PBL Chemistry and processes: impact on Mercury deposition over Marine and Coastal Areas

Mercury (Hg) is a global health threat due to its toxicity and ubiquitous presence in all environmental compartments. Its elemental form, Gaseous Elemental Mercury (GEM or Hg 0), reacts relatively slowly with the major oxidants present in the atmosphere, and therefore can be transported long distances before being oxidised to readily deposited HgII compounds. Within the biogeochemical cycle of Hg, oceans are by far the greatest source of Hg to the atmosphere. A number of model and field studies suggest that more than 5000 Mg of Hg escape from oceans every year [1]. A two layer gas exchange model has been proposed to describe the evasion of Hg0 from the marine environment to the atmosphere, although the knowledge of the exact mechanism is still lacking. The fate of Hg emitted from the oceans depends on several factors and processes occurring within the Planetary Boundary Layer (PBL). Bromine compounds are released from sea salt aerosols and it has been proposed that Br is the major oxidant of mercury within the Marine Boundary Layer (MBL) [2]. This potentially means that Hg is rapidly oxidised in the MBL and coastal regions, thus resulting in significant Hg deposition over marine and coastal areas worldwide. The dry deposition of Hg0 is uncertain, due the difficulties in quantifying the deposition velocities of this species over different types of land surface / vegetation [3]. It has only recently been included in some models, and leads to a modelled atmospheric lifetime for Hg of between 0.5 and 0.7 years, which is shorter than previous estimates. Moreover its inclusion has an impact on the geographical distribution of the Hg deposition flux, and likely enhances fluxes near emissions sources. In this study we use the global Hg chemical transport model ECHEMERIT to trace Hg from the oceans to final receptors. Different Hg oxidation mechanisms, one based on Bromine and another using a combination of O3 and OH, as well as different deposition schemes, were investigated to assess how they influence the final deposition of Hg over land areas closest to marine environments.