近沙尘源区气溶胶光学特性的季节变化及其统计学描述

Seasonal Variation and Statistic Characteristics of Aerosols Optical Properties near Dust Region

利用天空辐射计测定了敦煌地区1999年1月至2001年3月期间太阳直接辐射和散射辐射,采用“SKYRAD”反演模式同时反演了敦煌地区气溶胶光学厚度、体积尺度分布和折射指数实部,分析了其季节变化及统计学特性。结果表明,光学厚度存在明显季节变化,大致从12月开始存在明显的上升,3月或4月达到最大值,5—8月光学厚度逐渐减小,9—10月又有所上升,11月达到最小值,而波长指数的变化与光学厚度的变化基本相反;敦煌地区气溶胶光学厚度和波长指数的概率分布与其季节变化具有很好的相关性,并近似满足对数正态分布和正态分布;4个季节的气溶胶波长指数与光学厚度表现出一种相似的关系,并可以简单地利用一个指数函数加以描述;气溶胶尺度分布表现出双峰型结构,一种是位于半径0.25μm附近的积聚态,另一种是半径7.7μm左右的粗模态,且春季积聚态与粗模态之间存在着一个假模态;折射指数实部春季明显升高,对波长的敏感性较低,且4个季节的概率分布最大值均处在1.54～1.56范围内;两种情况下对实部值的概率分布进行拟合,发现其概率分布同样可以利用高斯模型加以描述,但是两种情况下春季与其他季节明显不同。

A ground-based sky radiometer was used to measure the direct and diffuse solar irradiances from Jan. 1999 to Mar. 2001 in Dunhuang area, one near dust region in China. The aerosol optical depth （AOD）, volume size distributions, the real part of refraction index were simultaneously retrieved using the ‘SKYRAD＇ model and their seasonal variation and statistic characteristics were analyzed. The results revealed that the aerosol optical depths varied seasonally, which increased from December, reaching its maximum value in May or April, and decreased from May to August, then also increased from September to October and with a November minimum. And the variation in Angstrom exponent （alpha） was opposite to that in AOD. The probability distributions of AOD and alpha were consistent with their seasonal variation and approximately follow a log-normal probability distribution and a normal probability distribution, respectively. The relationship between alpha and AOD showed a similar relationship in four seasons and could be characterized by one simple exponential function. The aerosol volume size distributions could be characterized by the sum of two log-normals distributions, and represented an accumulation mode with a radius of about 0.25 μm, and a coarse mode with a radius of about 7.7 μm, and there was a pseudo-mode with a radius of about 1.69μm between the accumulation mode and coarse mode in spring. The real parts of the refraction index rose dramatically in spring, and exhibited a low sensitivity to wavelengths, meanwhile, the biggest values of probability distribution in four seasons were range between 1.54- 1.56. The probability distributions of the real parts of the refraction index were fitted with Gauss Model under two conditions. The results showed that the probability distributions of the real parts of the refraction index could be better described by Gauss model, but the characteristic of which in spring was contrary with those in other seasons under two conditions.