Impacts of anthropogenic emissions and cold air pools on urban to montane gradients of snowpack ion concentrations in the Wasatch Mountains, Utah

Urban montane valleys are often characterized by periodic wintertime temperature inversions (cold air pools) that increase atmospheric particulate matter concentrations, potentially stimulating the deposition of major ions to these snow-covered ecosystems. We assessed spatial and temporal patterns of ion concentrations in snow across urban to montane gradients in Salt Lake City, Utah, USA, and the adjacent Wasatch Mountains during January 2011, a period of several persistent cold air pools. Ion concentrations in fresh snow samples were greatest in urban sites, and were lower by factors of 4-130 in a remote high-elevation montane site. Adjacent undeveloped canyons experienced significant incursions of particulate-rich urban air during stable atmospheric conditions, where snow ion concentrations were lower but not significantly different from urban sites. Surface snow ion concentrations on elevation transects in and adjacent to Salt Lake City varied with temporal and spatial trends in aerosol concentrations, increasing following exposure to particulate-rich air as cold air pools developed, and peaking at intermediate elevations (1500-1600m above sea level, or 200-300m above the valley floor). Elevation trends in ion concentrations, especially NH₄⁺ and NO₃^-, corresponded with patterns of aerosol exposure inferred from laser ceilometer data, suggesting that high particulate matter concentrations stimulated fog or dry ion deposition to snow-covered surfaces at the top of the cold air pools. Fog/dry deposition inputs were similar to wet deposition at mid-elevation montane sites, but appeared negligible at lower and higher-elevation sites. Overall, snow ion concentrations in our urban and adjacent montane sites exceeded many values reported from urban precipitation in North America, and greatly exceeded those reported for remote snowpacks. Sodium, Cl^-, NH₄⁺, and NO₃^- concentrations in fresh snow were high relative to previously measured urban precipitation, with means of 120, 117, 42, and 39μeql^-1, respectively. After exposure to atmospheric particulate matter during cold pool events, surface snow concentrations peaked at 2500, 3600, 93, and 90μeql^-1 for these ions. Median nitrogen (N) deposition in fresh urban snow samples measured 0.8kgNha^-1 during January 2011, with similar fog/dry deposition inputs at mid-elevation montane sites. Wintertime anthropogenic air pollution represents a significant source of ions to snow-covered ecosystems proximate to urban montane areas, with important implications for ecosystem function.