Sources, fate and transport of atmospheric mercury from Asia

Asian anthropogenic emissions of mercury to the atmosphere contribute 54% of all anthropogenic emissions. For this reason, it is important to understand how these emissions impact both the regional and global mercury cycle. The largest share of mercury emissions in Asia is due to coal combustion and smelting. Approximately half of the Asian anthropogenic emissions are emitted as Hg-0. The remainder are emitted as gaseous Hg2+ compounds and particle-bound Hg (PHg). These latter forms of Hg are susceptible to relatively rapid removal from the atmosphere. The Asian emissions lead to very high concentrations of airborne Hg in urban and rural areas of Asia. Modelling the fate and transport of the Asian emissions using the Goddard Earth Observing System (GEOS)-Chem model of global tropospheric chemistry shows that these emissions are associated with high deposition of Hg within Asia, but there is little data with which to evaluate this model result. Observations downwind of Asia show that Hg-0 is the dominant form in Asian plumes, with generally small, but variable amounts of other forms. Thus we conclude that most of the Asian Hg2+ and PHg have been removed in the source region. In addition, downwind observations show that total emissions are significantly larger than indicated by the anthropogenic emission inventory. This likely reflects either an underestimate of the anthropogenic emissions or emissions of Hg from land. Plumes containing enhancements in Asian Hg-0, but not other species, have been detected as far away as North America. Because only Asian Hg-0 is transported long distances, the deposition is distributed relatively uniformly in the northern hemisphere. In North America, Asian anthropogenic emissions account for 7-20% of all deposition, with an average of 16%.