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Aerosol optical properties from 2005 to 2008 at the Longfengshan regional background station in Northeast China were measured and analyzed. The annual mean of aerosol optical depth (AOD) at 440 nm for the four years was about 0.27 +/- 0.25, 0.39 +/- 0.37, 0.35 +/- 0.34, and 0.38 +/- 0.38, respectively, and the corresponding annual mean for the Angstrom exponent between 440 nm and 870 nm was about 1.43 +/- 0.48, 1.23 +/- 0.37, 1.53 +/- 0.47, and 1.55 +/- 0.42. The average monthly AOD(440nm) showed similar seasonal variation with a maximum in spring and a minimum in autumn. The monthly means of AOD at 440, 675, 870 and 1020 nm increase from the January to March with the maxima about 0.77 +/- 0.04, 0.65 +/- 0.04, 0.58 +/- 0.06, 0.57 +/- 0.07. respectively and decrease from September to February with the minima about 0.32 +/- 0.12, 0.22 +/- 0.09, 0.15 +/- 0.08, and 0.13 +/- 0.07 in January. The monthly mean of Angstrom exponent shows a minimum in March (0.97 +/- 0.52) and a maximum in September (1.66 +/- 0.29). Both the AOD and Angstrom exponent presents single peak distributions of occurrence frequencies. The Longfenshan data showed high AODs (>1.00) both clustering in the fine mode growth wing and the coarse mode. Two typical cases under dust and haze conditions showed that the AOD under dusty day decreased from 2.20 to 1.20 and the Angstrom exponent increased from 0.10 to 1.00. On the contrast, the AOD under haze day remained relatively stable about 0.90 and the Angstrom exponent was around 1.40. The 3-day backtrajectory analysis at Longfengshan illustrated that the air-masses near ground on the dust day were from Bohai Sea and passed through Liaodong Peninsula and Northeast plain in China. But the air-masses on 500 m AGL were originated from western Mongolia and crossed Gobi deserts, Otindag Sand Land and Horqin Sand Land in Northeast China. The air-masses at Longfengshan near ground 500 m and 1000 m AGL on the haze days were from North China Region and passed through Northeast Heavy Industrial Base in Northeast China. Crown Copyright (C) 2010 Published by Elsevier Ltd. All rights reserved.