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
Authors: Dunne Erin , Galbally Ian E. , Cheng Min , Selleck Paul , Molloy Suzie B. , Lawson Sarah J. 
Published: January 2018

Abstract. Understanding uncertainty is essential for utilizing atmospheric volatile organic compound
(VOC) measurements in robust ways to develop atmospheric science. This study
describes an inter-comparison of the VOC data, and the derived uncertainty
estimates, 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-performance
liquid chromatography)
during routine monitoring as part of the Sydney Particle Study (SPS) campaign in 2012. Benzene, toluene, C8 aromatics, isoprene,
formaldehyde and acetaldehyde were selected for the comparison, based on
objective selection criteria from the available data. Bottom-up uncertainty
analyses were undertaken for each compound and each measurement system.
Top-down uncertainties were quantified via the inter-comparisons. In all
seven comparisons, the correlations between independent measurement
techniques were high with R2 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 regression
line with a median of 0.11 (range 0.04–0.23 ppbv). These results
give a high degree of confidence that for each comparison the response of the
two independent techniques is dominated by the same constituents. The slope
and intercept as determined by reduced major axis (RMA) regression gives
a different story. The slopes varied considerably with a median of 1.25 and
a range of 1.16–2.01. The intercepts varied with a median of 0.04 and a range
of −0.03 to 0.31 ppbv. An ideal comparison would give a slope of 1.00
and 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 the
under-reporting of formaldehyde, acetaldehyde and acetone by the DNPH technique. As well as these, this study has identified a specific
interference 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-comparison
studies for the same compounds. Overall, for the light aromatics, isoprene and the C1–C3 carbonyls, the uncertainty
in a set of measurements varies by a factor of between 1.5 and 2. These uncertainties (∼50 %) are significantly higher
than uncertainties estimated using standard propagation of error methods, which in this case were ∼22 % or less, and are
the result of the presence of poorly understood or neglected processes that affect the measurement and its uncertainty. The
uncertainties in VOC measurements identified here should be considered when assessing the reliability of VOC measurements from routine
monitoring 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.