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

Abstract

<p><strong>Abstract.</strong> Current understanding of mercury (Hg) behaviour in the atmosphere contains significant gaps. Some key characteristics of Hg processes including anthropogenic and geogenic emissions, atmospheric chemistry, and air-surface exchange are still poorly known. This study provides a complex analysis of processes governing Hg fate in the atmosphere involving both measurement 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. Contribution of natural and secondary emissions enhances the south-to-north gradient but their effect is less significant. The atmospheric chemistry does not affect considerably both spatial distribution and temporal variation of GEM concentration in the surface air. On the other hand, RM air concentration and wet deposition are largely defined by oxidation chemistry. The Br oxidation mechanism allows successfully reproducing observed seasonal variation of the RM / GEM ratio in the near-surface layer, whereas it predicts maximum in wet deposition in spring instead of summer as observed at monitoring sites located in North America and Europe. Model runs with the OH chemistry correctly simulate both the periods of maximum and minimum values and the amplitude of observed seasonal variation but lead to shifting the maximum RM / GEM ratios from spring to summer. The O<sub>3</sub> 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.</p>

Recent Advances in Atmospheric Chemistry of Mercury

“…Despite theoretical doubts concerning the thermal stability of gaseous HgOH and HgO products, many modeling results using O 3 /OH as the main atmospheric Hg oxidants showed good agreement with the observed Hg 0 concentration and wet deposition flux. However, more complex atmospheric Hg oxidation reactions, particularly heterogeneous reactions involving O 3 and OH, have been implied in recent studies ( [57] and references therin).…”

“…Travnikov et al [57] compared simulation results from four state-of-the-art chemical transport models with field data from various global and regional monitoring networks. It was found that models using the Br oxidation mechanism correctly simulated the observed seasonal variation of the Hg II /Hg 0 ratio in the near-surface layer but failed to predict the observed wet deposition maximum in summer at monitoring sites in North America and Europe.…”

Sensitivity model study of regional mercury dispersion in the atmosphere

“…Some of these tests have been studied for other regions (e.g. Travnikov et al, 2016 andBieser et al, 2016) while many other studies have investigated Hg oxidation by ozone or Br (Hynes et al, 2009;Subir et al, 2011Subir et al, , 2012WeissPenzias et al, 2015).…”

“…In a recent review Ariya et al (2015) give a exhaustive summary of modelling progress and of the uncertainties still present concerning the atmospheric Hg cycle. To date only a limited number of model-to-model intercomparisons have been carried out (for the US: Bullock et al, 2008Bullock et al, , 2009Zhang et al, 2012;for Europe: Ryaboshapko et al, 2007; for global models: Travnikov et al, 2010Travnikov et al, , 2016AMAP/UNEP, 2013), where it was found that are often significant differences in Hg concentrations and deposition estimated by different models. Previous European studies (Ryaboshapko et al, 2007) performed a model intercomparison for the year 1999, using eight different models and data from 11 measurement stations with the aim to characterise the ability of CTMs to predict atmospheric Hg concentration and deposition fields.…”

“…The work presented here was performed in the framework of the GMOS Mercury Modelling Task Force (MMTF; Travnikov et al, 2016).…”