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Toluene was measured using both a gas chromatographic system (GC), with aflame ionization detector (FID), and a proton transfer reaction-massspectrometer (PTR-MS) at the AIRMAP atmospheric monitoring station ThompsonFarm (THF) in rural Durham, NH during the summer of 2004. Simultaneousmeasurements of monoterpenes, including α- and β-pinene, camphene, Δ3-carene, and d-limonene, by GC-FID demonstrated large enhancements inmonoterpene mixing ratios relative to toluene, with median and maximumenhancement ratios of ~2 and ~30, respectively. A detailedcomparison between the GC-FID and PTR-MS toluene measurements was conductedto test the specificity of PTR-MS for atmospheric toluene measurements underconditions often dominated by biogenic emissions. We derived quantitativeestimates of potential interferences in the PTR-MS toluene measurementsrelated to sampling and analysis of monoterpenes, including fragmentation ofthe monoterpenes and some of their primary carbonyl oxidation products viareactions with H3O+, O2+ and NO+ in the PTR-MSdrift tube. The PTR-MS and GC-FID toluene measurements were in goodquantitative agreement and the two systems tracked one another well from theinstrumental limits of detection to maximum mixing ratios of ~0.5 ppbv. A correlation plot of the PTR-MS versus GC-FID toluene measurementswas described by the least squares regression equation y=(1.13±0.02)x−(0.008±0.003) ppbv, suggesting a small ~13% positivebias in the PTR-MS measurements. The bias corresponded with a ~0.055ppbv difference at the highest measured toluene level. The two systemsagreed quantitatively within the combined 1σ measurement precisions for60% of the measurements. Discrepancies in the measured mixing ratios werenot well correlated with enhancements in the monoterpenes. Betterquantitative agreement between the two systems was obtained by correctingthe PTR-MS measurements for contributions from monoterpene fragmentation inthe PTR-MS drift tube; however, the improvement was minor (<10%).Interferences in the PTR-MS measurements from fragmentation of themonoterpene oxidation products pinonaldehyde, caronaldehyde and α-pineneoxide were also likely negligible. A relatively large and variable toluenebackground in the PTR-MS instrument likely drove the measurement bias;however, the precise contribution was difficult to accurately quantify andthus was not corrected for in this analysis. The results from THF suggestthat toluene can be reliably quantified by PTR-MS using our operatingconditions (drift tube pressure, temperature and voltage of 2.0 mbar, 45 °C and 600 V, respectively) under the ambient compositions probed. This workextends the range of field conditions under which PTR-MS validation studieshave been conducted

A comparison of GC-FID and PTR-MS toluene measurements in ambient air under conditions of enhanced monoterpene loading