Multi-model study of mercury dispersion in the atmosphere: Atmospheric processes and model evaluation

Current understanding of mercury (Hg) behaviour in the atmosphere contains significant gaps. Some key characteristics of Hgprocessesincludinganthropogenicandgeogenicemissions,atmosphericchemistry,andair-surfaceexchangearestillpoorly known.ThisstudyprovidesacomplexanalysisofprocessesgoverningHgfateintheatmosphereinvolvingbothmeasurement data from ground-based sites and simulation results of chemical transport models. A variety of long-term measurements of gaseous elemental Hg (GEM) and reactive Hg (RM) concentration as well as Hg wet deposition flux has been compiled from different global and regional monitoring networks. Four contemporary global-scale transport models for Hg were applied both in their state-of-the-art configurations and for a number of numerical experiments aimed at evaluation of particular processes. Results of the model simulation were evaluated against measurements. As it follows from the analysis the inter-hemispheric gradient of GEM is largely formed by the spatial distribution of anthropogenic emissions which prevail in the Northern Hemisphere.Contributionofnaturalandsecondaryemissionsenhancesthesouth-to-northgradientbuttheireffectislesssignificant. TheatmosphericchemistrydoesnotaffectconsiderablybothspatialdistributionandtemporalvariationofGEMconcentration in the surface air. On the other hand, RM air concentration and wet deposition are largely defined by oxidation chemistry. The BroxidationmechanismallowssuccessfullyreproducingobservedseasonalvariationoftheRM/GEMratiointhenear-surface layer, whereas it predicts maximum in wet deposition in spring instead of summer as observed at monitoring sites located in NorthAmericaandEurope.ModelrunswiththeOHchemistrycorrectlysimulateboththeperiodsofmaximumandminimum valuesandtheamplitudeofobservedseasonalvariationbutleadtoshiftingthemaximumRM/GEMratiosfromspringtosummer. The O3 chemistry does not provide significant seasonal variation of Hg oxidation. Thus, performance of the considered Hg oxidation mechanisms differs in reproduction of different observed parameters that can imply possibility of more complex chemistry and multiple pathways of Hg oxidation occurring concurrently in various parts of the atmosphere.