A synthesis review on atmospheric wet deposition of particulate elements: Scavenging ratios, solubility, and flux measurements

Atmospheric dry and wet deposition of particulate matter controls its lifetime in air and contributes to the environmental burden of toxic pollutants and thus has important implications on human and ecosystem health. This synthesis review focuses on atmospheric wet deposition of particulate elements and analyzed their scavenging ratios (i.e., ratio of concentration in precipitation to that in ambient air), solubility, and wet deposition flux measurements based on published studies in the literature, aiming to gather updated knowledge that can be used for modeling their wet deposition. Our analysis finds that scavenging ratios of a specific element have a narrow range. Overall, elemental scavenging ratios for snow are approximately three times higher than those for rain. Elements that are bound to coarse (PM2.5–10) particles have larger scavenging ratios than those bound to fine (PM2.5) particles, except for Fe and Si. Solubility of elements in rainwater ranges from 8% (Fe) to 94% (Ca). Solubility is moderately correlated with scavenging ratio, possibly explaining the lower scavenging ratios of Fe and Si compared with other elements with similar fine fractions. Data collected from North America, Europe, the Middle East, and Asia show that the wet fluxes of Al and Fe are orders of magnitude greater than those of routinely monitored anthropogenic elements (Zn, Pb, Cu, Ni, Cd, Cr). Wet deposition fluxes of particulate elements in the Middle East exceed those in other regions, likely due to regional transport of dust and soil resuspension. Fluxes from all regions are a factor of two to three times greater in industrialized and urban locations than in rural and remote locations because of industrial, vehicular, and soil and mineral dust emissions. Dry deposition fluxes are usually greater than wet deposition fluxes, although to varying degrees according to co-located measurements. Based on the relationships between scavenging ratio and the elemental PM2.5 fraction under rain and snow conditions, we derived regression equations for estimating scavenging ratios of particulate elements for which measurements are limited. Such knowledge and data improve the quantification of atmospheric deposition fluxes for an expanded list of metals and metalloids and the understanding of pathways contributing to ecological risk.