Aerosols at ten sites in northwestern China are classified in this study: (1) by using the aerosol optical depth (AOD), the Angstrom exponent (α) and the Angstrom 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 080 in all four seasons at Tazhong, Hotan, Hami, Ejina, Dunhuang, Minqin, and Jiuquan, but higher than 080 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 AOD440mm and α show that α smaller than 0.80 decrease with increasing AOD440mm at all ten sites.Aerosols are classified into four types (Type Ⅰ-Ⅳ) according to the total mean τ440 ((τ)440) and total mean Angstrom exponent ((α)) of each site.Aerosols with a τ440 smaller than (τ)440, but greater than or equal to (α) (τ440 < (τ)440;α ≥ (α)) are classified as Type Ⅰ;aerosols with τ440 ≥ (τ)440 and α ≥ (α) are Type Ⅱ;those with τ440 < (τ)440 and α < (α) are Type Ⅲ;and those with τ440 ≥ (τ)440 and α < (α) 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.