Positive sampling artifacts in particulate organic carbon measurements in roadside environment

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 PM₁₀, ranging from 0.6 to 2.7 μg C m^-3 for PM_1.0, from 0.7 to 2.7 μg C m^-3 for PM_2.5, and from 1.1 to 2.7 μg C m^-3 for PM₁₀. They were respectively ∼2.8%, ∼2.4%, and ∼1.6% of the particulate mass. The most abundant species on the backup quartz filters were OC2 (250°C) and OC3 (450°C), accounting for ∼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°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 ∼10% higher than the corrected OC/EC ratios by positive organic artifacts in winter, and it is ∼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. [Figure not available: see fulltext.]