Comparison of VOC measurements made by PTR-MS, adsorbent tubes–GC-FID-MS and DNPH derivatization–HPLC during the Sydney Particle Study, 2012: a contribution to the assessment of uncertainty in routine atmospheric VOC measurements

摘要

Understanding uncertainty is essential for utilizing atmospheric volatile organic compound(VOC) measurements in robust ways to develop atmospheric science. This studydescribes an inter-comparison of the VOC data, and the derived uncertaintyestimates, measured with three independent techniques (PTR-MS,proton-transfer-reaction mass spectrometry; GC-FID-MS,gas chromatography with flame-ionization and mass spectrometric detection;and DNPH–HPLC, 2,4-dinitrophenylhydrazine derivatization followed by analysis by high-performanceliquid chromatography)during routine monitoring as part of the Sydney Particle Study (SPS) campaign in 2012. Benzene, toluene, C aromatics, isoprene,formaldehyde and acetaldehyde were selected for the comparison, based onobjective selection criteria from the available data. Bottom-up uncertaintyanalyses were undertaken for each compound and each measurement system.Top-down uncertainties were quantified via the inter-comparisons. In allseven comparisons, the correlations between independent measurementtechniques were high with values with a median of 0.92 (range 0.75–0.98)and small root mean square of the deviations (RMSD) of the observations from the regressionline with a median of 0.11 (range 0.04–0.23 ppbv). These resultsgive a high degree of confidence that for each comparison the response of thetwo independent techniques is dominated by the same constituents. The slopeand intercept as determined by reduced major axis (RMA) regression givesa different story. The slopes varied considerably with a median of 1.25 anda range of 1.16–2.01. The intercepts varied with a median of 0.04 and a rangeof −0.03 to 0.31 ppbv. An ideal comparison would give a slope of 1.00and an intercept of 0.Some sources of uncertainty that are poorly quantified by the bottom-up uncertainty analysis method were identified, including:contributions of non-target compounds to the measurement of the target compound for benzene, toluene and isoprene by PTR-MS as well as theunder-reporting of formaldehyde, acetaldehyde and acetone by the DNPH technique. As well as these, this study has identified a specificinterference of liquid water with acetone measurements by the DNPH technique.These relationships reported for Sydney 2012 were incorporated into a larger analysis with 61 similar published inter-comparisonstudies for the same compounds. Overall, for the light aromatics, isoprene and the C–C carbonyls, the uncertaintyin a set of measurements varies by a factor of between 1.5 and 2. These uncertainties ( ∼ 50 %) are significantly higherthan uncertainties estimated using standard propagation of error methods, which in this case were  ∼ 22 % or less, and arethe result of the presence of poorly understood or neglected processes that affect the measurement and its uncertainty. Theuncertainties in VOC measurements identified here should be considered when assessing the reliability of VOC measurements from routinemonitoring with individual, stand-alone instruments; when utilizing VOC data to constrain and inform air quality and climate models;when using VOC observations for human exposure studies; and for comparison with satellite retrievals.