Sulfur and reactive nitrogen deposited in the alpine of the Southern Canadian Rockies: quantification and assessment of the main factors influencing deposition
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Sulfur (S) and reactive nitrogen (Nr = nitrate + ammonium) deposited at alpine sites in the Front Range of the Southern Canadian Rocky Mountains were evaluated by analyzing ion concentrations and stable isotope ratios in glacial snowpack and summer bulk precipitation samples. Deposition was 0.74 ± 0.18 kg S ha-1yr-1 (mean ± standard deviation among 3 sites) and 1.10 ± 0.18 kg Nr ha-1 yr-1. Seasonal contributions were disproportionate with summer rain, which represented 24 ± 8 % of annual precipitation, bringing 41- 46% of annual S and Nr loads. Conditions at the highest elevation site represented regional background S deposition (0.9 kg S ha-1 yr-1) and N deposition (1.3 kg Nr ha-1yr-1). The orographic influence on snow accumulation gave increasing snowpack S and Nr loads with rising elevation, with deposition mainly from distant sources. Summer S deposition was also dominantly from distant sources, while local to regional pollutants strongly enhanced Nr deposition. Distinct differences in deposition were seen between the NNW facing Robertson Valley and the opposing SE facing Haig Valley. Preferential transport and deposition to the Robertson Valley from local to regional pollution during NW-upslope synoptic conditions was probably the main factor for enhanced deposition in the Robertson Valley. The deposition rates measured here indicate that N-saturation is an ecologic concern for high alpine ecosystems in the region. δ34S-SO42- values in summer precipitation indicated three isotopically distinct sources: two regional to local anthropogenic sources having a similar isotope composition (A δ34S-SO42- ≥ +9 ‰, and C δ34S-SO42- ~ +8 ‰) that mixed with relatively clean air (B) with a δ34S-SO42- value ~ +4 ‰. Snowpack δ34S-SO42- values also indicated three isotopically distinct sources with the dominant source having a δ34S-SO42- value ~+2‰. Evidence suggests a seasonal difference in the relative magnitude of pathways of SO2 to SO42- oxidation is responsible for the rain vs. snowpack difference of ≥ 7 ‰. In the summer, homogeneous oxidation by OH and heterogeneous oxidation by H2O2 were most important, while oxidation by O2 catalysed by transition metal ions in a radical chain reaction pathway (TMI) was substantially more significant in the winter.