Atmospheric chemistry

Halogens from trees and soils

Terrestrial vegetation and soils are an important source of methyl iodide, with emission rates comparable in magnitude to the oceans

Credit: Courtesy of The US National Park Service

Methyl iodide, although present in the atmosphere only at trace levels, influences aerosol formation, ozone chemistry and the atmosphere's oxidizing capacity. Oceanic emissions, although uncertain in magnitude, were thought to be the main source of this chemical compound, but new research shows that mid-latitude vegetation and soils contribute on a par.

Barkley Sive from the University of New Hampshire and colleagues1 compared methyl iodide concentrations in the air over the North Atlantic ocean and over forests in the United States, and measured fluxes from trees and soils. Because the compound is destroyed by light within about four days, atmospheric methyl iodide will only be found close to its source. High concentrations of methyl iodide were found far inland, suggesting terrestrial and not oceanic origins. In addition, observed methyl iodide concentrations vary on daily and seasonal scales similarly to the concentrations of known plant-emitted compounds. Of the terrestrial emissions, vegetation contributed about twice as much methyl iodide to the atmosphere as soils.

Surprisingly, methyl iodide emission rates from the terrestrial biosphere appear to be as large on a global scale as oceanic emission rates, with implications for aerosol formation and ozone chemistry over the continents that are yet to be explored.

References

  1. 1

    Sive, B. C. et al. A large terrestrial source of methyl iodide. Geophys. Res. Lett. 34, L17808 (2007).

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Thompson, A. Halogens from trees and soils. Nature Geosci (2007). https://doi.org/10.1038/ngeo.2007.15

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