LAUSR

The Moderate Resolution Imaging Spectroradiometer (MODIS) sensor onboard NASA’s Aqua satellite has been collecting valuable data about the Earth system for more than 14 years, and one of the benefits of this is that it has made it possible to detect the long-term variation in aerosol loading across the globe. However, the long-term aerosol optical depth (AOD) trends derived from MODIS need careful validation and assessment, especially over land. Using AOD products with at least 70 months’ worth of measurements collected during 2002–15 at 53 Aerosol Robotic Network (AERONET) sites over land, Mann–Kendall (MK) trends in AOD were derived and taken as the ground truth data for evaluating the corresponding results from MODIS onboard Aqua. The results showed that the AERONET AOD trends over all sites in Europe and North America, as well as most sites in Africa and Asia, can be reproduced by MODIS/Aqua. However, disagreement in AOD trends between MODIS and AERONET was found at a few sites in Australia and South America. The AOD trends calculated from AERONET instantaneous data at the MODIS overpass times were consistent with those from AERONET daily data, which suggests that the AOD trends derived from satellite measurements of 1–2 overpasses may be representative of those from daily measurements.摘 要MODIS (Moderate Resolution Imaging) 气溶胶光学厚度 (Aerosol Optical Depth, AOD) 产品为研究全球范围的 AOD 变化趋势提供了很好的数据平台. 然而, 由于陆地下垫面的复杂和陆地上空气溶胶时空分布的多变, MODIS 陆地气溶胶产品获得的 AOD 变化趋势亟需广泛的验证和评估. 本文选取了全球 53 个 (准连续观测 70 个月以上) AERONET (Aerosol Robotic Network) 测站, 用 Mann–Kendall (MK) 非参数检验方法验证评估了 MODIS/Aqua 陆地气溶胶光学厚度 2002∼2015 年的变化趋势. 以 AERONET 日平均 AOD 计算得到的月中值 AODRE 趋势作为参考, 将其与 MODIS/Terra, MODIS/Aqua 过境前后半小时的 AERONET 瞬时资料计算得到的 AODAM, AODPM 月中值 MK 趋势进行比较, AODAM, AODPM 与 AODRE 的变化趋势一致. 这说明, 上, 下午卫星过境前后半小时的 AERONET AOD 观测可以代表日平均 AOD 的变化趋势. 此外, 将地基 AERONET AODRE MK 趋势和 MODIS/Aqua 气溶胶产品得到的 AODMOD 变化趋势做了比较, 结果显示: 欧洲和北美的所有测站, 亚洲和非洲的大多数测站, MODIS 气溶胶产品得到的 AODMOD 变化趋势与 AERONET AODRE 变化趋势相同. 但是, 澳大利亚和南美的测站, 由于 MODIS 气溶胶产品反演精度低, MODIS AODMOD 变化趋势与地基 AODRE 的变化趋势不一致.