Estimates of Surface Re-Emissions, Wildfire and Anthropogenic Contributions to Total Gaseous Mercury (TGM) Using Positive Matrix Factorization Modeling

Mercury (Hg) is a hazardous pollutant known for causing severe neurological and reproductive damage. Reducing Hg emissions to air is imperative for these reasons with many countries implementing domestic regulations and supporting international treaties notably the Minamata Convention on Mercury. Environment and Climate Change Canada’s Canadian Air and Precipitation Monitoring Network (CAPMoN) measures total gaseous mercury (TGM) at three regionally representative sites in the western, southeastern, and Atlantic Canada regions. Using the Positive Matrix Factorization (PMF) model and long-term TGM and supplementary observational data, we estimated the anthropogenic and natural surface emission (geogenic and re-emitted Hg) contributions to TGM over various time scales. Trends analyses were applied to TGM concentrations and the PMF-derived source contributions to examine potential causes of the long-term TGM trends. TGM source attribution was conducted for the sites at Saturna, British Columbia; Egbert, Ontario; and Kejimkujik National Park, Nova Scotia. In addition to TGM, ancillary measurements of inorganic ions, SO₂, CO, total carbon, and air temperature were obtained from CAPMoN and other monitoring networks. The datasets were inputted into the PMF model for estimating factor profiles and contributions. At Saturna, natural surface emission contributions to annual TGM accounted for 65% with the remainder from anthropogenic emission contributions. These contributions include terrestrial surface re-emissions (14%), oceanic evasion (9.5%), wildfires (5.6%), and a portion of background Hg (36%). The remaining anthropogenic contributions comprise shipping emissions and sea-salt processing (10%), secondary sulfate (4.6%), local combustion (3.8%), and a portion of background Hg (17%). The TGM trend was -0.05 ng m^-3 yr^-1 during 2010-2015. The negative trend was attributed to decreasing Hg emissions from shipping and terrestrial re-emissions. At Egbert, natural surface emission contributions to annual TGM were 72.5%, comprising terrestrial re-emissions (15%), crustal/soil dust (9%), road salt (4%), wildfires (1%), and a portion of background Hg (43%). Anthropogenic contributions were mainly from local combustion (6%), secondary sulfate (2%), and a portion of background Hg (20%). The TGM trend was -0.026 ng m^-3 yr^-1 during 2005-2018 with the decline in background Hg contributions having the most impact on the long-term trend. At Kejimkujik, 65% of the annual TGM was apportioned to natural surface emissions, including from terrestrial re-emissions (8%), oceanic evasion (7%), wildfires (2%), and a portion of background Hg (49%). The remaining anthropogenic contributions comprise regional emissions and sea-salt processing (10%), local combustion (2%), secondary sulfate (1%), and a portion of background Hg (22%). The TGM trend was -0.014 ng m^-3 yr^-1 during 2005-2016. Decreasing contributions from background Hg, regional emissions, and terrestrial re-emissions were the main drivers of the long-term trend. Local combustion sources do not contribute significantly to annual TGM or long-term trends at the three sites. The study highlights the importance of reducing background Hg by way of reducing global Hg emissions. Re-emitted Hg from land and aquatic surfaces are also important sources that will become increasingly significant as anthropogenic Hg emissions are reduced. Wildfire impacts on annual TGM are on the order of a few percent; however, it can reach as high as 10% of the TGM in the warm season at Saturna. As global temperatures continue to rise, the impact on TGM from wildfire and surface re-emissions is likely to amplify.