Abstract. The chemical composition of 39 cloud samples and droplet size distributions in 24 cloud events were investigated at the summit of Mt. Tai from July to October 2014. Inorganic ions, organic acids, metals, HCHO, H 2 O 2 , sulfur(IV), organic carbon, and elemental carbon as well as pH and electrical conductivity were analyzed. The acidity of the cloud water significantly decreased from a reported value of pH 3.86 during (Guo et al., 2012 to pH 5.87 in the present study. The concentrations of nitrate and ammonium were both increased since 2007-2008, but the overcompensation of ammonium led to an increase in the mean pH value. The microphysical properties showed that cloud droplets were smaller than 26.0 µm and most were in the range of 6.0-9.0 µm at Mt. Tai. The maximum droplet number concentration (N d ) was associated with a droplet size of 7.0 µm. High liquid water content (LWC) values could facilitate the formation of larger cloud droplets and broadened the droplet size distribution. Cloud droplets exhibited a strong interaction with atmospheric aerosols. Higher PM 2.5 levels resulted in higher concentrations of water-soluble ions and smaller sizes with increased numbers of cloud droplets. The lower pH values were likely to occur at higher PM 2.5 concentrations. Clouds were an important sink for soluble materials in the atmosphere. The dilution effect of cloud water should be considered when estimating concentrations of soluble components in the cloud phase.
Soil organic matter (SOM) is the major factor affecting sequestration of heavy metals in soil. The mean free binding energy and the mean free adsorption energy and speciation of Zn in soil, as affected by SOM, were determined by employing Wien effect measurements. The presence of SOM markedly decreased the Zn binding energy in soils in the following order: Top (5.86 kJ mol(-1)) < Bottom (8.66 kJ mol(-1)) < Top OM-free (9.44 kJ mol(-1)) ≈ Bottom OM-free (9.50 kJ mol(-1)). The SOM also significantly decreased the adsorption energy of Zn on black soil particles by reducing nonspecific adsorption of Zn on their surfaces. The speciation of Zn in soils was elucidated by extended X-ray absorption fine structure spectroscopy and microfocus X-ray fluorescence. The results obtained by linear combination fitting of EXAFS spectra revealed that the main forms of Zn in soil were outer-sphere Zn, Zn-illite, Zn-kaolinite, and HA-Zn. As the SOM content increased, the proportion of HA-Zn among the total immobilized Zn increased, and the proportion of nonspecific adsorbed Zn decreased. The present results implied that SOM is an important controlling factor for the environmental behavior of Zn in soils.
Wien effect measurements were used to study the effect of organic matter on the interactions between divalent cations and soil clay particles of two black soil samples containing organic matter (OM) at 54.4 and 12.3 g kg−1 in the top (0–20‐cm) and bottom (100–120‐cm) horizons, respectively, and a sample of OM‐free black soil, all saturated with Cd2+, Cu2+, Pb2+, and with Ca2+ as a reference cation. The weak‐field electrical conductivities of suspensions of the top and bottom horizons and OM‐free black soil samples were 0.021 to 0.033, 0.011 to 0.021, and 0.0065 to 0.0082 mS cm−1, respectively. The mean free binding energies of the cations in the same soil sample suspensions were 5.5 to 7.3, 7.3 to 9.3, and 9.6 to 10 kJ mol−1, respectively. The mean free adsorption energies of all cations increased with field strength and were in the order OM‐free > bottom horizon > top horizon. At field strengths >100 kV cm−1, in the top‐horizon soil, the adsorption energies of Ca were 0.21 to 0.72 kJ mol−1, those of Cd and Cu were similar to one another at 0.01 to 0.25 kJ mol−1, and those of Pb were close to zero, while in the bottom horizon soil, the adsorption energies of the various cations were in descending order: Ca > Cd > Pb > Cu, and in the OM‐free soil the order of the adsorption energies of the various cations were Cd ≈ Cu ≈ Ca > Pb. The humus basically increased the negative electrokinetic potentials of the clay‐size‐fraction particles of the three black soil samples saturated with Ca, Cd, Cu, or Pb.
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