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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
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Volume 15, issue 10 | Copyright
Atmos. Chem. Phys., 15, 5359-5376, 2015
https://doi.org/10.5194/acp-15-5359-2015

Research article 18 May 2015

Research article | 18 May 2015

Mercury vapor air–surface exchange measured by collocated micrometeorological and enclosure methods – Part II: Bias and uncertainty analysis

W. Zhu1,2, J. Sommar1, C.-J. Lin1,3,4, and X. Feng1 W. Zhu et al.
• 1State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550002, China
• 2University of Chinese Academy of Sciences, Beijing 100049, China
• 3Department of Civil Engineering, Lamar University, Beaumont, TX 77710, USA
• 4College of Environment and Energy, South China University of Technology, Guangzhou 510006, China

Abstract. Dynamic flux chambers (DFCs) and micrometeorological (MM) methods are extensively deployed for gauging air–surface Hg0 gas exchange. However, a systematic evaluation of the precision of the contemporary Hg0 flux quantification methods is not available. In this study, the uncertainty in Hg0 flux measured by the relaxed eddy accumulation (REA) method, the aerodynamic gradient method (AGM), the modified Bowen ratio (MBR) method, as well as DFC of traditional (TDFC) and novel (NDFC) designs, are assessed using a robust data set from two field intercomparison campaigns.

The absolute precision in Hg0 concentration difference (ΔC) measurements is estimated at 0.064 ng m−3 for the gradient-based MBR and AGM systems. For the REA system, the parameter is Hg0 concentration (C) dependent at 0.069 + 0.022C. During the campaigns, 57 and 62 % of the individual vertical gradient measurements are found to be significantly different from 0, while for the REA technique, the percentage of significant observations is lower. For the chambers, non-significant fluxes are confined to a few night-time periods with varying ambient Hg$^{0}$ concentrations. Relative bias for DFC-derived fluxes is estimated to be ~ ±10, and ~ 85% of the flux bias is within ±2 ng m−2 h−1 in absolute terms. The DFC flux bias follows a diurnal cycle, which is largely affected by the forced temperature and irradiation bias in the chambers. Due to contrasting prevailing micrometeorological conditions, the relative uncertainty (median) in turbulent exchange parameters differs by nearly a factor of 2 between the campaigns, while that in ΔC measurement is fairly consistent. The estimated flux uncertainties for the triad of MM techniques are 16–27, 12–23 and 19–31% (interquartile range) for the AGM, MBR and REA methods, respectively. This study indicates that flux-gradient-based techniques (MBR and AGM) are preferable to REA in quantifying Hg0 flux over ecosystems with low vegetation height. A limitation of all Hg0 flux measurement systems investigated is their inability to obtain synchronous samples for the calculation of ΔC. This reduces the precision of flux quantification, particularly in the MM systems under non-stationarity of ambient Hg0 concentration. For future applications, it is recommended to accomplish ΔC derivation from simultaneous collected samples.