Observational Study on Aerosol Scattering Phase Function at Raoyang of Hebei, China
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摘要:
利用2014年夏季在河北饶阳开展的“华北区域光化学立体试验”观测得到气溶胶辐射特性,根据三波长角散射浊度仪Aurora 4000测量获取的前向角散射系数(10°~90°)和后向散射系数,提出了一种基于浊度仪直接测量的气溶胶角散射系数和改进的HG相函数近似获取气溶胶散射相函数、不对称因子的拟合估算方法。分析了2014年6月16日—8月18日夏季观测试验期间河北饶阳地区大气气溶胶细粒子(PM2.5)在635 nm,525 nm,450 nm 3个波段的不对称因子、相函数等的变化特征。结果表明:用改进的HG相函数近似能够较好地拟合河北饶阳实际观测的PM2.5的气溶胶散射相函数。拟合得到河北饶阳地区观测期间干气溶胶细粒子在3个波段的平均不对称因子(g)分别为0.53±0.04(635 nm),0.57±0.05(525 nm)和0.57±0.07(450 nm)。
Abstract:Aerosol radiative properties are key factors in the process of aerosol radiative forcing calculation. The scattering phase function and asymmetry factor facilitate the illustration of the direction of radiative transfer and the estimation of parameters in remote sensing.In the early summer of 2014, aerosol radiative parameters are obtained at Raoyang Meteorology Administration (an agricultural district) with a latest three-wavelength polar nephelometers (Aurora 4000) developed by EcoTech, Australia. Compared with the previous model, the instrument is unique in that it can measure the volume scattering coefficient from Θ through to 170° degrees and Θ is 10°, 15°, …, 90°. Based on the volume scattering coefficient, the scattering phase function can be computed with the traditional formula between them. What's more, one improved approach is proposed to calculate the asymmetry factor with the combination of scattering phase function and backscattering ratio. The backscattering ratio used can be calculated from the backscattering coefficient and total scattering coefficient as defined. Aerosol radiative properties of PM2.5 including calculated scattering phase function and the fitted asymmetry factor are presented during the observation between 16 June and 18 August in 2014. The result suggests that the improved HG aerosol particle phase function can fit the aerosol scattering phase function observed at Raoyang Meteorology Administration of Hebei well. Observed results of forward scattering phase function (15°-20°) and the backscattering ratio are all in good agreement with the numerical results. The average asymmetry factors at the wavelength of 635 nm, 525 nm and 450 nm are 0.53, 0.57 and 0.57, respectively, with no significant difference possibly due to the size distribution of aerosol. The value of it at 525 nm are accordant with analysis of previous experiments, which indicate the asymmetry factor of dry aerosol are in the range of 0.55 and 0.63 (550 nm). Two examples of different pollution status (dirty period and clean one) are chosen, depending on the scattering coefficient, and the radiative properties are different. During two dirty periods, values of asymmetry factor are between 0.55 and 0.63, which is larger than those in clean periods. For asymmetry factor, mean values at 635 nm, 525 nm and 450 nm wavelength are in the range of 0.51-0.53, 0.54, 0.54, respectively. Such results provide more details and supports for further study of radiative effects of aerosol.
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Key words:
- aerosol;
- radiative property;
- asymmetry factor;
- scattering phase function
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表 1 不同λ下15°~20°的角散射系数和后向散射比回归系数
Table 1 Regression coefficient for angularscattering at 15°-20° and backscattering ratio with different λ
λ A1 A2 0 0.978 1.083 0.1 0.981 1.079 0.2 0.985 1.075 0.3 0.990 1.069 0.4 0.997 1.063 0.5 1.007 1.056 0.6 1.022 1.05 0.7 1.052 1.019 0.8 1.071 1.001 0.9 1.072 1 1 1.072 1 注:回归公式PRHG=A1×P15°~20°, bRHG=A2×b,其中bRHG和b分别为后向散射比的拟合值和观测值,表中的参数A1, A2分别为线性回归系数。 表 2 2014年6月16日—8月18日气溶胶不对称因子(g)的统计特征
Table 2 Statistical property of asymmetry factor from 16 Jun to 18 Aug in 2014
统计量 g 635 nm 525 nm 450 nm 平均值 0.53 0.57 0.57 方差 0.04 0.05 0.07 最大值 0.61 0.66 0.67 最小值 0.35 0.28 0.24 中值 0.53 0.58 0.59 表 3 中国饶阳地区不对称因子与其他地区的对比
Table 3 The asymmetry factor(g) at Raoyang of China and other regions
地区 不对称因子 波长/nm 计算方法 文献出处 饶阳(中国,农村) 0.57 525 本研究 俄克拉荷马(美国,平原) 0.55~0.63 550 MIE散射、直接测量 文献[18] 库亚巴(巴西,城市) 0.54 550 MIE散射 文献[39] 合肥(中国,城市) 0.75 550 经验公式 文献[40] 兰州(中国,城市) 0.7 520 经验公式 文献[41] 维也纳(奥地利,城市) 0.628 532 直接测量 文献[42] 京都(日本,城市) 0.59 532 直接测量 文献[15] 注:经验公式是利用地面测量的散射、吸收系数,根据不对称因子(g)与单次散射反照率的经验公式计算;MIE散射是利用粒子谱计算得到的相函数与不对称因子(g);直接测量是利用多角度浊度仪测量角散射系数获得相函数和不对称因子(g)。 表 4 2014年7—8月饶阳污染过程、清洁时段不对称因子统计特征
Table 4 The asymmetry factor(g) in dirty and clean periods from Jul to Aug in 2014
过程 时段 g 635 nm 525 nm 450 nm 污染过程1 07-06—07 0.56 0.62 0.63 污染过程2 07-31—08-02 0.58 0.62 0.63 清洁时段1 07-09—10 0.53 0.54 0.54 清洁时段2 08-04—06 0.51 0.54 0.54 -
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