Measurements of fractionated gaseous mercury concentrations over northwestern and central Europe, 1995-99

Although it makes up only a few per cent, of total gaseous mercury (TGM) in the atmosphere, the fraction of oxidised (divalent) mercury plays a major role in the biogeochemical cycle of mercury due to its high affinity for wa ter and surfaces. Quantitative knowledge of this fraction present in mixing ratios in the parts-per-10(15) (ppq) range is currently very scarce. This work is based on approximate to 220 data for divalent gaseous mercury (DGM) collected during 1995-99 in ambient air. Over the course of the measuremen ts, the sampling and analytical methods were modified and improved. This is described here in detail and includes transition from wet leaching and red uction procedures to thermo-reductive desorption, the use of annular as wel l as tubular denuders and adoption of an automated sampling system. The con centration of DGM exhibited a strong seasonal behaviour in contrast to atom ic gaseous mercury, with low values in winter and maximum values in summer. The DGM/TGM ratios were frequently found to be below the detection limit ( less than or equal to 1%) and in the range 1-5%. A trend of diurnal DGM pat terns was observed and implies photolytically induced sources. Scavenging o f DGM during rain events was also noticed.