Microscopic Evaluation of Trace Metals in Cloud Droplets in an Acid Precipitation Region

Abstract

Mass concentrations of soluble trace metals and size, number, and mixing properties of nanometal particles in clouds determine their toxicity to ecosystems. Cloud water was found to be acidic, with a pH of 3.52, at Mt. Lu (elevation 1,165 m) in an acid precipitation region in South China. A combination of Inductively Coupled Plasma Mass Spectrometry (ICPMS) and Transmission Electron Microscopy (TEM) for the first time demonstrates that the soluble metal concentrations and solid metal particle number are surprisingly high in acid clouds at Mt. Lu, where daily concentrations of SO2, NO2, and PM10 are 18 μg m(-3), 7 μg m(-3), and 22 μg m(-3). The soluble metals in cloudwater with the highest concentrations were zinc (Zn, 200 μg L(-1)), iron (Fe, 88 μg L(-1)), and lead (Pb, 77 μg L(-1)). TEM reveals that 76% of cloud residues include metal particles that range from 50 nm to 1 μm diameter with a median diameter of 250 nm. Four major metal-associated particle types are Pb-rich (35%), fly ash (27%), Fe-rich (23%), and Zn-rich (15%). Elemental mapping shows that minor soluble metals are distributed within sulfates of cloud residues. Emissions of fine metal particles from large, nonferrous industries and coal-fired power plants with tall stacks were transported upward to this high elevation. Our results suggest that the abundant trace metals in clouds aggravate the impacts of acid clouds or associated precipitation on the ecosystem and human health.

Trace Metals in Cloud Water Sampled at the Puy De Dôme Station

“…Regarding Cu, concentrations from continental air masses are comparable to References [14,37] and are larger than marine Cu concentrations. However, marine Cu concentrations at the PUY are in the same range as for the ones in References [5,13,15,16,38]. Mn and Fe in both continental and marine origin samples presented in this work show low concentrations: this is probably due to the fact that these trace metals are mostly insoluble when they come from crustal particles, while they are more soluble if adsorbed in carbonaceous aerosol, associated with anthropogenic emissions.…”

“…Arsenic (As) concentrations in cloud waters collected at the puy de Dôme site from marine and continental origins are low and are comparable to those reported for References [5,13,14,16,37] (marine and continental sites), whereas References [15,38] which are related to polluted sampling sites, show concentrations 15-20 times higher. Cd concentrations are close to those reported by References [5,16], but lower than those reported by other works.…”

“…In addition, there is no significant difference between marine and continental air masses. Cr, a typical marker of pollution, is not present in our samples and is reported to be below the detection limit as in References [14,16], while it is measured for the other works, in particular for References [15,38] in China. Regarding Cu, concentrations from continental air masses are comparable to References [14,37] and are larger than marine Cu concentrations.…”

“…The ratio of Zn to Sb for cloud samples collected at Mt. Plynlimon (United Kingdom) between 1992 [37]: filtered 0.22 µm; Cini et al [13]: filtered 0.4 µm; Ghauri et al [16]: filtered 0.45 µm; Fomba et al [13]: filtered 0.45 µm; Li et al [15]: filtered 0.45 µm; Liu et al [38]: filtered 0.45 µm; Plessow et al [5]: not filtered. ); (Site characteristics are reported in Table S2.…”

“…Cini et al [14] reports that the Vallombrosa region (Tuscan Appennines, Italy) is influenced by sea components (marine origin air masses) but also exposed to significant pollution sources [14]: this observation can explain the high trace metals concentration found in comparison to this work and to other studies. References [15,38] collected samples in a continental region in China characterized by high emissions of pollutants. Mt.…”