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 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.

Long-term air concentrations, wet deposition, and scavenging ratios of inorganic ions, HNO₃, and SO₂ and assessment of aerosol and precipitation acidity at Canadian rural locations

This study analyzed long-term air concentrations and annual wet deposition of inorganic ions and aerosol and precipitation acidity at 31 Canadian sites from 1983 to 2011. Scavenging ratios of inorganic ions and relative contributions of particulate- and gas-phase species to NH4+, NO3−, and SO42− wet deposition were determined. Geographical patterns of atmospheric Ca2+, Na+, Cl−, NH4+, NO3−, and SO42− were similar to wet deposition and attributed to anthropogenic sources, sea-salt emissions, and agricultural emissions. Decreasing trends in atmospheric NH4+ (1994–2010) and SO42− (1983–2010) were prevalent. Atmospheric NO3− increased prior to 2001 and then declined afterwards. These results are consistent with SO2, NOx and NH3 emission trends in Canada and the USA. Widespread declines in annual NO3− and SO42− wet deposition ranged from 0.07 to 1.0 kg ha−1 a−1 (1984–2011). Acidic aerosols and precipitation impacted southern and eastern Canada more than western Canada; however, both trends have been decreasing since 1994. Scavenging ratios of particulate NH4+, SO42− and NO3− differed from literature values by 22 %, 44 %, and a factor of 6, respectively, because of the exclusion of gas scavenging in previous studies. Average gas and particle scavenging contributions to total wet deposition were estimated to be 72 % for HNO3 and 28 % for particulate NO3−, 37 % for SO2 and 63 % for particulate SO42−, and 30 % for NH3 and 70 % for particulate NH4+.

Long-term air concentrations, wet deposition, and scavenging ratios of inorganic ions, HNO₃ and SO₂ and assessment of aerosol and precipitation acidity at Canadian rural locations

This study analyzed long-term air concentrations and annual wet deposition of inorganic ions and aerosol and precipitation acidity at 30 Canadian sites from 1983–2011. Scavenging ratios of inorganic ions and relative contributions of particulate- and gas-phase species to NH4+, NO3−, and SO42− wet deposition were determined. Long-term median atmospheric NH4+, NO3−, and SO42− between sites ranged from 0.1–1.7, 0.03–2.0, and 0.6–3.5 μg m−3, respectively. Their median annual wet deposition varied from 0.2–5.8, 0.8–23.3, and 0.8–26.6 kg ha−1 a−1. Geographical patterns of atmospheric Ca2+, Na+, Cl−, NH4+, NO3−, and SO42− were similar to wet deposition and attributed to anthropogenic sources, sea-salt emissions, and agricultural emissions. Decreasing trends in atmospheric NH4+ (1994–2010) and SO42− (1983–2010) were prevalent. Atmospheric NO3− increased from 1991–2001 and declined from 2001–2010. These results are consistent with SO2, NOx and NH3 emission trends in Canada and the U.S. Widespread declines in annual NO3− and SO42− wet deposition ranged from 0.07–1.0 kg ha−1 a−1 (1984–2011). Acidic aerosols and precipitation impacted southern and eastern Canada more than western Canada; however both trends have been decreasing since 1994. Scavenging ratios of particulate NH4+, SO42− and NO3− differed from literature values by 22 %, 44 % and a factor of 6, respectively, because of the exclusion of gas scavenging. Average gas and particle scavenging contributions to wet NO3− deposition were 72±23 % for HNO3 and 28±23 % for particulate NO3−. SO2 and particulate SO42− contributed 37±20 % and 63±20 % to wet SO42− deposition, respectively. NH3 and particulate NH4+ contributed 30±19 % and 70±19 % to wet NH4+ deposition.

Scavenging ratios of polycyclic aromatic compounds in rain and snow in the Athabasca oil sands region

The Athabasca oil sands industry in northern Alberta, Canada, is a possible source of polycyclic aromatic compounds (PACs). Monitored PACs, including polycyclic aromatic hydrocarbons (PAHs), alkylated PAHs, and dibenzothiophenes (DBTs), in precipitation and in air at three near-source sites in the Fort MacKay and Fort McMurray area during January 2011 to May 2012, were used to generate a database of scavenging ratios (Wt) for PACs scavenged by both snow and rain. Higher concentrations in precipitation and air were observed for alkylated PAHs and DBTs compared to the other PACs. The sums of the median precipitation concentrations over the period of data analyzed were 0.48 μ g L−1 for the 18 PAHs, 3.38 μ g L−1 for the 20 alkylated PAHs, and 0.94 μ g L−1 for the 5 DBTs. The sums of the median air concentrations for parent PAHs, alkylated PAHs, and DBTs were 8.37, 67.26, and 11.83 ng m−3, respectively. Median Wt over the measurement period were 6100 – 1.1 × 106 from snow scavenging and 350 – 2.3 × 105 from rain scavenging depending on the PAC species. Median Wt for parent PAHs were within the range of those observed at other urban and suburban locations, but Wt for acenaphthylene in snow samples were 2–7 times higher compared to other urban and suburban locations. Wt for some individual snow and rain samples exceeded literature values by a factor of 10. Wt for benzo(a)pyrene, dibenz(a,h)anthracene, and benzo(g,h,i)perylene in snow samples had reached 107, which is the maximum for PAH snow scavenging ratios reported in the literature. From the analysis of data subsets, Wt for particulate-phase dominant PACs were 14–20 times greater than gas-phase dominant PACs in snow samples and 7–20 times greater than gas-phase dominant PACs in rain samples. Wt from snow scavenging were ~ 9 times greater than from rain scavenging for particulate-phase dominant PACs and 4–9.6 times greater than from rain scavenging for gas-phase dominant PACs. Gas-particle fractions of each PAC, particle size distributions of particulate-phase dominant PACs, and the Henry's law constant of gas-phase dominant PACs explained, to a large extent, the different Wt values among the different PACs and precipitation types. The trend in Wt with increasing alkyl substitutions may be attributed to their physico-chemical properties, such as octanol–air and particle partition coefficients and subcooled vapor pressure, which increases gas-particle partitioning and, subsequently, the particulate mass fraction. This study verified findings from a previous study of Wang et al. (2014) that suggested that snow scavenging is more efficient than rain scavenging of particles for equivalent precipitation amounts, and also provided new knowledge of the scavenging of gas-phase PACs and alkylated PACs by snow and rain.

