Elsevier

Geochimica et Cosmochimica Acta

Volume 125, 15 January 2014, Pages 186-195
Geochimica et Cosmochimica Acta

Stable bromine isotopic composition of methyl bromide released from plant matter

https://doi.org/10.1016/j.gca.2013.10.016Get rights and content

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 isotopes81Br) 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 the heavier 81Br was 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 90 h at 180 °C the δ81Br values increased from −2‰ to 0‰ for pectin and to −1‰ for Salicornia. These δ81Br source signatures of CH3Br formation from plant matter combine with similar data for carbon isotopes to facilitate multidimensional isotope diagnostics of the CH3Br budget.

Introduction

Methyl bromide (CH3Br) is the most important source of Br radicals in the stratosphere. It accounts for 15% of the ozone depletion potential caused by halogen species (Butler, 2000). Consequently, the use of CH3Br as a fumigant is scheduled to be phased out by 2015 under amendments to the Montreal Protocol (Montzka et al., 2011). However, methyl bromide is also released naturally from oceans (King et al., 2002), biomass burning (Andreae and Merlet, 2001), salt marshes (Rhew et al., 2000), wetlands (Varner et al., 1999), fungi (Harper, 1985), and several plant species (Gan et al., 1998, Wishkerman et al., 2008). The main sinks are believed to be uptake by oceans (Butler et al., 2007) and soils (Shorter et al., 1995) as well as reaction with OH radicals in the atmosphere (Saltzman et al., 2004). Although most of these source and sink processes appear to be relatively well investigated, albeit facing upscaling challenges common to most bottom-up approaches, the global budget shows an imbalance of 32 Gg/a corresponding to ca. 25% of the known annual emissions (Yvon-Lewis et al., 2009). Hence, sources are either underestimated and/or sinks are overestimated.

Emissions from plants may add considerably to the CH3Br atmospheric budget. Warwick et al. (2006) ran an inverse model of currently observed mixing ratios in the atmosphere and identified a terrestrial source from tropical vegetation. Biotic and abiotic reaction mechanisms have been described. For the biotic reaction pathway, living plants are producing methyl halides in their cells as a result of enzymatic reactions (Saito and Yokouchi, 2006 and references therein). Blei et al. (2010) performed flux measurements on branches and leaves in a southeast Asian tropical rainforest and found emissions up to 3 ng h−1 gdw−1 CH3Br (gram dry weight). Large scale airborne measurements over the South American tropical rainforest showed small net fluxes because of local sinks such as photolysis and soil uptake (Gebhardt et al., 2008). Both studies demonstrate that vegetation in the tropics might be an important source of methyl bromide to the atmosphere.

Apart from biotic production of living plants, an abiotic reaction pathway has been described for CH3Br following a nucleophilic substitution reaction (SN2) (Hamilton et al., 2003, Keppler et al., 2004). The methoxy groups of pectin and lignin, both abundant cell-stabilizing macromolecules, react with halide ions dissolved in the tissue water of the plants (Khan et al., 2001) (Fig. 1a) or present as hydrogen bonded ions on the pectin molecule (Fig 1b). Up to 52% of the halide ions may be H-bonded to functional groups of the organic molecules (Myneni, 2002). These halide ions form acids, either with protons from the unesterified carboxylic acid group of the pectin molecule (McRoberts, 2011) (Fig 1a) or from the surrounding water (Fig 1b). The acids can cleave the methoxy group from the pectin molecule in a nucleophilic substitution reaction with a methyl halide as one product. The same reaction pathway has been suggested for bromide salts (Hamilton et al., 2003).

This abiotic reaction pathway was suggested to be an important source to the atmospheric methyl halide budget (Keppler et al., 2005, Wishkerman et al., 2008). It accounts for both low temperature emissions from dead and senescent plant material (Derendorp et al., 2012) and, together with the lignin methoxy groups, for biomass burning (Andreae and Merlet, 2001, Van der Werf et al., 2006). For methyl chloride ∼20–25% of the estimated total budget might come from biomass burning and a similar amount from senescent plants and plant litter (Keppler et al., 2005, Saito and Yokouchi, 2008). For methyl bromide approximately 10% of the atmospheric budget might originate from biomass burning (Yvon-Lewis et al., 2009). While vegetation-based emissions of CH3Br have the potential to reduce the estimated imbalance between sources and sinks in the atmospheric budget, low-temperature formation from senescent and dead plant matter has not yet been included in the global budget because quantification is difficult.

