Natural Surface Emissions Dominate Anthropogenic Emissions Contributions to Total Gaseous Mercury (TGM) at Canadian Rural Sites

Abstract. The Canadian Air and Precipitation Monitoring Network (CAPMoN) measures total gaseous mercury (TGM) at three rural-remote sites. Long-term TGM, ancillary measurements and the Positive Matrix Factorization (PMF) model were used to assess temporal changes in anthropogenic and natural surface emission (wildfires plus re-emitted Hg) contributions to TGM and examine the emission drivers of the observed TGM trends between 2005 and 2018. TGM showed decreasing trends at the three sites; the magnitudes (ng m^-3 yr^-1) were -0.050 at Saturna for 2010–2015, -0.026 at Egbert for 2005–2018, and -0.014 at Kejimkujik for 2005–2016. The increasing contributions from natural surface Hg emissions at Saturna (1.64 % yr^-1) and Kejimkujik (1.03 % yr^-1) resulted from declining anthropogenic Hg emissions and increasing oceanic and terrestrial Hg re-emissions. The mean relative contributions of natural surface emissions to annual TGM were 65 %, 72.5 % and 65 % at Saturna, Egbert and Kejimkujik. TGM at Saturna were mainly from background Hg (53 %), Hg re-emissions (14 %), and shipping (10 %); at Egbert, from background Hg (63 %), Hg re-emissions (15 %), and crustal/soil dust (9 %); and at Kejimkujik, from background Hg (71 %), regional point source emissions (10 %), and Hg re-emissions (8 %). Local combustion sources contributed a few percent of the annual TGM, while the percentage from oceanic Hg evasion was 6.6–9.5 % for the two coastal sites. Wildfire impacts on annual TGM were 5.6 % at Saturna, 1.3 % at Egbert, and 2.1 % at Kejimkujik. Background Hg contributions to TGM were greater in the cold season, whereas wildfire and surface re-emission contributions can be significant in the warm season.