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The objective of the study was to investigate the main factors that contribute the variation of PM10 concentration of Seoul and to quantify their effects using generalized additive model (GAM). The analysis was performed with 3year air pollution data (2004~2006) measured at 27 urban sites and 7 roadside sites in Seoul, a background site in Gangwha and a rural site in Pocheon. The diurnal variation of urban PM10 concentrations of Seoul showed a typical bimodal pattern with the same peak times as that of roadside, and the maximum difference of PM10 level between urban and roadside was about 14㎍/㎥ at 10 in the morning. The wind direction was found to be a major factor that affects PM10 level in all investigated areas. The overall PM10 level was reduced when air came from east, but background PM10 level in Gangwha was rather higher than the urban PM10 level in Seoul, indicating that the PM10level in Gangwha is considerably influenced by that in Seoul metropolitan area. When hourly variations of PM10 were analyzed using GAM, wind direction and speed explained about 34% of the variance in the model where the variables were added as a 2-dimensional smoothing function. In addition, other variables, such as diurnal variation, difference of concentrations between roadside and urban area, precipitation, month, and the regression slope of a plot of carbon monooxide versus PM10, were found to be major explanatory variables, explaining about 64% of total variance of hourly variations of PM10 in Seoul.