6. Analysis of the Causes of Influencing Factors of Air Pollution in Lanzhou

Dust weather is a kind of small probability and large hazard weather caused by the development of specific large-scale circulation background and specific weather system under specific geographical environment and underlying surface conditions, mainly in arid areas, in semi-arid areas, in desertified areas, and in farming-pastoral ecotone. Dust weather has a great destructive power, and the economic losses caused to the country and people by sand and dust weather every year are huge and incalculable (Liu et al. 2009; Qi et al. 2011). As a meteorological disaster and ecological environment problem, dust weather has already attracted the attention of scientists from all over the world. In different regions, dust weather has been studied from the aspects of synoptic, climatology, moving path, and transmission mechanism (Qian et al. 2004; Wang et al. 2005; Yin et al. 2007; Kaskaoutis et al. 2014; Dimitriou et al. 2017). Dust aerosol had a climate effect, simultaneously diffusing the incident visible light and the emergent long-wave radiation on the ground. On one hand, the dust aerosol directed scattering and absorption of radiation, resulting in direct climate effect (parasol effect). On the other hand, dust aerosol particles could change cloud microphysical processes, radiation characteristics, and precipitation, resulting in indirect climate effects (Su 2008). In addition, dust aerosols could also endanger people’s physical and mental health (Chen and Yang 2001; Meng et al. 2007) and have an impact on global ecological effects (Zhang et al. 1997; Jickells et al. 2005; Huang et al. 2007; Fan 2013; Ta et al. 2013).