The current estimates for plant emissions of CH3Br are based largely on small-scale concentration or flux measurements (Gebhardt et al., 2008, Blei and Heal, 2011) and are not well constrained. Hence stable isotope techniques are starting to be explored to improve the source apportionment of methyl halides. Carbon-13 analysis on atmospheric samples was first shown for CH3Cl by Rudolph et al. (1997) and since then it was applied in various studies to characterize source (Thompson et al., 2002, Keppler et al., 2004, Saito and Yokouchi, 2008) and sink signatures (Gola et al., 2005, Sellevag et al., 2006). A first δ13C isotope based budget estimate was accomplished by Keppler et al. (2005). Recently, the hydrogen isotopic composition of CH3Cl released from halophyte plant species was investigated (Greule et al., 2012).

Isotope studies on CH3Br are much scarcer mainly because of its low atmospheric mixing ratio and small emission rates from its sources. Signatures of δ13C have been determined for fumigation products (McCauley et al., 1999), salt marshes (Bill et al., 2002) and tropospheric air (Bill et al., 2004, Bahlmann et al., 2011). However, δ13C values of salt marsh emissions (−43‰ VPDB) and the troposphere (−42.3.1‰ VPDB) are difficult to distinguish. Industrial CH3Br products have more negative δ13C values (−54‰ VPDB).

Bromine has two stable isotopes 79Br (51.69%) and 81Br (49.31%) with an average 81Br/79Br ratio of 0.954 (e.g. Wieser et al., 2013). The small mass difference of 2.5% causes only minor but measureable isotope fractionation. Two previous studies reported δ81Br values of −0.80‰ to +3.35‰ SMOB (Standard Mean Ocean Bromide) for deep groundwaters of the Siberian Platform (Shouakar-Stash et al., 2007) and −4.3‰ to −0.4‰ SMOB for industrially produced brominated organic compounds (Carrizo et al., 2011) thus giving a δ81Br range of ca. 8‰. Two recent articles reported isotope enrichment factors (ε) of −0.20‰ to −0.76‰ for microbial debromination of brominated phenols (Bernstein et al., 2013) and −0.5‰ to −2.7‰ observed for Grignard reagent formation (Szatkowski et al., 2013) showing the potential to identify certain processes and reactions by using Br isotope analysis.

A method to measure bromine isotopes in CH3Br was recently established (Horst et al., 2011) and the first δ81Br isotope values for ambient tropospheric CH3Br are in the range of −0.5‰ to +1.8‰ SMOB (Horst et al., 2013). Despite the small δ81Br range, the high-precision values could be used to identify a degradation trend and to start considering the influence of potential sources on the tropospheric δ81Br composition of CH3Br.

Here we report the first measurements of bromine isotopes in CH3Br derived from incubations with thermally-treated plant matter to explore its feasibility to aid in addressing the abiotic plant source. We suggest a potential δ81Br source range and investigate the fractionation process caused by this abiotic production pathway of CH3Br in senescent plants and dry plant litter.

Section snippets

Plant sample and pectin

For our experiments we used KBr-fortified apple pectin (Sigma Aldrich) and dried leaf samples of the halophyte Salicornia fruticosa collected in tidal areas on Sardinia, Italy. The fortified pectin was prepared in the following way: Potassium bromide (KBr, Sigma Aldrich) solution (1.5 mg KBr, 100 mL) was placed in a beaker and heated to 50 °C. Pectin (10 g) was slowly added while stirring. Then the formed gel was mixed (2 min) with an Ultra-Turrax® blender and transferred onto an aluminum foil tray

Plant matter and blank

Salicornia and pectin were analyzed for methoxy and (blank) salt concentrations prior to the incubation experiments (Table 1). We monitored both Cl (emitted as CH3Cl) and Br (emitted as CH3Br) concentrations during our experiments (Fig. EA-1, Electronic Annex) although the subsequent interpretation focuses on Br and its isotopes. The chloride content for pectin was estimated from the cumulative yields of the time series experiment assuming 100% conversion of Cl to CH3Cl. The bromide content

