2018). Atmospheric contaminants are one of the major sources of heavy metal(loid)
deposition and accumulation in soil (Ahmed and Ishiga 2006). The deposition of
metal-PM on soil has become a global issue due to the rapid increase in industrialization and urbanization.
Soil deposition of metal-PM is higher near industrial and mining areas. The
average concentration of heavy metal(loid)s in soils may exceed their background
levels due to the deposition of metal-PMs from the atmosphere (Hu et al. 2018;
Liang et al. 2017b; Pratte et al. 2018). Different authors reported heavy metal(loid)
concentrations in soil after atmospheric deposition of metal-PM (Table 4).
Deposition of metal-PM on soil can be wet or dry deposition (Gunawardena et al.
2013; Lynam et al. 2015). Wet and dry depositions are considered important
phenomena of atmosphere cleaning. Wet deposition represents the absorption into
droplets followed by precipitation of these droplets by rain (Lynam et al. 2015; Zhu
et al. 2016b). Wet deposition also takes places via impaction on the earth’s surface
(dew formation, fog, and mists) (Amodio et al. 2014). Wet deposition results in the
washout of both PM-bound metal(loid)s and vapor phase. Dry deposition represents
the uptake at the earth’s surface (water, soil, or vegetation) (Amodio et al. 2014). Dry
deposition of PM-bound metal(loid)s involves the removal of all the particles
suspended in the air. Dry deposition occurs with several mechanisms like interception, sedimentation, turbulent diffusion, Brownian diffusion, diffusiophoresis,
inertial forces, thermophoresis, and electrical migration (Amodio et al. 2014;
Zufall et al. 1999). Deposition rates of atmospheric pollutants (including
PM-bound metal(loid)s) are governed by PM characteristics (size and shape),
surface properties (microscale roughness, friction velocity, and temperature), and
meteorological dynamics (relative humidity, wind velocity) (Amodio et al. 2014;
Zufall et al. 1999). Several previous studies have evaluated wet and dry deposition of
PM-bound metal(loid)s at global scale. For example, Pan and Wang (2015) evaluated dry and wet depositions of PM-bound metal(loid)s at ten sites in Northern
China. They reported that atmospheric deposition of Cu, Pb, Zn, Cd, As, and Se
represents the same intensity as their accumulations/increases in the topsoil.
The flux of dry and wet depositions may vary with particle size, season, and area.
Pan and Wang (2015) reported that the dry deposition was more consistent with
spatial distribution of the total (dry plus wet) deposition flux compared to wet
deposition. They also reported that dry deposition dominated the total flux for the
majority of heavy metal(loid)s that exist as coarse particles (Pan and Wang 2015).
Azimi et al. (2003) carried out sampling and analysis of atmospheric deposition of
heavy metals (Al, Cd, Cr, Cu, Fe, Na, Pb, and Zn) at the University of Paris XII.
They reported that the prevalent deposition type was dry deposition which represents
80%, 60%, and 40% for Pb, Cu, and Cd, respectively. Ye et al. (2018) evaluated
atmospheric deposition based on insoluble/soluble fractions of five heavy metals
(Cr, Pb, Cu, Cd, and Zn) at Dinghushan (suburban) and Guangzhou (urban) sites.
They reported that the ratios of wet/dry deposition fluxes showed that wet deposition
mainly governed the heavy metal deposition compared to dry deposition. They
also reported that wet deposition fluxes significantly vary with seasonal variation
between winter and summer monsoon seasons in this region. Previously, Sakata and
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M. Shahid et al.
deposition and accumulation in soil (Ahmed and Ishiga 2006). The deposition of
metal-PM on soil has become a global issue due to the rapid increase in industrialization and urbanization.
Soil deposition of metal-PM is higher near industrial and mining areas. The
average concentration of heavy metal(loid)s in soils may exceed their background
levels due to the deposition of metal-PMs from the atmosphere (Hu et al. 2018;
Liang et al. 2017b; Pratte et al. 2018). Different authors reported heavy metal(loid)
concentrations in soil after atmospheric deposition of metal-PM (Table 4).
Deposition of metal-PM on soil can be wet or dry deposition (Gunawardena et al.
2013; Lynam et al. 2015). Wet and dry depositions are considered important
phenomena of atmosphere cleaning. Wet deposition represents the absorption into
droplets followed by precipitation of these droplets by rain (Lynam et al. 2015; Zhu
et al. 2016b). Wet deposition also takes places via impaction on the earth’s surface
(dew formation, fog, and mists) (Amodio et al. 2014). Wet deposition results in the
washout of both PM-bound metal(loid)s and vapor phase. Dry deposition represents
the uptake at the earth’s surface (water, soil, or vegetation) (Amodio et al. 2014). Dry
deposition of PM-bound metal(loid)s involves the removal of all the particles
suspended in the air. Dry deposition occurs with several mechanisms like interception, sedimentation, turbulent diffusion, Brownian diffusion, diffusiophoresis,
inertial forces, thermophoresis, and electrical migration (Amodio et al. 2014;
Zufall et al. 1999). Deposition rates of atmospheric pollutants (including
PM-bound metal(loid)s) are governed by PM characteristics (size and shape),
surface properties (microscale roughness, friction velocity, and temperature), and
meteorological dynamics (relative humidity, wind velocity) (Amodio et al. 2014;
Zufall et al. 1999). Several previous studies have evaluated wet and dry deposition of
PM-bound metal(loid)s at global scale. For example, Pan and Wang (2015) evaluated dry and wet depositions of PM-bound metal(loid)s at ten sites in Northern
China. They reported that atmospheric deposition of Cu, Pb, Zn, Cd, As, and Se
represents the same intensity as their accumulations/increases in the topsoil.
The flux of dry and wet depositions may vary with particle size, season, and area.
Pan and Wang (2015) reported that the dry deposition was more consistent with
spatial distribution of the total (dry plus wet) deposition flux compared to wet
deposition. They also reported that dry deposition dominated the total flux for the
majority of heavy metal(loid)s that exist as coarse particles (Pan and Wang 2015).
Azimi et al. (2003) carried out sampling and analysis of atmospheric deposition of
heavy metals (Al, Cd, Cr, Cu, Fe, Na, Pb, and Zn) at the University of Paris XII.
They reported that the prevalent deposition type was dry deposition which represents
80%, 60%, and 40% for Pb, Cu, and Cd, respectively. Ye et al. (2018) evaluated
atmospheric deposition based on insoluble/soluble fractions of five heavy metals
(Cr, Pb, Cu, Cd, and Zn) at Dinghushan (suburban) and Guangzhou (urban) sites.
They reported that the ratios of wet/dry deposition fluxes showed that wet deposition
mainly governed the heavy metal deposition compared to dry deposition. They
also reported that wet deposition fluxes significantly vary with seasonal variation
between winter and summer monsoon seasons in this region. Previously, Sakata and
76
M. Shahid et al.