Assessment of a passive sampling method and two on-line gas chromatographs for the measurement of benzene, toluene, ethylbenzene and xylenes in ambient air at a highway site

Accurate, continuous long-term measurements of aromatic volatile organic compounds with known health impacts are needed for comprehensive assessments of air quality in the vicinity of industrial and vehicular emission sources. Historically, evacuated canister sampling and on-line gas chromatography (GC) have been the most popular approaches applied by government institutions in North America to measure benzene, toluene, ethylbenzene and xylenes (BTEX) in ambient air. Canister sampling is labour-intensive and typically does not provide the continuous data needed for epidemiological studies, while on-line GCs are costly and require shelter and power. In this work, the accuracy and suitability of three techniques for generating long-term ambient BTEX datasets are assessed. The first is a passive sampling method involving sorbent-packed thermal desorption tubes, the second is an on-line miniature gas chromatograph (miniGC) coupled with a photoionization detector (PID) and the third is a more traditional on-line gas chromatograph coupled with a flame ionization detector (GC-FID). These three techniques were deployed at an air quality monitoring station located adjacent to a busy highway for six weeks in summer 2018 to compare method performance and accuracy. Ambient mean concentrations of BTEX determined from the three methods for the entire study period were found to agree within 7% for benzene and within 30% for the other species. Interestingly, a temperature dependent positive bias previously identified for 14-day passive tube sampling for ethylbenzene and xylenes under cold wintertime conditions was found to be negligible under summertime conditions.