## 3 Results  
### Wet Deposition Trends  
Rates of both annual wet Hg deposition and precipitation-weighted mean Hg concentrations at both MN16 and MN18 decreased over the time period 1998-2018 (Figure 2; Table 3); much of the decreases occurred prior to 2009, with relatively flat trends since 2009. Of interest from an ecosystem perspective is the change in wet-depositional loading. For reasons previously noted, removal of outliers, then use of regression-predicted weekly Hg\({}_{\text{precip}}\) values to calculate weekly and annual wet Hg deposition rates, was preferable to using NADP/MDN-reported annual deposition rates. Expected values, from locally weighted regression analysis, of annual wet deposition rates decreased by 20 and 24 percent, respectively for MN16 and MN18, over the 1998-2018 period. These percentage declines are smaller than those reported for 1998-2012 [24]; the change in magnitude may be due to a different analytical approach and effect of removing influential outliers, as well as a flattening of the trends starting around 2009. The flatter trends since \(\sim\)2009 may reflect a leveling-off of emissions-reductions in the U.S. and Canada over the last decade; i.e., large reductions in mercury emissions occur early in this time period, with more modest reductions in recent years (Table 1).  
Because precipitation amount is used in the calculation of wet Hg deposition rate, a trend in precipitation could drive a trend in Hg deposition. Linear regression of precipitation depth versus time shows no significant trend for MN16 (\(p\) = 0.80) and a weak positive trend at MN18 (\(p\) = 0.10) (see Figure S2: precipitation volume trend plots). The sites display considerable interannual variability in total precipitation depth (ranges: 554-908 and 508-832 mm yr\({}^{-1}\) for MN16 and MN18, respectively), driving interannual variability in wet Hg deposition. The weak, positive trend in precipitation amount at MN18 likely drove the relatively larger decline in precipitation-weighted mean concentrations at that site, compared to MN16, as larger precipitation events tend to have lower Hg\({}_{\text{precip}}\).  
Given the lack of a significant trend in precipitation volume, we conclude that the observed overall trend of declining wet Hg deposition rate is likely driven by reductions of mercury emissions in North America and not trends in precipitation. In addition, the observed declines in mercury deposition are synchronous with known declines in North  
Figure 2: Trend plots of annual wet mercury (Hg) deposition rates for MDN sites MN16 (**A**) and MN18 (**B**); and precipitation-weighted annual mean Hg concentrations for MDN sites MN16 (**C**) and MN18 (**D**) (Trend line is locally weighted regression; shaded region is 95-percent confidence interval. LOESS smoothing parameters are as follows: (**A**) 0.833; (**B**) = 1.0; (**C**) 1.0; and (**D**) 1.0.  
American mercury emissions since 1990 [13; 41], although global emissions have been comparatively constant [13]. A recent modeling study indicated that in northern Minnesota, emission reductions in North America are roughly equally important in comparison to emission reductions in the rest of the world in determining wet Hg deposition trends [15].