## 3 Results and Discussion
The comparison among Mn's AAQCs reveals that the AAQC for the TSP fraction is two times that of the AAQC for the PM\({}_{10}\) fraction and four times that of the AAQC for the PM\({}_{2.5}\) fraction. Therefore, the 24 h AAQC for the PM\({}_{2.5}\) fraction were calculated from the TSP fraction using a conversion factor of 4 as shown in Table S1. No target elements were found to exceed these calculated 24 h AAQCs (in ng/m\({}^{3}\)) for the PM\({}_{2.5}\) fraction in any of the air samples. Trace elements in PM\({}_{2.5}\) were routinely monitored at five stations in southern Ontario [6; 28]. Average concentrations from June 2015 to February 2018 for eight select elements at the Ring of Fire monitoring station were much lower than those observed in southern Ontario (Figure 5). Multiple-year measurements of trace elements in PM\({}_{2.5}\) showed that potential sources of these elements in southern Ontario included local and transboundary sources of coal combustion (enriched in sulphate, nitrate, and Se), non-ferrous metal smelting (As and Pb), and steel making (Fe, Mn and Zn) [31]. Fe is also emitted from traffic in urban communities like Toronto and Hamilton [21]. Ambient levels of As, Se, Pb, and Sb were found to be highly correlated when these elements were concurrently detected (Figure 6A-C), suggesting common emission sources. Worldwide, coal burning is an anthropogenic source of many toxic elements in air. [PERSON] et al. [39] previously reported good correlation between As and Se in PM\({}_{10}\) collected in Taiyuan where raw coals are largely produced and consumed. As, Se, and Sb were found in airborne particles near coal-fired power generation stations [40; 41]. Similarly, Mn and Fe were found to correlate well when both were detected (Figure 6D), potential sources of which include iron and steel industries [42; 43], coal-fired power generation [44], and coal mining [5]. In the present study, these elements are attributable to long-range atmospheric transport input from source regions since there are currently no industrial activities in the Ring of Fire area. Figure 4: **(A)** Clusters of air mass back-trajectories when the highest PM\({}_{2.5}\) concentration of 19 \(\upmu\)g/m\({}^{3}\) was observed on 5 July 2015, and **(B)** active wildfires near the air station on the same day ([[http://cwfis.cfs.nrcan.gc.ca/interactive-map](http://cwfis.cfs.nrcan.gc.ca/interactive-map)]([http://cwfis.cfs.nrcan.gc.ca/interactive-map](http://cwfis.cfs.nrcan.gc.ca/interactive-map)), (accessed on 30 June 2021)). Each colour in **(A)** indicates an air mass back-trajectory cluster and time percentage of that air mass originating from that cluster by using 24 air mass back-trajectories. Yellow stars in **(B)** represent active wildfires. La and Ce are light rare earth elements (REEs) naturally found in the upper continental crust (UCC). Wind can blow small particles containing REEs from the earth's surface to the atmosphere. La and Ce have similar physical-chemical properties and they are often found together. The ratio of La to Ce in UCC was reported to be around 0.5 [45]. La and Ce are also contained in cracking catalysts used by petroleum refineries [46; 47]. The catalyst material is released to air during the refinery process. Consequently, La and Ce were found in airborne particulate matter [46; 48; 49; 50]. The La/Ce ratio from fluid catalytic cracking (FCC) units is enriched and generally higher than 1 [46; 51]. La-enrichment factors in particulate matter were used to trace emissions from the petroleum refining industry [51]. Both La and Ce were detected in 16 out of the 40 samples collected in the Ring of Fire area and they were linearly correlated (Figure 6E). La/Ce ratios ranged from 0.5 to 1.5 with mean 0.8  
Figure 5: Average concentrations of trace elements during the monitoring period at the Ring of Fire (RoF) station and five stations in southern Ontario. The total number of samples was: 46 at RoF, 409 at Simcoe, 249 at Ottawa, 480 at Toronto, 384 at Windsor, and 433 at Hamilton.