中国西北地区气溶胶的三维分布特征及其成因

利用CALIPSO卫星遥感资料研究中国西北地区气溶胶的三维分布，分区域按类别讨论气溶胶的出现频率和气溶胶光学厚度（AOD），并在此基础上，结合MERRA-2再分析数据探讨形成原因。结果表明：中国西北地区气溶胶含量较高，以沙尘和污染性沙尘为主，主要分布在塔克拉玛干沙漠、古尔班通古特沙漠和甘肃-内蒙古一带。气溶胶的出现频率季节变化不大，大气中气溶胶含量及其三维分布则有明显的区域性差异和季节变化。夏季，由于大气上升运动明显，气溶胶分布较高，部分气溶胶可以被输送到天山山脉、青藏高原主体，甚至8 km的高空。塔克拉玛干沙漠的气溶胶以自然沙尘为主，主要分布在4 km以下，受地形影响该地区局地环流明显，三维分布主要取决于局地排放和垂直输送，AOD的季节变化与沙尘排放一致，春季>夏季>秋季>冬季。古尔班通古特沙漠和甘肃-内蒙古一带的气溶胶消光系数较小且分布零散，气溶胶种类复杂，除了自然沙尘还有大量的污染性沙尘（出现频率约为0.15，AOD约为0.03，甚至可以大于自然沙尘）、污染性大陆气溶胶和烟尘。这是因为，这些地区受局地排放和远距离输送的共同影响，上游气溶胶在西北风的作用下输送而来和局地排放的气溶胶混合并继续向下游输送，沙尘气溶胶经过远距离输送变性为污染性沙尘。

Three-dimensional distribution and formation causes of aerosols over Northwest China

Using the CALIPSO observations and MERRA-2 data， the three-dimensional distribution of aerosols over Northwest China are constructed， and the aerosol occurrence frequency and aerosol optical depth （AOD） are also discussed by region and category. On this basis， the formation causes are explored. There are a large amount of aerosols over Northwest China， mainly dust and polluted dust， which are primarily distributed over the Taklimakan Desert， Gurbantunggut Desert and Gansu-Inner Mongolia. The occurrence frequencies of these aerosols vary little with season， but there are obvious regional and seasonal variations of aerosol content and its three-dimensional distribution in the atmosphere. In summer， the vertical distributions of these aerosols are high due to the obvious ascending movements and some aerosols can be transported to the Tianshan Mountains， the Tibetan Plateau， and even the altitude of 8 km. The aerosols over the Taklimakan Desert are dominated by natural dust， mainly distributed below 4 km. Affected by the topography， the local circulation over the Taklimakan Desert is obvious， and thus the three-dimensional distribution of aerosols is mainly caused by local emission and vertical transport. The seasonal change of AOD is consistent with the dust emission， spring>summer>autumn>winter. In contrast， the aerosol extinction coefficients over the Gurbantonggut Desert and Gansu-Inner Mongolia are small and scattered. The aerosol types are complex. In addition to natural dust， there are a large amount of polluted dust aerosols （with occurrence frequency of 0.15 and AOD of 0.03， even larger than natural dust）， polluted continent aerosols and smoke. These areas are not only affected by local emissions， but also by long-distance transport. The upstream aerosols transported by the northwest wind mix with the local aerosols and continue to transport downstream. The natural dust aerosols are transformed into polluted dust during the long-distance transport.