To investigate the seasonal and day-night variation of PM10 chemical species,daytime and nighttime PM10 mass,water-soluble ions (Na+,NH4 +,K+,Mg2+,Ca2+,F-,Cl-,NO3 -,and SO4 2-),organic carbon (OC),elemental carbon (EC),and water-soluble carbon (WSOC) were acquired at a urban site of Xian from 20 December 2006 to 12 November 2007.No dramatic differences were found in PM10 or chemical species loadings between daytime and nighttime.In general,PM10 concentrations were in excess of 1.5 times to China Ambient Air Quality Standard (CAAQS) for 24-h average PM10 (150 μg m-3).Carbonaceous fractions and water-soluble ions accounted for nearly one-third and 12.4% of the PM10 mass.Relative Low NO3 -/SO4 2- ratio suggested that stationary source emissions were more important than the vehicle emissions in the source areas in Xian,China.Good correlation between WSOC and OC were observed,which indicated that they might derive from the similar origin,such as primary emission sources or secondary processes.Moreover,a strong relationship between WSOC and K+ was observed in winter,which implies winter WSOC were mainly from biomass burning.This was also highlighted by the chemical components results of biomass burning events.In contrast,poor correlation between WSOC and K+ in summer indicated WSOC was mainly of Secondary organic carbon contribution from strong photochemical activity.