利用 AVHRR 数据反演陆地气溶胶光学厚度

摘要

The moderate-resolution imaging spectroradiometer (MODIS)onboard NASA EOS Terra and Aqua satellites,advanced very high resolution radiometer (AVHRR)onboard NOAA series provide important aerosol measurements.MODIS provides atmosphere aerosol optical depth (AOD)product since 2000,and AVHRR also provides AOD product since 1981 but only over ocean.Developing AOD retrieval algorithm which can also obtain AOD from AVHRR over land is very important for establishing a long term AOD da-ta record for climate studies.As 2.1 μm band is absent,an algorithm which is different from MODIS is in-troduced to retrieve AOD over land from AVHRR.With this method,the surface target is assumed to re-main radiometrically invariant over a certain time period and some of observations are made under clear-sky background aerosol conditions.When background aerosol conditions are given,surface reflectance can be estimated by extracting the second minimum reflectance during the previous 22 days and the future 22 days.The second darkest reflectance is chosen to reduce cloud shadow contamination.After surface reflec-tance is selected,AOD is retrieved from a look up table (LUT)generated with the second simulation of the satellite signal in the solar spectrum (6S)radiative transfer model.The AOD over part of China (15°—45°N,75°—135°E)from AVHRR in 2009 is obtained based on this algorithm.The distribution pattern of AOD from this work is consistent with that of MYDO04 from MODIS in North China and East China,but has some difference in Northwest China.The daily regional mean AOD from AVHRR in the Yangtze Delta (28°—36°N,112°—122°E)agrees well with MODIS AOD with all correlation coefficients larger than 0.5 for four seasons,even up to 0.8 in winter.The correlation coefficients are 0.70 in Beijing,0.63 in Xiang-he and 0.61 in Taihu when AOD from AERONET are used to validate the AVHRR AOD retrievals.To compare temporally varying AERONET data with spatially varying AVHRR,the time match window is limited within 30 minutes and the spatial distance is limited within 0.10.The monthly variation of AOD from AVHRR in the Yangtze River Delta is consistent with that from MODIS,but the former is larger. Error sources about this retrieving algorithm are also discussed,including different satellite zenith angles in the selected period,surface reflectance,aerosol types,background AOD,calibration and sensor noise and so on.According to these results,this algorithm has the potential for deriving long-term AOD climate data record over land from AVHRR although some uncertainties still exist.Quality control and error char-acterization will be further investigated in the future.%开发 AVHRR 可见光通道反演陆地气溶胶光学厚度（AOD）的算法对于研究长时间序列 AOD 的变化有重要意义。AVHRR 由于缺少2．1μm 通道而不能采用 MODIS 的暗背景算法，该文利用背景合成算法进行陆地 AOD反演。背景合成算法是指假设一段时间内地表反射率变化不大且会出现相对清洁大气，采用最小值合成即可得到地表反射率，再通过辐射传输模式6S 制作的查算表查算得到 AOD 的反演结果。将此算法应用到2009年AVHRR 中国部分陆地区域（15°～45°N，75°～135°E）得到 AOD 的时空分布，将反演结果与同期 Aqua／MODIS 的MOD04 AOD 产品进行对比分析表明，华北和华东地区的反演效果较好，西北地区结果较差。以长江三角洲地区为例可知，AVHRR AOD 产品与 MODIS AOD 产品以及 AERONET 观测的 AOD 相比相关系数基本在0．6以上，从时间变化规律来看，AVHRR AOD 和 MODIS AOD 产品年变化趋势具有很好的一致性。该文为建立长时间序列 AVHRR AOD 数据集提供了一个较为可行的方法。