LAUSR

Abstract Emissions from fuel combustion and agricultural activities contribute significantly to the continuous atmospheric entrainment of pollutants into protected and vulnerable ecosystems where long-term monitoring is often a challenge. Here, results are presented from a 26-year study (1989–2015) of wet precipitation collected at Paradise Station (1654 m above sea level), Mount Rainier National Park, Washington, USA. Weekly samples were analyzed for pH, conductivity and major anions and cations. While precipitation concentrations for sulfate, nitrate, protons and conductivity peaked in the early 2000s, overall trends decreased by 54%, 46%, 41%, and 37%, respectively. Associated pH values increased from 5.2 to 5.6, and were largely controlled by non-sea-salt contributions of sulfate and neutralizing calcium, potassium and magnesium. Between 1999 and 2015, nitrogen (N) deposition rates from ammonium increased by a factor of 3.6, from 0.27 to 0.96 kg N ha−1 yr−1 (p = 0.02), while nitrate deposition did not change statistically (0.91–0.74 kg N ha−1 yr−1, p = 0.30). Combined, these N sources are reaching reported critical loads of 2.0 kg N ha−1 yr−1. Results indicate that emission regulations focused on stationary sources have effectively decreased apparent acid precipitation, however, increased nitrogen deposition from ammonium may lead to further fertilization and acidification of delicate soils and waters. Continued long-term monitoring is thus imperative to track continued anthropogenic inputs to vulnerable ecosystems.