Seasonal variation, source, and regional representativeness of the background aerosol from two remote sites in western China.

Using observations from two remote sites during July 2004 to March 2005, we show that at Akdala (AKD, 47 degrees 06' N, 87 degrees 58' E, 562 m asl) in northern Xinjiang Province, there were high wintertime loadings of organic carbon (OC), elemental carbon (EC), and water-soluble (WS) SO4(2-), NO3(2-), and NH4+, which is similar to the general pattern in most areas of China and East Asia. However, at Zhuzhang (ZUZ, 28 degrees 00' N, 99 degrees 43' E, 3,583 m asl) in northwestern Yunnan Province, the aerosol concentrations and compositions showed little seasonal variation except for a decreasing trend of OC from August to autumn-winter. Additionally, the OC variations dominated the seasonal variation of PM10 (particles<or=10 microm diameter) level. Chemical characteristics combined with transport information suggested sea salt origin of ionic Na+, Mg2+, and Cl- at ZUZ. At AKD, ionic Ca2+, Mg2+, Na+, and Cl- primarily originated from salinized soil. Furthermore, the WS Ca2+ contributions (5.4-6%) to the PM10 mass during autumn, winter, and early spring reflected a constant dust component. The results of this study indicated that both sites were regionally representative. However, the representative regions and scales of these background sites may vary seasonally as the regional atmospheric transport patterns change. Seasonal variations in the background aerosol levels from these two areas need to be considered when evaluating the regional climate effects of the aerosols.