Evaluation of isotope fractionation behavior

The isotopic values changed over the course of the time-series experiment (Fig. 4c). The most likely reason is the gradual emptying of the salt substrate pool and the accompanying 81Br enrichment in the remaining substrate pool. To investigate this, the Rayleigh Eqs. (2), (3) were used to calculate the δ81Br value of the remaining salt substrate and the δ81Br value of the CH3Br instantaneously emitted at any given time. The initial delta value of the KBr salt was known (δ0s = +0.06‰ SMOB), as was

Conclusions

This study investigated the stable bromine isotopic composition of abiotically emitted CH3Br from plant matter. Results show that the Br isotopic composition of the formed methyl bromide can be expected to be relatively constant over a wide temperature range and for incomplete reactions. The enrichment factor of ca. −2‰ (incomplete reaction) makes it a useful tool to identify emissions from halophytic plants. Similar fractionations may also be assumed for non-halophytic plants. Emissions from

Acknowledgements

This research was funded by the Swedish Research Council (VR Grant 311-2007-8381). Örjan Gustafsson acknowledges financial support as an Academy Research Fellow from the Swedish Royal Academy of Sciences through a grant from the Knut and Alice Wallenberg Foundation. Brett Thornton acknowledges post-doc funding from the Bolin Centre for Climate Research. Frank Keppler was supported by the ESF (EURYI Award to F.K.) and DFG (KE 884/2-1) and by the DFG research unit 763 ‘Natural Halogenation

References (55)

  • E. Bahlmann et al.

    A high volume sampling system for isotope determination of volatile halocarbons and hydrocarbons

    Atmos. Meas. Technol.

    (2011)
  • A. Bernstein et al.

    Kinetic bromine isotope effect: example from the microbial debromination of brominated phenols

    Anal. Bioanal. Chem.

    (2013)
  • M. Bill et al.

    Carbon isotope ratios of methyl bromide and methyl chloride emitted from a coastal salt marsh

    Geophys. Res. Lett.

    (2002)
  • M. Bill et al.

    Stable carbon isotope composition of atmospheric methyl bromide

    Geophys. Res. Lett.

    (2004)
  • E. Bjorkman et al.

    Release of chlorine from biomass at pyrolysis and gasification conditions

    Energy Fuels

    (1997)
  • J.H. Butler

    Atmospheric chemistry: better budgets for methyl halides?

    Nature

    (2000)
  • J.H. Butler et al.

    Oceanic distributions and emissions of short-lived halocarbons

    Glob. Biogeochem. Cycle

    (2007)
  • D. Carrizo et al.

    Compound-specific bromine isotope composition of industrially synthesized and one natural organobromine substances

    Environ. Chem.

    (2011)
  • M. Elsner et al.

    A new concept linking observable stable isotope fractionation to transformation pathways of organic pollutants

    Environ. Sci. Technol.

    (2005)
  • J. Gan et al.

    Production of methyl bromide by terrestrial higher plants

    Geophys. Res. Lett.

    (1998)
  • S. Gebhardt et al.

    Halogenated organic species over the tropical South American rainforest

    Atmos. Chem. Phys.

    (2008)
  • A.A. Gola et al.

    Kinetic isotope effects in the gas phase reactions of OH and Cl with CH3Cl, CD3Cl, and 13CH3Cl

    Atmos. Chem. Phys.

    (2005)
  • J. Hamilton et al.

    Chloride methylation by plant pectin: an efficient environmentally significant process

    Science

    (2003)
  • D.B. Harper

    Halomethane from halide ion – a highly efficient fungal conversion of environmental significance

    Nature

    (1985)
  • J. Hoefs

    Stable Isotope Geochemistry

    (2004)
  • A. Horst et al.

    Compound-specific bromine isotope analysis of methyl bromide using gas chromatography hyphenated with inductively coupled plasma multiple-collector mass spectrometry

    Rapid Commun. Mass Spectrom.

    (2011)
  • Horst A., Thornton B. F., Holmstrand H., Andersson P., Crill P. M. and Gustafsson O. Stable bromine isotopic...
  • Cited by (0)

    1

    Present address: IRTA-SCR, Carretera Poble Nou Km 5.5, E-43540 Sant Carles de la Ràpita, Catalonia, Spain.

    View full text