Determination of gaseous and particulate selenium over a rural grassland in the U.K.

Abstract

The atmosphere is identified as playing an important role in the mass balance of selenium (Se) in grassland ecosystems, potentially governing the critical levels for grazing livestock. This paper presents the first known measurements of gas phase Se in the U.K. atmosphere along with simultaneous particulate measurements. Sampling and analytical determination are difficult and detailed consideration was therefore given to method development. Particulate Se was trapped on 0.45 μm Millipore HA filters using a low-velocity sampling method and determination by acid digestion and hydride atomic absorption. Gaseous Se is particularly difficult to trap and charcoal and nitric acid methods were compared. It was found that nitric acid was the most favourable for our purposes, primarily because of compatibility with analysis by atomic absorption. Total concentrations ranged from 0.5 to 4.0 ng Se m−3, with 20–50% present in the gas phase (i.e. < 0.45 μm fraction). Simultaneous gas-particle partitioning was undertaken on a transect of 0.1, 0.35 and 3.00 m above a grassland surface in rural northwest England. Despite analytical difficulties, these results were used to estimate volatilisation fluxes at 100–200μg m−2 yr−1, whilst dry deposition, calculated from the dry deposition velocity (Vg) of particulate Se, contributed ca. 2 μg 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 that wet deposition far exceeds dry deposition of Se, probably representing an input in excess of 200 μg m−2 yr−1.