Aerosols characteristics, sources, and drive factors analysis in typical megacities, NW China

Cities in arid and semi-arid regions often confront harsher natural environments and severer industrial pollution. So these are frequently subjected to heavy air pollution, which has critical consequences for the health and production activities and well-being of local citizens. To curb the aggravation of air pollution, the Chinese government conducted an air pollution control action plan with unprecedented strength in September 2013. In this study, based on the fine-resolution aerosol optical depth (AOD) data, combined meteorological and other auxiliary data, utilizing information entropy, backward trajectory, source analysis, and multi-layer perceptron methods, we comprehensively analyzed the spatiotemporal characteristics and variations of AOD in typical arid and semi-arid areas megacities (Lanzhou and Urumqi) from 2010 to 2019, and further explore the potential pollution sources and nature and anthropic influences on AOD. The main results indicate that Lanzhou AOD remains higher than Urumqi in almost all different time windows, especially in the spring. Specifically, the AOD shows a single-peaked distributed trend on the monthly scale, but the peak occurred one month later in Lanzhou than in Urumqi. In the seasons, Lanzhou and Urumqi all show a trend that spring and winter AOD are higher than summer and fall. Lanzhou and Urumqi overall show a tortuous decreasing and then rebounded trend between 2010 and 2019, but the AOD variation in the Urumqi is more dramatic than in Lanzhou. The high AOD mainly concentrates in low altitude areas and there is a downward tendency of AOD in high-AOD areas, but an upward trend in low-AOD areas. The AOD intensity of change is higher in Lanzhou than in Urumqi, but Lanzhou has more concentration. The seasonal differences in backward trajectories are stronger in Lanzhou than in Urumqi, generally, Lanzhou's potential trajectory is mainly distributed in the Hexi Corridor, while Urumqi is along the northern slope of the Tianshan Mountains urban cluster distribution. The potential source area of AOD has a significant seasonal variation, and the closer to the object, the greater the contribution. Generally, Lanzhou pollution is more sensitive to the atmosphere condition and has a wider source area than Urumqi. The AOD was influenced by a combination of natural and anthropogenic factors, but natural variables seem to be more dominant. The impact of natural factors in Urumqi and Lanzhou is statistically obtained up to 75.04% and 52.01% respectively.