influenced by the wind speed.From July 2015 to October 2016,the extinction coefficient of the surface atmospheric aerosols at 532 nm wavelength reached 0.04 km~(-1) with no wind,while it increased to 0.1 km~(-1) with gusts.In this period,the extinction coefficients of the surface atmospheric aerosol at 532 nm wavelength,obtained by the CALIPSO,change from 0.01 to 0.07 km~(-1),which is less than the values obtained the theoretical calculation and larger than the average of Tibetan Plateau in 2006-2016.These calculations and experimental evidence provide important arguments to the model of atmospheric aerosol to be applied in the calibration of LHAASO.Our results suggest that the extinction coefficients over LHAASO require further study,including research on the size distribution,shape,concentration of aerosols particles,wind dependence,relative humidity dependence,etc.     

Evidence for particle size–shape correlations in the optical properties of silicate clay aerosol

Accurate modeling of the optical properties of atmospheric mineral dust is important for climate modeling calculations and remote sensing data retrievals. Atmospheric mineral dust in the accumulation mode size range is often rich in silicate clays including kaolinite and illite. This is important because dust optical properties depend on particle shape, and fundamental clay particles are known to consist of very thin flakes.In this combined laboratory and modeling study, we investigate the optical properties (IR extinction and visible light scattering) of two samples of silicate clay dust aerosol, kaolinite and illite. Particle size distributions are measured simultaneously with the optical properties. T-Matrix theory based simulations using a spheroidal particle approximation are compared with experimental data. We find that the full range of visible scattering and polarimetry data, and IR extinction profiles are not well fit by assuming a single size–shape distribution for the aerosol. In contrast, a simple bimodal distribution model that treats small particles (fundamental clay flakes) in the distribution as highly eccentric oblate spheroids with axial ratio parameters ≥5, but approximates larger particles by a more moderate shape distribution with axial ratio parameters <3, gives better agreement with the full range of experimental data. These conclusions are consistent with mineralogical data on the dimensions of fundamental clay particles.     

Adjacency Effect in the Atmospheric Correction of Satellite Remote Sensing Data: Evaluation of the Influence of Aerosol Extinction Profiles

In the atmospheric correction of satellite data in the visible and near-infrared bands, it is necessary to remove the adjacency effect due to the reflection from contiguous pixels. Evaluation of the influence of aerosol vertical distributions on the adjacency effect is done by calculating the single-scattering light intensity which, after the reflection at the ground surface, reaches the satellite sensor via a single scattering with a molecule or an aerosol particle. In the simulation, we assume aerosol vertical profiles similar to those used in the MODTRAN radiation transfer code, and those having a mixed layer with a uniform value of the aerosol extinction coefficient. We assume for the ground surface a simple model representing a border of land/sea surfaces. In spite of the single scattering approximation, it is confirmed that even if the optical thickness is the same, we have a larger adjacency effect when the extinction coefficient is large at higher altitudes. We also discuss the dependence of the adjacency effect on the aerosol optical thickness and that on the difference in the reflectances of the land and sea surfaces along the border.     

一种无需定标的地基激光雷达气溶胶消光系数精确反演方法

如何对低云下雾霾的激光雷达探测数据进行准确定标,一直是米散射激光雷达数据反演中一个有待解决的问题.对于低云和雾霾同时出现的天气,激光很难穿透云层,不能利用大气清洁层对激光雷达信号定标.而对于探测高度小于6 km的便携式米散射激光雷达,由于探测高度较低,也很难利用大气清洁层对激光雷达数据进行定标.本文根据Fernald前向积分方程的特点,提出了一种气溶胶消光系数迭代算法.通过对反演过程进行特定设置,每经过一次迭代,利用气溶胶消光系数迭代算法得到的气溶胶消光系数反演值与其真实值之间的差值就会相应减小.经过几次迭代后,气溶胶消光系数反演值与真实值之间的差值就会小到可以忽略不计.初步反演结果表明:利用气溶胶消光系数迭代算法,无需对激光雷达探测数据定标就能精确反演出气溶胶消光系数廓线.