A whole-air relaxed eddy accumulation measurement system for sampling vertical vapour exchange of elemental mercury

An apparatus relying on relaxed eddy accumulation (REA) methodology has been designed and developed for continuous-field measurements of vertical Hg⁰ fluxes over cropland ecosystems. This micro-meteorological technique requires sampling of turbulent eddies into up- and downdraught channels at a constant flow rate and accurate timing, based on a threshold involving the sign of vertical wind component (w). The fully automated system is of a whole-air type drawing air at a high velocity to the REA sampling apparatus and allowing for the rejection of samples associated with w-fluctuations around zero. Conditional sampling was executed at 10-Hz resolution on a sub-stream by two fast-response three-way solenoid switching valves connected in parallel to a zero Hg⁰ air supply through their normally open ports. To suppress flow transients resulting from switching, pressure differentials across the two upstream ports of the conditional valves were minimised using a control unit. The Hg⁰ concentrations of the up- and downdraught channel were sequentially (each by two consecutive 5-minute gas samples) determined after enhancement collection onto gold traps by an automated cold vapour atomic fluorescence spectrophotometer (CVAFS) instrument. A protocol of regular reference sampling periods was implemented during field campaigns to continuously adjust for bias that may exist between the two conditional sampling channels. Using a 5-minute running average was conditional threshold, nearly-constant relaxation coefficients (β _s) of ~0.56 were determined during two bi-weekly field deployments when turbulence statistics were assured for good quality, in accordance with previously reported estimates. The fully developed REA-CVAFS system underwent Hg⁰ flux field trial runs at a winter wheat cropland located in the North China Plain. Over a 15-d period during early May 2012, dynamic, often bi-directional, fluxes were observed during the course of a day with a tendency of emission (Hg⁰ median flux of 77.1 ng m⁻² h⁻¹) during daytime and fluctuation around zero (Hg⁰ median flux of 9.8 ng m⁻² h⁻¹) during nighttime.