Arsenic, a toxic element from natural and human sources, enters surface environments through atmospheric cycling and deposition. The transport mechanisms and sources of arsenic species are still poorly understood, as most research has focused on arsenic in groundwater and soils. However, an estimated 31 gigagrams (Gg) of arsenic are cycled annually which can represent an important source of arsenic to surface ecosystems and food chains. In addition, arsenic can travel long distances in the atmosphere, making its impact not limited to local environments.

Esther S. Breuninger, Julie Tolu, Lenny H. E. Winkel, and colleagues, Eawag, Swiss Federal Institute of Aquatic Science and Technology, Dübendorf, and ETH Zurich, Switzerland, have optimized methods to detect arsenic at extremely low concentrations. They found traces of the element in aerosols, clouds, and rainwater during measurements at the Pic du Midi Observatory, located 2877 m above sea level in the Pyrenees, France. The station is mostly exposed to the free troposphere, which, therefore, enables measurements that are largely unaffected by local sources of pollution.

The researchers identified terrestrial and marine sources for different arsenic species. Their findings reveal that clouds contain higher concentrations of arsenic than rainwater, much of which originates from natural biological processes like bacterial, algae, or plant metabolism.

Through the addition of a methyl group, inorganic arsenic undergoes (bio)transformation and volatilization by various fungi, bacteria, and algae. The species monomethylarsine (CH₃As^IIIH₂), dimethylarsine ((CH₃)₂As^IIIH), and trimethylarsine ((CH₃)₃As^III) are formed, with trimethylarsine being the predominant species emitted from (rice) paddy soils. Volatile arsenic species have a half-life of about 8 hours under daytime conditions, with longer lifetimes for inorganic arsines and in the dark. These gaseous species transform into non-volatile, oxidized pentavalent arsenic compounds, such as arsenate (As^V) and methylated arsenic species such as monomethylarsonic acid (CH₃As^VO₃H₂), dimethylarsinic acid ((CH₃)₂As^VO₂H), and trimethylarsine oxide ((CH₃)₃As^VO), which can undergo gas-particle transformation.

Until now, it was believed that human activities such as mining, smelting of non-ferrous metals, landfills, and the burning of fossil fuels were the main sources of atmospheric arsenic. However, in some cloud samples, methylated compounds made up the majority of the arsenic detected, suggesting that biological processes play a more important role in the global distribution of atmospheric arsenic than previously thought. Therefore, the researchers recommend that the influence of biological processes on the global arsenic cycle is given more consideration in the models.

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• Marine and terrestrial contributions to atmospheric deposition fluxes of methylated arsenic species,
  Esther S. Breuninger, Julie Tolu, Franziska Aemisegger, Iris Thurnherr, Sylvain Bouchet, Adrien Mestrot, Rachele Ossola, Kristopher McNeill, Dariya Tukhmetova, Jochen Vogl, Björn Meermann, Jeroen E. Sonke, Lenny H. E. Winkel,
  Nature Communications 2024.
  https://doi.org/10.1038/s41467-024-53974-z