干旱区生态环境知识资源中心

Aerosols at ten sites in northwestern China are classified in this study: (1) by using the aerosol optical depth (AOD), the Angstrom exponent (alpha) and the Angstrom exponent difference (delta alpha); and (2) by using the total means of AOD(440nm) and alpha. The seasonal variations of the AOD and alpha show that the maximum AODs occur in spring, except at Urumqi and Lanzhou. The seasonal mean alpha 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 AOD(440nm) and alpha show that alpha smaller than 0.80 decrease with increasing AOD(440nm) at all ten sites. Aerosols are classified into four types (Type I-IV) according to the total mean tau(440) ((tau) over bar (440)) and total mean Angstrom exponent ((alpha) over bar) of each site. Aerosols with a tau(440) smaller than (tau) over bar (440), but greater than or equal to (alpha) over bar (tau(440) < <(tau)over bar>(440) ; alpha >= (alpha) over bar) are classified as Type I; aerosols with tau(440) >= (tau) over bar (440) and alpha >= (alpha) over bar are Type II; those with tau(440) < <(tau)over bar>(440) and alpha < <(alpha)over bar> are Type III; and those with tau(440) >= (tau) over bar (440) and alpha < <(alpha)over bar> 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.