Decline in atmospheric mercury deposition in London.

Bulk atmospheric deposition samples were continuously collected using a standard IVL-type mercury (Hg) bulk deposition collector from January 1999 to December 2005 in order to monitor Hg deposition in London. The volume-weighted annual Hg concentrations in deposition gradually declined from 76.0 ng L(-1) in 1999 to 43.8 ng L(-1) in 2005. Correspondingly, Hg fluxes in deposition declined from 45.3 microg m(-2) yr(-1) in 1999 to 15.0 microg m(-2) yr(-1) in 2005. However, this decline in Hg deposition does not agree with trends in UK Hg emissions which are relatively stable over the sampling period. Comparison with contemporaneous data collected at Lochnagar, a remote site in Scotland, suggests that the high Hg concentrations in London deposition are likely to be due to local or regional sources. Surface sediments taken from lakes across London show that the environment has been heavily contaminated by Hg and suggest that Hg re-emission from depositional sinks (e.g. soils, water bodies) may be an important source to London's atmosphere, thereby delaying response to the major reductions in direct emissions to the atmosphere since the 1970s.