| 其他摘要 | Mercury (Hg) is a global distributed persistent toxic pollutant. In order to protect the human health and the environment from anthropogenic emissions and release of mercury and mercury compounds, the UNEP Minamata Convention on Mercury was signed by 94 countries in the October of 2013, whereof China was one of the most important signatories. Anthropogenic and natural emission constitute the sources of atmospheric Hg, where natural emission was estimated up to two times of anthropogenic emission. Accurate estimate natural Hg emission inventory is essential to understand the environmental effect of global Hg reduction. However, due to the complicated mechanisms in the air-surface exchange of Hg and high uncertainty in the measurement techniques, here is remain strong debate in the natural Hg emission for two decades. In this study, we developed a novel dynamic flux chamber, evaluated the comparability and uncertainties of contemporary Hg flux measurement techniques, investigated Hg emission from a typical oilseed-rice rotated agricultural land and developed empirical models using laboratory factorial experiments and field data. The main conclusions can be summarized as follow: (1). we performed an integrated modeling, laboratory and field study to design a DFC capable of producing a steady and uniform air flow over a flat surface. Laboratory flux measurements demonstrated that the new design improves data reproducibility as compared to a conventional DFC, and reproduces the model-predicted flux trend with increasing sampling flow rates. A mathematical relationship between the flushing flow rate and surface friction velocity, a variable commonly parameterized in atmospheric models, was developed for field application. For the first time, the internal shear property of a DFC can be precisely controlled using the sampling flow rate, and the flux under atmospheric condition can be inferred from the measured flux and surface shear properties. (2). we deployed a collocated set of micro-meteorological (MM) and enclosure measurement systems to quantify Hg0 flux over bare soil and low standing crop in an agricultural field. The techniques include relaxed eddy accumulation (REA), modified Bowen-ratio (MBR), aerodynamic gradient (AGM) as well as dynamic flux chambers of traditional (TDFC) and novel (NDFC) designs. Fluxes measured by the MM and DFC methods showed distinct temporal trends. The former exhibited a highly dynamic temporal variability while the latter had much gradual temporal features. The correlations between NDFC and TDFC fluxes and between MBR and AGM fluxes were significant, but the correlation between DFC and MM instantaneous fluxes were from weak to moderate. Statistical analysis indicated that the median of turbulent fluxes estimated by the three independent MM-techniques were not significantly different. Cumulative flux measured by TDFC is considerably lower (42% of AGM and 31% of MBR fluxes) while those measured by NDFC, AGM and MBR were similar (～5.3% difference). This implicates that the NDFC technique, which accounts for internal friction velocity, effectively bridged the gap in measured Hg0 flux compared to MM techniques. Due to the restriction of REA resolution, gradient methods was suggested to measure the low stand ecosystem flux. (3). during the comparison, relative bias for DFC-derived fluxes was estimated to be ~ ±10%, although ~85% of the flux observations within ±2 ng m-2 h-1 in absolute term. The DFC flux bias showed a diurnal pattern similar to those of temperature and sunlight. For MM methods, the quantification of concentration difference (delta C) and micrometeorological conditions (e.g., the level of atmospheric turbulence) is the most critical. The precision in measurements was estimated to be 0.064 ng m-3 for MBR and AGM method. For Hg0-REA system, delta C was depending on Hg0 concentration (C) at 0.069+0.022C. The estimated flux uncertainties for the triad of MM-techniques were 16 - 27%, 12 - |
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