Natural surface emissions dominate anthropogenic emissions contributions to total gaseous mercury at Canadian rural sites

The Canadian Air and Precipitation Monitoring Network (CAPMoN) measures total gaseous mercury (TGM) at three rural–remote sites. Long-term TGM measurements, 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 all 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 (1.8 % yr−1) and Kejimkujik (0.3 % yr−1) resulted from declining anthropogenic Hg emissions and increasing oceanic re-emissions. The mean relative contributions of natural surface emissions to annual TGM were 71 %, 77.5 %, and 64 % at Saturna, Egbert, and Kejimkujik. TGM at Saturna was mainly from the Hg pool (50 %), terrestrial re-emissions (26 %), and shipping (10 %); at Egbert, from the Hg pool (66 %), terrestrial re-emissions (24 %), and crustal/soil dust (5 %); and at Kejimkujik, from the Hg pool (81 %), regional Hg emissions (11 %), and oceanic re-emissions (8 %). Local combustion contributed a few percent of the annual TGM, while the oceanic re-emission contribution was 6 %–8 % for the two coastal sites. Wildfire impacts on annual TGM were 3.7 % for Saturna and 1.6 % for Egbert. The Hg pool contributions to TGM were greater in the cold season, whereas wildfire and surface re-emission contributions can be significant in the warm season.