A dual-inlet, single detector relaxed eddy accumulation system for long-term measurement of mercury flux

Abstract

Abstract. The fate of anthropogenic emissions of mercury (Hg) to the atmosphere is influenced by the exchange of elemental Hg with the earth surface. This exchange holds the key to a better understanding of Hg cycling from local to global scales, which has been difficult to quantify. To advance research about land–atmosphere Hg interactions, we developed a dual-inlet, single detector relaxed eddy accumulation (REA) system. REA is an established technique for measuring turbulent fluxes of trace gases and aerosol particles in the atmospheric surface layer. Accurate determination of gaseous elemental mercury (GEM) fluxes has proven difficult due to technical challenges presented by extremely small concentration differences (typically < 0.5 ng m−3) between updrafts and downdrafts. We present an advanced REA design that uses two inlets and two pairs of gold cartridges for continuous monitoring of GEM fluxes. This setup reduces the major uncertainty created by the sequential sampling in many previous designs. Additionally, the instrument is equipped with a GEM reference gas generator that monitors drift and recovery rates. These innovations facilitate continuous, autonomous measurement of GEM flux. To demonstrate the system performance, we present results from field campaigns in two contrasting environments: an urban setting with a heterogeneous fetch and a boreal peatland during snowmelt. The observed average emission rates were 15 and 3 ng m−2 h−1, respectively. We believe that this dual-inlet, single detector approach is a significant improvement of the REA system for ultra-trace gases and can help to advance our understanding of long-term land–atmosphere GEM exchange.

Global observations and modeling of atmosphere–surface exchange of elemental mercury: a critical review

“…The REA method has been deployed for flux measurements for agricultural lands, forest canopies, wetlands and urban settings (Cobos et al, 2002;Olofsson et al, 2005;Bash and Miller, 2009;Osterwalder et al, 2016;Sommar et al, 2013b). The AGM method has been used over grasslands, agricultural lands, salt marshes, landfills and snow (Lee et al, 2000;Kim et al, , 2003Fritsche et al, 2008b, c;Baya and Van Heyst, 2010).…”

Fluxes of gaseous elemental mercury (GEM) in the High Arctic during atmospheric mercury depletion events (AMDEs)

“…Phys. Discuss., https://doi.org /10.5194/acp-2017- (Osterwalder et al, 2016). Furthermore, Osterwalder et al (2016) found significant difference between GEM fluxes during unstable, stable, and neutral conditions with a near-zero flux during stable conditions, emission during unstable conditions and deposition during neutral conditions.…”

“…The sampling inlets are located 5.69 m above ground. Osterwalder et al (2016) and Zhu et al (2015b) describe the advantages of using dual inlets, where temporally synchronous concentration determination of updrafts and downdrafts is the most obvious advantage. The Teflon tubes were heated to 50°C and each tube is connected 30 to a three-way valve, which can either collect sample air or zero air.…”

“…Thus, GEM can be transported over longer distances to areas with 25 low natural and anthropogenic emissions. GEM concentrations in the Arctic are mainly due to long-range transportation from lower latitude sources (Dastoor et al, 2008;Pandey et al, 2011;. However, in the Arctic, during polar spring, the lifetime of GEM can be as short as roughly 10 hours Steffen et al, 2008;.…”

“…The relaxed eddy accumulation (REA) method (Businger and Oncley, 1990) is used for the flux measurement in a setup with a dual inlet (Cobos et al, 2002;Osterwalder et al, 2016) and dual detectors. GEM fluxes have been determined with REA previously over agricultural soil (Cobos et al, 10 2002), in a winter wheat cropland (Sommar et al, 2013;Zhu et al, 2015a), in an urban environment, and in boreal peatland (Osterwalder et al, 2016), but never in the Arctic. At the beginning of the measuring period at the end of April, the snow depth was 1.02-1.03 m. Little precipitation was observed and the snow depth varied between 0.94 m and 1.09 m during the campaign.…”