The positive artifacts in particulate organic carbon measurements in a roadside environment were characterized using two filters in tandem. The experiments were performed for PM(1.0), PM(2.5), and PM(10) at 24-h interval using a URG sampler, followed by organic carbon (OC)/elemental carbon (EC) analysis by the Interagency Monitoring of Protected Visual Environments thermal/optical reflectance carbon analysis protocol. The OC concentrations, derived from the quartz filter behind a front quartz filter, were quite similar for PM(1.0), PM(2.5), and PM(10), ranging from 0.6 to 2.7 microg C m(-3) for PM(1.0), from 0.7 to 2.7 microg C m(-3) for PM(2.5), and from 1.1 to 2.7 microg C m(-3) for PM(10). They were respectively approximately 2.8%, approximately 2.4%, and approximately 1.6% of the particulate mass. The most abundant species on the backup quartz filters were OC2 (250 degrees C) and OC3 (450 degrees C), accounting for approximately 80% of measured organic carbon on the backup quartz filters. It indicates the filter artifacts are mainly composed of adsorbed semi-volatile organics (below the analysis temperature of 450 degrees C) including gaseous and particulate phase; the loading of artifacts depends on the nature of vapor and its interaction with filter substrate, rather than particle sizes. The uncorrected OC/EC ratios on the front quartz filters were approximately 10% higher than the corrected OC/EC ratios by positive organic artifacts in winter, and it is approximately 20% higher in summer. Another finding is that the separation distance of the front and backup filters influence the level of artifacts assessed by the backup filter.