Rainout and Washout Contributions to Wet Atmospheric Deposition in Southern South America

Wet atmospheric deposition (AD) is a crucial process that determines nutrient inputs from the atmosphere to ecosystems. It consists of two distinct processes: rainout (RO) and washout (WO). RO occurs when aerosols and gases in the free troposphere are incorporated into rainwater, while WO happens when aerosols and gases present in the atmospheric boundary layer are washed out by rain droplets. Dissolved ions in rainwater have either terrestrial or marine origins, which are typically estimated using the enrichment factor. In this study, we propose two innovative methods to estimate the relative contribution of ions by RO or WO based on rain event collections, and we apply these methods to characterize wet AD in southern South America. We collected rain events with deposition samplers at four sites in a 700-km east–west transect during 2007. Our new methods yielded consistent and reasonable estimates of wet AD amounts by RO and WO. We found that some elements, mainly marine (Na⁺, Cl⁻, SO₄²⁻), were mostly deposited by RO, while others, including Ca²⁺, NO₃⁻, and Mg²⁺, had similar deposition rates by RO and WO. Two terrestrial ions, NH₄⁺ and K⁺, were mainly deposited by WO. These results suggest that the relative contribution of RO or WO in the region is related to the marine or terrestrial origin of the ions, the size of the compound formed after emission, the maximum atmospheric loading capacity of each ion, and the distance to the potential emission source. The stoichiometry and quantities of wet AD in the region suggest that both oceanic and agricultural sources contribute to wet AD.