Abstract
The mechanisms triggering the large variations in the mercury (Hg) multiple isotopic compositions of atmospheric particle-bound Hg worldwide still remain unclear. The comparison of Hg isotopic compositions in aerosols collected in urban and rural areas may help distinguish the effects of natural processes from those of anthropogenic inputs. We thus investigated the Hg isotopic compositions of PM10 aerosols collected seasonally during 2015 at two monitoring stations on Montreal Island, one located downtown and the other in its westernmost subrural part, barely impacted by the city anthropogenic emissions. Our results show that, while Hg isotopic compositions present no seasonality at the urban station, possibly due to constant anthropogenic emissions, the subrural samples display clear seasonal variations, with higher Δ199Hg and Δ200Hg values (up to 0.77 and 0.12‰, respectively) during summer and close to 0‰ during the rest of the year, that cannot solely be explained by anthropogenic primary emissions. Besides, Δ200Hg measured in the subrural aerosols display a positive correlation with O3 suggesting the implication of secondary processes involving ozone. We propose that the significant summer shift in the multiple Hg isotopic compositions may reflect a transition in the corresponding Hg0 oxidation pathway, from halogens-triggered to ozone-dominated reactions. Still, this hypothesis needs to be further tested. Nevertheless, it demonstrates that Hg isotopes are effective at characterizing secondary processes that control its atmospheric budget, even at a local scale (i.e., urban vs subrural) and could thus be used to better constrain its atmospheric chemistry in various environments.
Highlights
Mercury (Hg) is a toxic element that has the ability to be remobilized at large scales in the atmosphere following successive oxidation and reduction reactions
Our study demonstrates that the current scheme for atmospheric Hg that only considers anthropogenic emissions, oxidation by halogen atoms and photoreduction cannot account for the whole range of Hg multi-isotopic compositions we measured in atmospheric particles bound mercury (PBM) in Montreal
Our results demonstrate that an approach coupling Hg isotopic compositions and chemistry discriminates the different oxidation mechanisms controlling the Hg atmospheric budget
Summary
Mercury (Hg) is a toxic element that has the ability to be remobilized at large scales in the atmosphere following successive oxidation and reduction reactions. Gaseous elemental mercury (Hg0), the dominant Hg form in the atmosphere, is relatively stable and has a relatively long lifetime of 0.5 to 1 year that allows it to be transported over long distances worldwide (Selin 2009). HgII may be transformed into methylmercury in aquatic system (MeHg+) that can bioaccumulate in living aquatic organisms and impact Human health (Sunderland 2007). This emphasizes the need to better constrain the different oxidation pathways that Hg0 undergoes in the atmosphere. The dominant oxidation and reduction mechanisms controlling atmospheric Hg, and their respective reaction rates, are still subject to debate (Saiz-Lopez et al, 2018) as discrepancies between the observed and modelled residence time and spatiotemporal distribution of Hg0 remain (Horowitz et al, 2017)
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