Stable bromine isotopic composition of methyl bromide released from plant matter

Abstract

Methyl bromide (CH3Br) emitted from plants constitutes a natural source of bromine to the atmosphere, and is a component in the currently unbalanced global CH3Br budget. In the stratosphere, CH3Br contributes to ozone loss processes.Studies of stable isotope composition may reduce uncertainties in the atmospheric CH3Br budget, but require well-constrained isotope fingerprints of the source end members. Here we report the first measurements of stable bromine isotopes (δ 81 Br) in CH3Br from abiotic plant emissions. Incubations of both KBr-fortified pectin, a ubiquitous cell-stabilizing macromolecule, and of a natural halophyte (Salicornia fruticosa), yielded an enrichment factor (ε) of -2.00±0.23‰ (1σ, n=8) for pectin and -1.82±0.02‰ (1σ, n=4) for Salicornia (the relative amount of 81 Br decreased in CH3Br compared to the substrate salt). For short incubations, and up to 10% consumption of the salt substrate, this isotope effect was similar for temperatures from 30 up to 300°C. For longer incubations of up to 100 hours at 180°C the δ 81 Br values increased from -2‰ to 0‰ for pectin and to -1‰ for Salicornia. These δ 81 Br source signatures of CH3Br formation from plant matter combine with similar data for carbon isotopes to facilitate multidimensional isotope diagnostics of the CH3Br budget.

Isotope Fractionation Processes of Selected Elements

“…The CH 3 Br concentration in northern Sweden was 2-3 times lower than in the Stockholm area, possibly indicating industrial contamination of the latter area. CH 3 Br emissions from plants are about 2 ‰ depleted in 81 Br relative to bromine in the plant (Horst et al 2014). (Fietzke and Eisenhauer 2006;Krabbenhöft et al 2009;Neymark et al 2014).…”

Natural Variations of Stable Chlorine and Bromine Isotopes on Earth

“…Further at the Abisko site also soil-air samples were collected to determine isotopic fingerprinting and the importance of this source. The results obtained by Horst et al (2013) and indicated that, overall, the air samples showed similar δ 81 Br values for both field studies ranging from −0.47 to +1.75 ‰ versus SMOB for Stockholm samples and −0.14 to +0.48 ‰ versus SMOB for Abisko samples. However, in Stockholm the δ 81 Br range was larger and the mixing ratios of CH 3 Br were higher (14.4 ± 2.1 pptv) whereas for the Abisko samples the mixing ratios (9.8 ± 3.1 pptv) were statistically indistinguishable from global background (8 pptv).…”

“…The first bromine isotope measurements of atmospheric CH 3 Br samples were reported by Horst et al (2013) and . They used the method developed by Thornton et al (2013) to extract enough CH 3 Br from atmospheric samples from sites near Stockholm (an urban site) and Abisko (a rural site) in Sweden.…”

“…Due to the small number and premature character of these measurements Horst et al (2013) and were not able yet to give an explanation for these results.…”

Isotopic Characterization (2H, 13C, 37Cl, 81Br) of the Abiotic Sinks of Methyl Bromide and Methyl Chloride in Water and Implications for Future Studies.

“…Apart from carbon isotope analysis, hydrogen isotope 10 measurements have been demonstrated for CH3Cl (Nadalig et al, 2013;Keppler et al, 2018;Jaeger et al, 2018). For CH3Br bromine isotope analysis was presented in two studies (Horst et al, 2013;Horst et al, 2014).…”