Aerosol background at two remote CAWNET sites in western China.

The frequency distributions and some statistical features of background aerosol concentrations were investigated at two remote China Atmosphere Watch Network (CAWNET) stations. The estimated elemental carbon (EC) background at Akdala (AKD) in the mid-latitudes of northwestern China (approximately 0.15 microg m(-3)) was only half of that at Zhuzhang (ZUZ) in low-latitude southwestern China (approximately 0.30 microg m(-3)). The contributions of EC to the aerosol mass also differed between sites: EC contributed 3.5% of the PM(10) mass at AKD versus 5.1% at ZUZ. Large percentages of the total organic carbon (OC) apparently were secondary organic carbon (SOC); SOC/OC averaged 81% at ZUZ and 68% at AKD. The OC/EC ratios in PM(10) (ZUZ: 11.9, AKD: 12.2) were comparable with other global background sites, and the OC/EC ratios were used to distinguish polluted periods from background conditions. The SO(4)(2)(-), NH(4)(+) and soil dust loadings at AKD were higher and more variable than at ZUZ, probably due to impacts of pollution from Russia and soil dust from the Gobi and adjacent deserts. In contrast to ZUZ, where the influences from pollution were weaker, the real-time PM(10) mass concentrations at AKD were strongly skew right and the arithmetic mean concentrations of the aerosol populations were higher than their medians. Differences in the aerosol backgrounds between the sites need to be considered when evaluating the aerosol's regional climate effects.