Atmospheric Measurement Techniques Discussions www.atmos-meas-tech-discuss.net 1867-8610 3 1 2010 10.5194/amtd-3-405-2010 http://www.atmos-meas-tech-discuss.net/3/405/2010/ http://www.atmos-meas-tech-discuss.net/3/405/2010/amtd-3-405-2010.html http://www.atmos-meas-tech-discuss.net/3/405/2010/amtd-3-405-2010.pdf 405 429 2010-02-08 Lag time determination in DEC measurements with PTR-MS R. Taipale risto.taipale@helsinki.fi T. M. Ruuskanen J. Rinne University of Helsinki, Department of Physics, P.O. Box 64, 00014 University of Helsinki, Finland University of Innsbruck, Institute of Ion Physics and Applied Physics, Technikerstr. 25, 6020 Innsbruck, Austria 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 conundrum, 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 resting 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. 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