DETERMINATION OF GASEOUS AND PARTICULATE SELENIUM OVER A RURAL GRASSLAND IN THE UK

The atmosphere is identified as playing an important role in the mass balance of selenium (Se) in grassland ecosystems, potentially governin g the critical levels for grazing livestock. This paper presents the f irst known measurements of gas phase Se in the U.K. atmosphere along w ith simultaneous particulate measurements. Sampling and analytical det ermination are difficult and detailed consideration was therefore give n to method development. Particulate Se was trapped on 0.45 mu m Milli pore HA filters using a low-velocity sampling method and determination by acid digestion and hydride atomic absorption. Gaseous Se is partic ularly difficult to trap and charcoal and nitric acid methods were com pared. It was found that nitric acid was the most favourable for our p urposes, primarily because of compatibility with analysis by atomic ab sorption. Total concentrations ranged from similar to 0.5 to 4.0 ng Se m(-3), with similar to 20-50% present in the gas phase (i.e. < 0.45 m u m fraction). Simultaneous gas-particle partitioning was undertaken o n a transect of 0.1, 0.35 and 3.00 m above a grassland surface in rura l northwest England. Despite analytical difficulties, these results we re used to estimate volatilisation fluxes at similar to 100-200 mu m(- 2) yr(-1), whilst dry deposition, calculated from the dry deposition v elocity (V-g) of particulate Se, contributed ca. 2 mu g m(-2) yr(-1). Previous investigations have indicated that there has been a long-term increase of Se in other agricultural soils in the U.K. This implies t hat wet deposition far exceeds dry deposition of Se, probably represen ting an input in excess of 200 mu g m(-2) yr(-1).