Abstract
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.
Highlights
The volatility of atomic mercury (Hg0) adds to the complexity of the element biogeochemical cycle
This study indicates that flux-gradient-based techniques (MBR and aerodynamic gradient method (AGM)) are preferable to relaxed eddy accumulation (REA) in quantifying Hg0 flux over ecosystems with low vegetation height
Turbulent Hg0 fluxes determined by Eqs. (3)–(5) include parameters derived from OPEC flux, whose precision improves by a factor of 1 √taverage by increasing the flux averaging time
Summary
The volatility of atomic mercury (Hg0) adds to the complexity of the element biogeochemical cycle. An advantage of the MM techniques compared to chambers is the measurement under undisturbed conditions. This implies practical disadvantages in that Hg0 has to be detected at ambient level, and in that small temporal concentration fluctuations or vertical gradients have to be resolved. A DFC system derives flux from a steady-state mass balance and, after deployment, there is build-up of an excess (or deficit) of Hg0 concentration inside the enclosure compared to ambient air. DFCs of different sizes, shapes and operation flow rates yield different Hg0 fluxes under identical environmental conditions (Wallschläger et al, 1999; Zhang et al, 2002; Eckley et al, 2010). It is important to characterize the effects of enclosure on the microclimate over diurnal cycles, temperature and radiation balance, that may lead to erroneous flux quantification as observed for other trace gases (Denmead, 2008)
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