An automated system for selective and continuous measurements of vertical thoron profiles for the determination of transport times near the ground

摘要

The quantification of in-canopy transport times is of major importance forthe investigation of sources, sinks and net fluxes of reactive trace gaseswithin plant canopies. The Damk?hler number, which compares timescales ofchemical reactions with transport times, is a widely applied measure toevaluate flux divergences. In this study we present and evaluate a novelautomated measurement system for selective vertical thoron (Tn) profiles nearthe Earth's surface and demonstrate its suitability for the direct andreliable determination of transport times within a natural grassland canopy.For the first time, we perform a rigorous determination of systematic andrandom uncertainties of Tn (and Rn) concentrations under field conditions forthis type of measurement system. The obtained median precisions for threeconcentration classes (> 100 Bq m?3,100–15 Bq m?3, < 15 Bq m−3) were 8.8%,23.2% and 132.1% for Tn (and 16.6%, 25.0%, 99.2% forRn). We calculate in-canopy transport times (τ) and propagate theiruncertainty from the individual errors of the Tn concentration measurements.A quality assessment of τ for the field experiment during a period of51 days revealed good data quality with 44% of the relativeuncertainties below 50%. The occurrence of transport time uncertaintieshigher than 100% was caused by absolute Tn gradients lower than70 Bq m?3 m?1, which was found for 22% of all determinedtransport times. In addition, the method was found to be highly sensitive tothe Tn concentrations at the upper of the two inlet heights (z_u).Low values of C_Tnzu result in high absoluteuncertainties of the transport time. A comparison with empiricalparameterizations revealed a much lower scatter for the τ valuesdetermined from our measurements. We found an excellent agreement with τ values obtained by the in-canopy resistance approach used, e.g., in theSURFATM model during daytime, while the SURFATM model significantlyoverestimated transport times during nighttime.