1、第六章第六章 气溶胶辐射特性气溶胶辐射特性p 单个粒子辐射特性p 粒子群辐射特性p 大气能见度Source:http:/www.epa.gov/air/visibilityThe Smoky Mountains on a clear day.and on a hazy one.Average monthly from MODIS on NASAs TerraAverage monthly from MODIS on NASAs Terra1 单个粒子辐射特性单个粒子辐射特性（Mie散射）散射）p 尺度参数p 截面p 效率p 相函数p 不对称因子p 折射指数单个粒子单个粒子Mie散射散射假设独立
2、散射粒子是球体介质均匀入射光为无偏振自然光米散射理论是球状粒子散射的通用理论球形粒子散射球形粒子散射Fig.9.11Ray A is reflectedRay B is refracted twiceRay C is diffractedRay D is refracted,reflected twice,then refractedRay E is refracted,reflected once,and refracted散射的分类散射的分类尺度参数尺度参数定义无量纲尺度参数当x1时：Rayleigh散射如空气分子对短波辐射的散射如空气分子对短波辐射的散射当0.1x50：几何光学：折射如大
3、雨滴对可见光的折射、反射。如大雨滴对可见光的折射、反射。2 ax散射的分类散射的分类尺度参数尺度参数散射的分类散射的分类尺度参数尺度参数Rrayleigh散射散射的分类散射的分类尺度参数尺度参数Fig.9.9Geometry of a Primary Rainbow截面截面对于单个粒子（分子、气溶胶粒子）对于单个粒子（分子、气溶胶粒子）截面（量纲：L2）F：辐射通量密度，单位：辐射通量密度，单位：Wm2f：辐射通量（功率）：辐射通量（功率），单位：，单位：W吸收截面+散射截面消光截面fF效率（因子）效率（因子）粒子的散射截面与粒子几何截面之比无量纲无量纲2scsc rQ单个粒子单个粒子Mie散
4、射参数散射参数消光效率、散射效率和吸收效率：球形粒子的散射特性的决定因素：尺度参数x粒子复折射指数m 221221221 Re221eennnssnnnnnaesQnabaxQna ab baxQQQ散射效率散射效率吸收效率、消光效率吸收效率、消光效率散射相函数散射相函数散射相函数散射相函数散射相函数散射相函数散射相函数散射相函数20Forward and BackscatteringFig.9.12Cloud dropletsScatter primarily in the forward directionGas moleculesScatter evenly in the forward
5、 and backward directions.不对称因子（无量纲）不对称因子（无量纲）表示前向散射的相对强度一般情况下不会小于一般情况下不会小于1，即介，即介于于0至至1之间之间Rayleigh散射的情况，前后向散射的情况，前后向对称，对称，g=0。11212421ReRe11nnnnnnnsn nnga ab ba bx Qnn n折射指数折射指数折射指数折射指数m=n-ik is defined by its chemical composition;n is responsible for scattering.k is responsible for absorption.If k
6、 is equal to 0 at a given wavelength thus a particle does not absorb radiation at this wavelength.折射指数折射指数Some refractive indices of atmospheric aerosol substances at =0.5 mm.SubstancenkWater1.3330Hematite2.61.0Elemental carbon1.960.66Organic carbon1.550NaCl(s)1.5440H2SO4(aq)1.530NH4HSO4(s)1.530(NH4
7、)2SO4(s)1.520SiO21.550NOTE:hematite(赤铁矿赤铁矿)is a mineral that is a main light absorbing components of mineral dust.折射指数折射指数折射指数折射指数Main absorbing species in the SW are black carbon(Soot)and Hematite(dust),but in the LW various species have high imaginary parts of the refractive index.But overall abso
8、rption is also controlled by particle size折射指数折射指数粒子混合粒子混合External MixingAll type of aerosol species are in complete isolation with another species optically,chemically and physically.Internal MixingDifferent type of aerosols species are arrayed in form of layers and optical properties of such mixtu
9、re is volume averaged.Core-Shell StructureOne aerosol species is coated over another species to from core-shell structure.折射指数折射指数粒子混合粒子混合2 粒子群辐射特性粒子群辐射特性p 散射系数p 单次散射反照率p 不对称因子p 光学厚度p 混浊度粒子群的积分光学特性粒子群的积分光学特性对截面积分，得到体积消光系数、吸收系数、散射系数（单位：mm2cm-3=10-6m-1）scsc02sc0(,)()d(,)()drm n rrr Qrm n rrExtinction
10、CoefficientFig 9.15Attenuation of incident radiance,Io,due to absorption as it travels through a column.Extinction coefficient(cm-1,m-1,or km-1)A measure of the loss of radiation per unit distance粒子群的积分光学特性粒子群的积分光学特性粒子群的积分光学特性粒子群的积分光学特性散射比单次散射反照率single scattering albedo散射占消光的比例se粒子群的积分光学特性粒子群的积分光学特性
