Aerosols at ten sites in northwestern China are classified in this study: (1) by using the aerosol optical depth (AOD), the Ångström exponent (α) and the Ångström exponent difference (δα); and (2) by using the total means of AOD440nm and α. The seasonal variations of the AOD and α show that the maximum AODs occur in spring, except at Urumqi and Lanzhou. The seasonal mean α values are lower than 0.80 in all four seasons at Tazhong, Hotan, Hami, Ejina, Dunhuang, Minqin, and Jiuquan, but higher than 0.80 in winter at Urumqi, Lanzhou and Yinchuan. The first classification method shows that coarse mode particles are found at all ten sites, but that fine mode growth only happens at Urumqi, Lanzhou, and Yinchuan. The relationship between AOD440nm and α show that α smaller than 0.80 decrease with increasing AOD440nm at all ten sites. Aerosols are classified into four types (Type I–IV) according to the total mean τ440 (τ̅440) and total mean Ångström exponent (ᾱ) of each site. Aerosols with a τ440 smaller than τ̅440, but greater than or equal to ᾱ (τ440 < τ̅440; α ≥ ᾱ) are classified as Type I; aerosols with τ440 ≥ τ̅440 and α ≥ ᾱ are Type II; those with τ440 < and α < ᾱ are Type III; and those with τ440 ≥ τ̅440 and α < ᾱ are Type IV. The second aerosol classification method shows that Type I and Type III aerosols are the most common at all ten sites. Type II aerosols are the least at Tazhong and Hotan, but are the most common at Urumqi, Lanzhou, and Yinchuan. On the contrary, Type IV aerosols are the most common at Tazhong and Hotan, but are the least common at Urumqi, Lanzhou and Yinchuan.