Abstract:

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.

Key words: Northwest China, aerosol classification, three-dimensional distribution, emission, transport