Supplementary material to "Long-term Atmospheric Deposition of Nitrogen and Sulfur and Assessment of Critical Loads Exceedances at Canadian Rural Locations"

S1 Daily average dry deposition velocities (Vd) of N and S species S1.1 Spatial patternsDaily average Vd for gas-phase compounds (SO2, HNO3) and particulate-phase compounds (pSO4 2-, pNH4 + and pNO3 -) for the 2000-2018 period are summarized in Table S2.For gaseous compounds, the mean daily Vd (cm/s) among 15 CAPMoN sites were 1.2 for HNO3 and 0.46 for SO2.For particulate sulfate (pSO4 2-), ammonium (pNH4 + ) and nitrate (pNO3 -), the mean daily Vd were 0.16, 0.15 and 0.21 cm/s, respectively.Vd of N and S compounds exhibited strong variability between sites.The regions with higher Vd for N and S compounds include the west coast, southeast and Atlantic (Table S2).According to the land use data surrounding a CAPMoN site (Table S1), the west coast and Atlantic sites have higher Vd likely because of nearby land use coverage that is associated with higher Vd.For example, water surfaces and forests.In particular, evergreen needleleaf or broadleaf trees are typically associated with larger leaf area index (LAI) and hence larger Vd.Meteorological conditions also differ substantially across Canadian sites, which can drive the spatial variability in Vd. S1.2 Cold vs. warm seasonal patternsVd of gaseous N and S compounds were slightly greater in the warm season than cold season at most of the sites based on the monthly variations in Fig. S1.This was also the case for particulate nitrate due to the higher fraction of nitrate in coarse PM during the warm season, which is based on size-fractionated measurements previously conducted at CAPMoN sites (Zhang et al., 2008).Given that Vd of coarse PM (PM2.5-10) is larger than that of fine PM (PM2.5), a higher fraction in coarse PM results in higher Vd.During the cold season, nitrate is predominantly associated with fine PM at CAPMoN sites (Zhang et al., 2008).Vd of sulfate and ammonium were slightly higher in the cold season than warm season.This pattern is likely attributed to meteorology perhaps higher wind speeds in the cold season. S1.3 Long-term annual trendsLong-term annual trends in Vd were estimated using Theil-Sen slopes of the seasonal average Vd, which have been seasonally adjusted using LOESS (locally estimated scatterplot smoothing).Statistically significant trends in Vd (p<0.05) are shown in Fig. S2.Among the sites, annual trends in Vd (cm/s of change per year) ranged from 2.4-8.8x 10 -3 for HNO3, 9.0 x 10 -4 to 5.1 x 10 -3 for SO2, 3.0 x 10 -4 to 1.0 x 10 - 3 for sulfate, 3.0-9.0x 10 -4 for ammonium and 2.7 x 10 -4 to 1.5 x 10 -3 for nitrate.Overall, the increasing annual trends in Vd were very small indicating that Vd of a chemical specie stays constant over time.Small fluctuations in Vd over time reflect meteorological variability.Thus, the main factor driving the long-term annual dry deposition trends are the ambient air concentrations.

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