Measurements of gaseous elemental mercury (GEM) in diffuse soil emission using the static closed-chamber method

Measurements aimed to evaluate the contribution of natural and anthropogenic gaseous elemental mercury (GEM) to the atmosphere are critical for monitoring and mitigating the impact of this highly toxic air contaminant. Notwithstanding, the amounts of GEM originating from different sources are difficult to be estimated, since specific protocols dictating the most appropriate sampling and analytical techniques are lacking. This study was aimed to test a methodological approach enables to measure GEM released through diffuse degassing from the soil, a mechanism that in volcanic and geothermal areas significantly contributes to the discharge of geogenic fluids to the atmosphere. A static closed-chamber (SCC), consisting of a plastic cylinder with a basal area of 201 cm2 and an inner volume of 1,810 cm3, was used in combination with a Lumex® (RA-915M). This instrument, i.e. a portable atomic absorption spectrometer with Zeeman effect, is able to measure Hg0 at high frequency, in real-time and at a wide range of concentrations (from 2 to 50,000 ng/m3). From an operative point of view, four aliquots of the soil gas accumulated in the SCC, were collected at fixed time intervals (when the SCC was positioned on the ground, and after 1, 2 and 3 min), through a pierceable rubber positioned on its top using a 60 cc syringe. The syringe was then connected to the Lumex inlet port through a three-way Teflon valve to allow the free entrance of air during the syringe injection. This external device was necessary to maintain stable the operative flux of the Lumex at 10 L/min during the injection operation, in order to prevent possible noise signals related to flux changes. Moreover, a carbon trap was set at the free air entrance to stabilize the baseline, which otherwise would be unstable, being dependent on the variable GEM concentrations in air. The amount (in ng) of GEM of each injection (KSYR) was calculated on the basis of a calibration curve previously constructed measuring known amounts of GEM saturated gas that were injected in the Lumex port set with the three-way valve device using a micro-syringe. By multiplying the KSYR values for the ratio between the volumes of (i) the syringe and (ii) the SCC, the amount of GEM present in the SCC (KSCC) during each sampling time-series was obtained. GEM fluxes are proportional to the variation in time of GEM amounts in the SCC, as follow: f(X) = (dKSCC/dt)/A, where A is the basal area of the SCC. This method was applied at Solfatara crater (Campi Flegrei, southern Italy), a hydrothermally altered tuff cone characterized by an anomalous diffuse emission of geogenic gases from the soil. The 214 measured fluxes, varying in a wide range of values (4 orders of magnitude), showed that the method was highly sensitive (detection limit: 5,000 ng m-2 day-1) and reproducible (±5 %).