Atmospheric Deposition Trends are Primarily Decreasing in Arkansas and Tennessee

McCarty, Anna

undergraduate honors thesis. ABSTRACT: Atmospheric deposition has likely changed with the influence of increasing global temperatures and subsequent changes in precipitation across the United States and globally. The objective of this project was to analyze atmospheric deposition across northern Arkansas and Tennessee. The three specific objectives were to: i) evaluate trends in atmospheric deposition of various element/compounds, ii) assess the relationship between atmospheric deposition and rainfall, and iii) evaluate if rainfall-adjusted atmospheric deposition is changing over time. Six sites were analyzed from 1980 to 2022, including AR16, AR27, TN00, TN04, TN11, and TN14, which span over northern Arkansas and across Tennessee. The Mann-Kendall Test (τ) was used to evaluate changes in atmospheric deposition over time. Locally weighted regression (LOESS) was used to fit a curve to the relationship between annual rainfall and mean concentrations. Atmospheric deposition was adjusted for rainfall using the difference of the observed data and the predicted data of LOESS, and the rainfall-adjusted depositional load was evaluated for changes over time. Policies and best management practices have decreased concentrations of rainfall constituents, such as NO3 and SO4, nationally since 1980. These national trends are reflected in the results from Arkansas and Tennessee. The largest decrease for NO3 occurred in eastern Tennessee (3.72% yr-1, TN04). Similarly, the largest decrease for SO4 occurred in eastern Tennessee (7.40% yr-1, TN04). However, urbanization and anthropogenic practices have led to an increase in Ca, Mg, K, Na, Cl, NH4 concentrations in more populated areas. For example, at TN14 near Memphis, TN concentrations of Ca, K, and NH4 increased over time (0.15% yr-1, 0.05% yr-1, and 0.19% yr-1). Many constituents’ mean concentrations had an effectively linear relationship with annual rainfall. Adjusting for rainfall showed some effect on percent change per year in concentration, where rainfall-adjusted concentrations of Ca, K, and NH4 increased at a larger rate over time (1.87% yr-1, 1.91% yr-1, and 0.96% yr-1) than compared to the rate of change of mean concentrations over time. Because anthropogenic practices have caused changes in trends of atmospheric deposition, continual study of anthropogenic effects is needed.