Figure. Schematic representation of the REA-CVAFS-measuring complex for vertical Hg0 turbulent flux (not to scale).

Mercury (Hg) is a global threat to human and environmental health. This fall numerous countries including China agreed to the UN’s Convention for reducing the emission and use of Hg. Important as this treaty is, it will neither restore the toxic legacy of past Hg emissions nor end the exposure of millions of people to harmful levels of this potent neurotoxin. The emissions, transport, transformation, and deposition continuously cycles Hg in different environmental compartments. Being volatile and rather chemical inert in elemental form, Hg⁰ undergoing atmospheric transport over large spatial scales that results in a rather homogeneous air mixing ratio over each hemisphere. Nevertheless, heavy industrialised regions or regions with geochemical deposits of Hg experience of course elevated air Hg concentration levels.

Atmospheric Hg interacts with terrestrial ecosystems, especially topsoil and aerial parts of vegetation, which are regarded as effective carriers of atmospheric Hg deposition. To better understand the biogeochemical cycling of Hg, it is important to quantify air-vegetation exchange of Hg⁰. Conventionally, the Hg⁰ gas exchange (flux, mass per surface and time unit) has been inferred from measurements of vertical concentration gradients. However, in many situations a technique which derives the flux from measurements at a single height would be advantageous. Therefore, members of the FENG Xinbin group at the State Key Laboratory of Environmental Geochemistry (SKLEG) of IGCAS, have developed and designed an apparatus (c.f. Figure) relying on relaxed eddy accumulation (REA) methodology for continuous-field measurements of vertical Hg⁰ fluxes over cropland ecosystems. This REA system is able to segregate turbulent air parcels flowing past a sampling point into up- and downdraft samples based on co-located measurements of vertical wind speed with a frequency resolution of 10 Hz. The up- and downdraft samples are chemically analysed on-line by an automated Hg⁰ gas analyser (cold vapour atomic fluorescence spectrophotometer, CVAFS) to obtain the concentration difference  and based on the equation  , the vertical flux (F) is calculated for an adequate averaging time (typically 20-30 min). (Note: β is the relaxation coefficient determined experimentally and σ_w is the standard deviation of vertical wind velocity.)

In the paper published in Tellus, Series B: Chemical and Physical Meteorology, the researchers introduce the configuration of our REA system and describe the coupling to a CVAFS instrument and describe the procedures for measurement of Hg⁰ fluxes based on investigations with respect to theoretical, methodological, environmental and instrumental requirements and limitations.

The Tellus B paper is related to the following publications:
1) Lin, C.-J.; Zhu, W.; Li, X.; Feng, X.; Sommar, J.; Shang, L., Novel Dynamic Flux Chamber for Measuring Air-Surface Exchange of Hg⁰ from Soils. Environ. Sci. Technol. 2012, 46, (16), 8910-8920.
2) Zhu, W.; Sommar, J.; Li, Z.; Feng, X.; Lin, C. J.; Li, G., Highly elevated emission of mercury vapor due to the spontaneous combustion of refuse in a landfill. Atmos. Environ. 2013, 79, 540-545.
3) Sommar, J.; Zhu, W.; Lin, C. J.; Feng, X., Field approaches to measure Hg exchange between natural surfaces and the atmosphere - A review. Crit. Rev. Environ. Sci. Technol. 2013, 43, 1657-1739.

(By Jonas Sommar)