A dual, 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 which holds the key to a better understanding of Hg cycling from local to global scales has been difficult to quantify. To advance and facilitate research about land–atmosphere Hg interactions, we developed a dual-intake, single analyzer 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 to technical challenges presented by extremely small concentration differences (typically < 0.5 ng m−3) between updrafts and downdrafts. To address this we present an advanced REA design that uses two inlets and two pair of gold cartridges for semi-continuous monitoring of GEM fluxes. They are then analyzed sequentially on the same detector while another pair of gold cartridges takes over the sample collection. We also added a reference gas module for repeated quality-control measurements. 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 mire during snow-melt. The observed emission rates were 15 and 3 ng m−2 h−1. We claim that this dual-inlet, single detector approach is a significant development of the REA system for ultra-trace gases and can help to advance our understanding of long-term land–atmosphere GEM exchange.