Background atmospheric sulfate deposition at a remote alpine site in the Southern Canadian Rocky Mountains

We report observations of stable isotope ratios and ion concentrations from seasonal snowpack and summer bulk precipitation from remote alpine sites in the Southern Canadian Rocky Mountains. Spatial deposition patterns for sulfur (S) and δ 34 S‐SO 4 2− values indicate dominantly distant sources with little impact from local to regional pollution. Comparable S loads and total snowpack δ 34 S‐SO 4 2− values for glacier snowpack indicates S emissions were well mixed prior to dry deposition or incorporation into snowfall. A uniform S load and similar δ 34 S‐SO 4 2− values in a detailed study of summer bulk precipitation implies well‐mixed distant emissions. We interpret the deposited 0.9 kg S ha −1 yr −1 as atmospheric background deposition in midlatitude Western Canada. This study will improve calculations for sites impacted by point source emissions and provide a baseline for attributing changes associated with climate change, industrialization, and urban growth. Field evidence from this study supports theoretical and laboratory research on the relative importance of oxidation pathways on atmospheric δ 34 S‐SO 4 2− values for long‐range transported sulfate. δ 34 S‐SO 4 2− of the dominant S source in summer bulk precipitation (~ +2‰) versus snowpack (≥ +9‰) cannot be explained by seasonal emission sources, temperature effects on fractionation, or Rayleigh distillation. The study supports a seasonal difference in the relative importance of the different SO 2 to SO 4 2− oxidation pathways with homogeneous oxidation by OH and heterogeneous oxidation by H 2 O 2 most important in summer, and O 2 catalyzed by transition metal ions in a radical chain reaction pathway more significant in winter.