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| Titel |
Mercury vapor air–surface exchange measured by collocated micrometeorological and enclosure methods – Part II: Bias and uncertainty analysis |
| VerfasserIn |
W. Zhu, J. Sommar, C.-J. Lin, X. Feng |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 15, no. 10 ; Nr. 15, no. 10 (2015-05-18), S.5359-5376 |
| Datensatznummer |
250119729
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| Publikation (Nr.) |
 copernicus.org/acp-15-5359-2015.pdf |
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| Zusammenfassung |
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. |
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