珠江三角洲气溶胶光学厚度的观测研究

Observation of aerosol optical depth over the Pearl River Delta

利用2004年1月至2007年6月的多波段太阳光度计数据反演珠江三角洲地区的气溶胶光学厚度(AOD),对仪器定标方法和反演结果进行了分析,并以反演结果为基准,比对检验MODIS的AOD产品.分析表明:在使用Langley法进行仪器定标时,用迭代方法进行数据筛选处理,定标结果更为合理.统计结果显示:珠三角区域春季AOD值较大,秋夏季次之,冬季较小;4个站点AOD的季节变化特征具有一致性;珠三角区域AOD的年平均值大于0.7,气溶胶造成的到达地表的直接可见光辐射透过率衰减至少有50%～60%,造成严重的霾天气;从频率分布看,AOD值主要集中在0.4～0.6区间.4个站点的α值在1.2～1.6区间内所占的比例很高,频率分布类似,表明此区域内气溶胶粒子平均有效半径较小且较一致,同属于城市-工业型气溶胶类型;α与AOD没有明显的可辨析关系,通过样本统计和典型个例分析,表明区域内清洁与污染过程气溶胶粒子模态稳定,平均半径变化不大,粒子数浓度上的差别是产生消光效果不同的主要原因.以地面太阳光度计反演的AOD为基准,验证MODIS卫星遥感的AOD,结果表明,MODIS卫星遥感AOD在珠三角区域具有较好的量化精度,并初步建立珠三角区域卫星遥感AOD的订正公式.

Multi-wavelength sun-photometers were used to retrieve data on the aerosol optical depth （AOD） over the Pearl River Delta （PRD） from January 2004 to June 2007. A method for calibrating the instruments is described and the AOD results are analyzed. The MODIS AOD products from NASA are compared against retrieved AOD from the sun-photometers. The results reveal that the data filtered by the iterative method is more reasonable for the Langley plot than that from other approaches. Some statistical results are as follows： AOD is largest in spring, followed by autumn and summer and smallest in winter over the PRD region. The seasonal trends of AOD are consistent for the four observation sites. The annual mean regional AOD is greater than 0.7, indicating that the aerosol can attenuate direct visible radiation by 50% - 60% , resulting in severely hazy weather. And AOD mainly stays between 0.4 and 0.6 in terms of frequency distribution. The Angstrom wavelength exponent for the four observation sites occupies high proportions between 1.2 and 1.6 with similar frequency distributions. Therefore the regional average effective radii of aerosol particles are small and also consistent and can be classified as urban-industrial aerosols. There is no significant relationship between the AngstrOm wavelength exponent and AOD. Through sample statistics and typical case analyses, it can be concluded that the type, size distribution and effective radius of aerosol particles do not change dramatically whether the day is clear or polluted, hut the number density of particles is responsible for the different effects on light extinction. MODIS AOD is also compared with the AOD retrieved by the sun-photometer. MODIS AOD is shown to have good precision in quantitative measurement over the PRD. A preliminary correction formula for MODIS AOD over the PRD is suggested.