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Methyl bromide (CH3Br) is the most abundant brominated organic compound in the atmosphere. It is known to originate from natural and anthropogenic sources, although many uncertainties remain regarding strengths of both sources and sinks and the processes leading to its formation. In this study a potential new CH3Br source from vegetation has been examined, analogous to the recently discovered abiotic formation of methyl chloride from plant pectin. Several plant samples with known bromine content, including ash (Fraxinus excelsior), saltwort (Batis maritima), tomato reference material (NIST-1573a), hay reference material (IAEA V-10), and also bromine enriched pectin, were incubated in the temperature range of 25-50 degrees C and analyzed for CH3Br emission using gas chromatography/mass spectrometry. All plant samples inspected showed an exponential increase in CH3Br emission as a function of temperature increase, i.e., emissions were observed to approximately double with every 5 degrees C rise in temperature. Next to temperature, it was found that emissions of CH3Br were also dependent on the bromine content of the plants. The highest CH3Br release rates were found for the saltwort which contained the highest bromine concentration. Arrhenius plots confirmed that the observed emissions were from an abiotic origin. The contribution of abiotic CH3Br formation from vegetation to the global budget will vary geographically as a result of regional differences in both temperature and bromide content of terrestrial plants.

To date, chloroethyne in the environment has been proposed to occur as a reactive intermediate during the reductive dechlorination of tri- and tetrachloroethene with zerovalent metals. Such artificial conditions might possibly be found at organohalide-contaminated sites that are surrounded by remediation barriers made of metallic iron. In this paper, it is shown that the highly reactive chloroethyne is also a product of natural processes in soil. Soil air samples from three differentterrestrial ecosystems of Northern Germany showed significant chloroethyne concentrations, besides other naturally produced monochlorinated compounds, such as chloromethane, chloroethane and chloroethene. Measured amounts range from 5 to 540 pg chloroethyne in air purged from 1 L of soil. A possible route of chloroethyne formation in soil is discussed, where chloroethyne is probably produced as a byproduct of the oxidative halogenation of aromatic compounds in soil. A series of laboratory studies, using the redox-sensitive catechol as a discrete organic model compound, showed the formation of chloroethyne when Fe3+ and hydrogen peroxide were added to the system. We therefore propose that the natural formation of chloroethyne in soil proceeds via oxidative cleavage of a quinonic system in the presence of the ubiquitous soil component chloride.