The measured mass concentrations of particulate matter less than 10 μm in diameter (PM10) by β-ray absorption method (BAM) and gravimetric method (GMM) at Gosan, Republic of Korea have shown consistent difference. By using a gas/particle equilibrium model, positive error by the absorption of water at the filter in BAM during the measurements were quantified. The positive error occupied about 50% of the measurement difference. The contributions from the volatilization of ionic species to the difference were minor. However, this estimated water content difference could fully explain the concentration difference by the two methods. Multiple regression analysis was applied to the PM10 mass concentration to find out the major factors that affected the concentration difference. Relative humidity, dust storm event and wind direction were identified as other major factors in addition to water absorption i.e., positive error. Still these factors could not fully account for the difference. Thus, it is advised that when using the PM10 concentration at Gosan, a supersite in Northeast Asia, possible difference should be considered for the data till June 2008 by BAM.