Atmospheric Measurement Techniques
www.atmos-meas-tech.net
1867-1381
1867-8548
3
4
2010
10.5194/amt-3-959-2010
http://www.atmos-meas-tech.net/3/959/2010/
http://www.atmos-meas-tech.net/3/959/2010/amt-3-959-2010.html
http://www.atmos-meas-tech.net/3/959/2010/amt-3-959-2010.pdf
959
980
2010-07-23
A comparison of GC-FID and PTR-MS toluene measurements in ambient air under conditions of enhanced monoterpene loading
J. L. Ambrose
jambrose@alumni.unh.edu
K. Haase
R. S. Russo
Y. Zhou
M. L. White
E. K. Frinak
C. Jordan
H. R. Mayne
R. Talbot
B. C. Sive
Department of Chemistry, University of New Hampshire, Durham, New Hampshire, USA
Climate Change Research Center, Institute for the Study of Earth Oceans and Space, University of New Hampshire, Durham, New Hampshire, USA
now at: Northern Essex Community College, Haverhill, Massachusetts, USA
now at: USMA Network Science Center, West Point, New York, USA
Toluene was measured using both a gas chromatographic system (GC), with a
flame ionization detector (FID), and a proton transfer reaction-mass
spectrometer (PTR-MS) at the AIRMAP atmospheric monitoring station Thompson
Farm (THF) in rural Durham, NH during the summer of 2004. Simultaneous
measurements of monoterpenes, including α- and β-pinene, camphene, Δ
<sup>3</sup>-carene, and <i>d</i>-limonene, by GC-FID demonstrated large enhancements in
monoterpene mixing ratios relative to toluene, with median and maximum
enhancement ratios of ~2 and ~30, respectively. A detailed
comparison between the GC-FID and PTR-MS toluene measurements was conducted
to test the specificity of PTR-MS for atmospheric toluene measurements under
conditions often dominated by biogenic emissions. We derived quantitative
estimates of potential interferences in the PTR-MS toluene measurements
related to sampling and analysis of monoterpenes, including fragmentation of
the monoterpenes and some of their primary carbonyl oxidation products via
reactions with H<sub>3</sub>O<sup>+</sup>, O<sub>2</sub><sup>+</sup> and NO<sup>+</sup> in the PTR-MS
drift tube. The PTR-MS and GC-FID toluene measurements were in good
quantitative agreement and the two systems tracked one another well from the
instrumental limits of detection to maximum mixing ratios of ~0.5 ppbv. A correlation plot of the PTR-MS versus GC-FID toluene measurements
was described by the least squares regression equation <i>y</i>=(1.13±
0.02)<i>x</i>−(0.008±0.003) ppbv, suggesting a small ~13% positive
bias in the PTR-MS measurements. The bias corresponded with a ~0.055
ppbv difference at the highest measured toluene level. The two systems
agreed quantitatively within the combined 1σ measurement precisions for
60% of the measurements. Discrepancies in the measured mixing ratios were
not well correlated with enhancements in the monoterpenes. Better
quantitative agreement between the two systems was obtained by correcting
the PTR-MS measurements for contributions from monoterpene fragmentation in
the PTR-MS drift tube; however, the improvement was minor (<10%).
Interferences in the PTR-MS measurements from fragmentation of the
monoterpene oxidation products pinonaldehyde, caronaldehyde and α-pinene
oxide were also likely negligible. A relatively large and variable toluene
background in the PTR-MS instrument likely drove the measurement bias;
however, the precise contribution was difficult to accurately quantify and
thus was not corrected for in this analysis. The results from THF suggest
that toluene can be reliably quantified by PTR-MS using our operating
conditions (drift tube pressure, temperature and voltage of 2.0 mbar, 45 °C and 600 V, respectively) under the ambient compositions probed. This work
extends the range of field conditions under which PTR-MS validation studies
have been conducted.
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