11、不对称因子绝对值越大，表示大气中含有大粒子较多 21212s2sddddddrrrrn rr Q grrgn rr Qrr粒子群光学厚度混浊度粒子群光学厚度混浊度对于Junge分布可导出：为Junge分布中的谱形常数当=3.3时，k被称为ngstrm浑浊度系数（ngstrm turbidity coefficient）2211saddzzzzzz(1)d,()dN r zn r zC z rr2bkk 粒子群光学厚度混浊度粒子群光学厚度混浊度例：若气溶胶光学厚度在0.5和0.8微米分别为0.17和0.1。试求形状因子为何值？得：=3.12899 37220.50.50.170.80.80.1m
12、kmkmm 220.50.170.80.10.6251.7 大气能见度大气能见度q 气象光学视程q 决定因素气象光学视程气象光学视程气象光学视程（Meteorological optical range）白炽灯发出的色温为2700K的平行光束的光通量在大气中削弱至初始值的5%所通过的路径长度。气象光学视程气象光学视程对比感阈对比感阈 contrast threshold目标物与背景亮度的相对比值目标物为黑体时，I0，c1，目标物看得最清楚在c减小到0之前，开始不能把目标物从背景中分辨出来的对比值称为对比感阈 bbIIcI对比感阈对比感阈 contrast threshold42人眼对单色光的相
13、对感光效率（实线：昼；虚线：夜）对比感阈对比感阈 contrast threshold对比感阈既反映人眼的生理特性，也和外界条件有关，包括：目标物的视角和视野亮度目标物的视角和视野亮度随着目标物视角和视野亮度的减小，对比感阈增大随着目标物视角和视野亮度的减小，对比感阈增大目标物持续时间目标物持续时间目标物时隐时现则对比感阈增大。识别目标物所需时间约目标物时隐时现则对比感阈增大。识别目标物所需时间约1/3秒秒，目标物显示时间小于该值则难以识别，除非对比度增大，目标物显示时间小于该值则难以识别，除非对比度增大目标物在视场中的位置目标物在视场中的位置目标在视场中央，对比感阈最小，反之加大。搜索位置不
14、确定目目标在视场中央，对比感阈最小，反之加大。搜索位置不确定目标物时，较大的对比度才宜发现标物时，较大的对比度才宜发现个人精神因素个人精神因素包括个人的健康、心理状态等包括个人的健康、心理状态等43气象光学视程气象光学视程对于正常视力，气象上取0.02Koschmieder能见度公式（能见度公式（1924）航空气象使用较高的0.05ext,ext,112.996ln0.05aRmm1.3060.766aaRRRR 或 mext,ext,ext,11113.912lnln0.02Rd dxcccex 能见度的决定因素能见度的决定因素 大气消光因素的叠加45ext,1,2Rayleigh scat
15、teringgas absorption,1,1,2,2particle+saasasa 能见度的决定因素能见度的决定因素 瑞利（Rayleigh）散射造成的消光46能见度的决定因素能见度的决定因素 气体吸收Fig.9.17能见度的决定因素能见度的决定因素 气溶胶粒子消光干气溶胶粒子、湿气溶胶粒子（霾）、云雾滴、冰晶、雨、雪、霰、雹等Reduction of visibility by aerosols.The visibility of an object is Reduction of visibility by aerosols.The visibility of an object i
16、s determined by its contrast with the background(2 vs.3).This determined by its contrast with the background(2 vs.3).This contrast is reduced by aerosol scattering of solar radiation into contrast is reduced by aerosol scattering of solar radiation into the line of sight(1)and by scattering of radia
17、tion from the the line of sight(1)and by scattering of radiation from the object out of the line of sight(4).object out of the line of sight(4).能见度的决定因素能见度的决定因素(Larson et al.,1984)Table 9.8Meteorological Range(km)Gas scatteringGas absorptionParticle scatteringParticle absorptionAllPolluted day366130
18、9.5949.77.42Less-polluted day35232615142167.1能见度的决定因素能见度的决定因素Individual contributions of scattering and absorptionby gases and particles in Pasadena,California,during the summer of 1984(Larson and Cass,1989)雾过程能见度雾过程能见度能见度能见度Trend maps for the extinction coefficient(Che et al.,2007)习题习题推导：其中k与消光效率的关
19、系为：2bkk 212e10d2()dzxxQxxkC zzx习题习题设云雾滴数浓度尺度谱分布可用Khrgian-Mazin分布表示：试用谱分布参数A、B表示云雾中的能见度。设云雾滴为纯水，忽略气体吸收和散射。2()BrdNn rAr edr习题习题(a)Derive Eq.(7.2.7)from Eqs.(7.2.5)and(7.2.6)and show thatwhere the extinction efficiency Qe is expressed in terms of the size parameter x.(b)Direct solar radiation measurements are made with a multiple wavelength radiometer.The aerosol optical depths derived from the observations are 0.17 and 0.1 at 0.63 and 0.86 m,respectively.ssuming a Junge size distribution and a constant k,what would be the shaping factor?
