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

Abstract

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 focused on atmospheric wet deposition of particulate elements and analyzed their scavenging ratios (i.e. concentration in precipitation to that in ambient air), solubility and wet deposition flux measurements based on published studies in 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 ~3 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 range from 8% (Fe) to 94% (Ca). Solubility is moderately correlated with scavenging ratio possibly explaining the lower scavenging ratios of Fe and Si compared to other elements with similar fine fraction. 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 2-3 greater in industrialized and urban locations than 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 elemental PM2.5 fraction under rain and snow conditions, we derived regression equations for estimating scavenging ratios of particulate elements whose measurements are limited. Such knowledge and data improves the quantification of atmospheric deposition fluxes for an expanded list of metals and metalloids and the understanding of pathways contributing to ecological risk.

Estimation of Atmospheric Dry and Wet Deposition of Particulate Elements at Four Monitoring Sites in the Canadian Athabasca Oil Sands Region

“… c Scavenging ratios (W) were taken from the database of literature values (Cheng et al., 2021), unless otherwise indicated. …”

“… e Scavenging ratios (W) were calculated by the regression equations and were bias‐corrected (Cheng et al., 2021). …”

“…For the insoluble elements, the scavenging ratios were calculated by the above formulas and were subsequently bias‐corrected by reducing the values by a factor of 2.6 for rain and 1.6 for snow (Cheng et al., 2021). Element‐specific rain and snow scavenging ratios obtained in this approach are summarized in Table 1 and the statistics are discussed in the following section.…”

“…For the insoluble elements, the scavenging ratios were calculated by the above formulas and were subsequently bias-corrected by reducing the values by a factor of 2.6 for rain and 1.6 for snow (Cheng et al, 2021 Fine fraction (f) values were taken from the database of literature values (Mamun et al, 2020), unless otherwise indicated. b Fine fraction (f) values were taken from Mamun et al (2021).…”

“…Elemental fluxes inMamun et al (2020) andCheng et al (2021) are the median values over urban/industrial sites worldwide. b Elemental fluxes estimated in this study correspond to PM 10 , whereas fluxes inMamun et al (2020) andCheng et al (2021) correspond to the total suspended PM. c Ratio of the fluxes in the literature to the mean fluxes over the four sites.…”