Daily particulate matter (PM2.5) aerosol samples were collected in Tongyu,a semi-arid area in northeastern China in spring.The concentrations of organic carbon (OC) and elemental carbon (EC) were determined with a thermal/optical carbon analyzer in the filter samples.The average concentrations of OC and EC in PM2.5 were 14.1 ± 8.7 and 2.0 ± 1.3 μg/m3,respectively.A good correlation between OC and EC was observed during the spring season,suggesting that they might be derived from similar sources.The correlation between OC and K+ was high (R = 0.74) and the K+/OC ratio,as determined from their linear regression slope,reached 2.57.The good correlation and the high K+/OC ratio indicated that biomass-burning was probably one of the major sources of OC in this region.The concentrations of estimated secondary organic carbon (SOC) in PM2.5 in Tongyu ranged from below detection limit to 26.1 μg/m3 (mean,5.9 μg/m3).The percentages of SOC in OC and in PM2.5 mass were 42.0% and 2.1%,respectively.The SOC concentrations during dust storms (DS) periods were higher than those during non-dust storm (NDS) periods,suggesting that chemical reaction processes involving gas-particle conversion should have occurred during the long-distance transport of aerosol particles.