Elemental carbon in snow at Changbai Mountain, Northeastern China: concentrations, scavenging ratios and dry deposition velocities

Light absorbing aerosol, in particular elemental carbon (EC), in snow and ice enhance absorption of solar radiation, reduce the albedo, and is an important climate driver. In this study, measurements of EC concentration in air and snow are performed concurrently at Changbai Station, Northeastern China, from 2009 to 2012. The mean EC concentration for surface snow is 987 ± 1510 ng g−1 with a range of 7 to 7636 ng g−1. EC levels in surface snow around (about 50 km) Changbai Mountain are lower than those collected on the same day at Changbai station, and decrease with distance from Changbai station, indicating that EC load in snow around Changbai Mountain is influenced by local source emissions. Scavenging ratios of EC by snow are calculated through comparing the concentrations of EC in fresh snow with those in air. The upper-limit of mean scavenging ratio is 137.4 ± 99.7 with median 149.4, which is smaller than those reported from Arctic areas. The non-rimed snow process may be one of significant factors for interpreting the difference of scavenging ratio in this area with the Arctic areas. Finally, wet and dry depositional fluxes of EC have been estimated, and the upper-limit of EC wet deposition flux is 0.46 ± 0.38 μg cm−2 month−1 during the three consecutive snow season, and 1.32 ± 0.95 μg cm−2 month−1 for dry deposition flux from December to February during study period. During these three years, 77% of EC in snow is attributed to the dry deposition, indicating that dry deposition processes play a major role for EC load in snow in the area of Changbai, Northeastern China. Based on the dry deposition fluxes of EC and hourly black carbon (BC) concentrations in air, the estimated mean dry deposition velocity is 2.81 × 10−3 m s−1 with the mean median of 3.15 × 10−3 m s−1. These preliminary estimates for the scavenging ratio and dry deposition velocity of EC on snow surface will be beneficial for numerical models, and improve simulations of EC transport, fate and radiative forcing in order to ultimately make better climate prediction.

Influences of in-cloud aerosol scavenging parameterizations on aerosol concentrations and wet deposition in ECHAM5-HAM

A diagnostic nucleation scavenging scheme, which determines stratiform cloud scavenging ratios for both aerosol mass and number distributions, based on cloud droplet, and ice crystal number concentrations, is introduced into the ECHAM5-HAM global climate model. This is coupled with a size-dependent in-cloud impaction scavenging parameterization for both cloud droplet-aerosol, and ice crystal-aerosol collisions. Sensitivity studies are presented, which compare aerosol concentrations, and deposition between a variety of in-cloud scavenging approaches, including prescribed fractions, several diagnostic schemes, and a prognostic aerosol cloud processing treatment that passes aerosol in-droplet and in-ice crystal concentrations between model time steps. For one sensitivity study, assuming 100% of the in-cloud aerosol is scavenged into the cloud droplets and ice crystals, the annual global mean accumulation mode number burden is decreased by 65%, relative to a simulation with prognostic aerosol cloud processing. Diagnosing separate nucleation scavenging ratios for aerosol number and mass distributions, as opposed to equating the aerosol mass scavenging to the number scavenging ratios, reduces the annual global mean sulfate burden by near to 10%. The annual global mean sea salt burden is 30% lower for the diagnostic approach, which does not carry aerosol in-droplet and in-crystal concentrations between model time-steps as compared to the prognostic scheme. Implementation of in-cloud impaction scavenging reduced the annual, global mean black carbon burden by 30% for the prognostic aerosol cloud processing scheme. Better agreement with observations of black carbon profiles from aircraft (changes near to one order of magnitude for mixed phase clouds), 210Pb surface layer concentrations and wet deposition, and the geographic distribution of aerosol optical depth are found for the new diagnostic scavenging as compared to prescribed ratio scavenging scheme of the standard ECHAM5-HAM.