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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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Volume 3, issue 4 | Copyright
Atmos. Meas. Tech., 3, 853-862, 2010
https://doi.org/10.5194/amt-3-853-2010
© Author(s) 2010. This work is distributed under
the Creative Commons Attribution 3.0 License.

  07 Jul 2010

07 Jul 2010

Lag time determination in DEC measurements with PTR-MS

R. Taipale1, T. M. Ruuskanen1,2, and J. Rinne1 R. Taipale et al.
  • 1University of Helsinki, Department of Physics, P.O. Box 64, 00014 University of Helsinki, Finland
  • 2University of Innsbruck, Institute of Ion Physics and Applied Physics, Technikerstr. 25, 6020 Innsbruck, Austria

Abstract. The disjunct eddy covariance (DEC) method has emerged as a popular technique for micrometeorological flux measurements of volatile organic compounds (VOCs). It has usually been combined with proton transfer reaction mass spectrometry (PTR-MS), an online technique for VOC concentration measurements. However, the determination of the lag time between wind and concentration measurements has remained an important challenge. To address this issue, we studied the effect of different lag time methods on DEC fluxes. The analysis was based on both actual DEC measurements with PTR-MS and simulated DEC data derived from high frequency H2O measurements with an infrared gas analyzer. Conventional eddy covariance fluxes of H2O served as a reference in the DEC simulation. The individual flux measurements with PTR-MS were rather sensitive to the lag time methods, but typically this effect averaged out when the median fluxes were considered. The DEC simulation revealed that the maximum covariance method was prone to overestimation of the absolute values of fluxes. The constant lag time methods, one based on a value calculated from the sampling flow and the sampling line dimensions and the other on a typical daytime value, had a tendency to underestimate. The visual assessment method and our new averaging approach utilizing running averaged covariance functions did not yield statistically significant errors and thus fared better than the habitual choice, the maximum covariance method. Given this feature and the potential for automatic flux calculation, we recommend using the averaging approach in DEC measurements with PTR-MS. It also seems well suited to conventional eddy covariance applications when measuring fluxes near the detection limit.

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