## 3 Results and Discussion
Figure 2: Seasonal wind roses from June 2015 to February 2018 measured by RM Young (**A–D**). Comparison of wind roses between concurrent measurements by RM Young and Vaisala WXT 520 from September 2016 to February 2018 (**E**,**F**). In each circular plot, directions the wind blew from are sorted into 36 equal arc segments with 10\({}^{\circ}\) for each segment and the length of each “spoke” around the circle indicates the frequency with which the wind blew from that direction. The 0\({}^{\circ}\) refers to the north. The maximum PM\({}_{2.5}\) concentration was observed on 5 July 2015. Potential source regions for this elevated event were assessed by analyzing air mass back-trajectories. The 24 three-day back-trajectories obtained for this day were then grouped into four clusters based on similar wind directions and speeds by K-means clustering. The four clusters suggested that the air mass mostly originated from the southwest and west before it arrived at the air station (Figure 4A). The Canadian Wildland Fire Information System showed that active wildfires were reported west of the air monitoring station during this time period (Figure 4B). The only element with elevated concentrations in this day was Cd, which has been reported to be related with forest fires emissions [35; 36; 37]. Therefore, the episodic PM\({}_{2.5}\) concentration likely arose from wildfires in northern Canada. The maximum 24 h PM\({}_{2.5}\) concentrations ranged from 13 to 53 \(\upmu\)g/m\({}^{3}\) in 2015-2018 among the AQHI stations in southern Ontario. The maximum PM\({}_{2.5}\) concentration (i.e., 19 \(\upmu\)g/m\({}^{3}\)) observed in the Ring of Fire area was within this range. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline  & \multicolumn{2}{c}{**PM\({}_{2.5}\)**} & \multicolumn{8}{c}{**Concentration (ng/m\({}^{3}\))**} \\  & **(\(\upmu\)g/m\({}^{3}\))** & **Mn** & **Fe** & **As** & **Se** & **Sb** & **La** & **Ce** & **Pb** \\ \hline Minimum & 0.75 & 0.13 & 3.0 & 0.012 & 0.010 & 0.015 & 0.0016 & 0.0016 & 0.055 \\ Maximum & 19 & 0.71 & 20 & 0.40 & 0.41 & 0.25 & 0.023 & 0.026 & 1.2 \\ Mean & 3.0 & 0.22 & 5.2 & 0.055 & 0.066 & 0.039 & 0.0046 & 0.0058 & 0.18 \\ Median & 2.2 & 0.13 & 3.0 & 0.012 & 0.010 & 0.015 & 0.0016 & 0.0034 & 0.055 \\ Standard deviation & 0.49 & 0.024 & 0.67 & 0.012 & 0.014 & 0.0079 & 0.00092 & 0.00095 & 0.036 \\ Interquartile range & 3.1 & 0.11 & 2.4 & 0.045 & 0.068 & 0.025 & 0.0031 & 0.0054 & 0.13 \\ MDL & 1.5 & 0.27 & 6.1 & 0.024 & 0.021 & 0.029 & 0.0031 & 0.0032 & 0.11 \\ \textgreater{}MDL & 63\% & 26\% & 26\% & 43\% & 48\% & 33\% & 40\% & 55\% & 37\% \\ \hline \end{tabular}
\end{table} 
Table 1: Statistics for air concentrations of PM\({}_{2.5}\) (\(\upmu\)g/m\({}^{3}\)) and select elements (ng/m\({}^{3}\)) measured at the Ring of Fire air monitoring station. Figure 3: Average PM\({}_{2.5}\) concentrations in 2015–2018 at the Ring of Fire (RoF) station and other AQHI stations in southern Ontario. The average at the RoF was calculated from 46 24 h samples and those at the AQHI stations were calculated from continuous hourly concentrations. Levels of elements were fairly low in all air samples and mostly below the method detection limits. Eight of them were detected in 25% or more of the samples, including Mn, Fe, As, Se, Sb, La, Ce, and Pb. Total chromium (including hexavalent chromium) was only detected in three samples and concentrations ranged from 0.46 to 0.62 ng/m\({}^{3}\) (Table S1), indicating that the Black Thor chromite deposit had little impact on regional air quality. Ontario's AAQC are based on protection against adverse effects on human health or the environment [38]. Twenty-four-hour AAQCs for the total suspended particulate (TSP) fraction are available for 25 of the 31 analyzed elements. In addition, 24 h AAQCs for the PM\({}_{10}\) fraction are available for Mn, Ni, and U, and a 24 h AAQC for the PM\({}_{2.5}\) fraction is available for Mn.