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Keywords:

  • chloromethane;
  • dipterocarp;
  • tree fern

Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Experiments
  5. 3. Results and Discussion
  6. Acknowledgments
  7. References
  8. Supporting Information

[1] Stable carbon isotope ratios of methyl chloride (CH3Cl) were measured in foliar emissions from 14 species of tropical plants growing in a glasshouse. The isotopic ratio of CH3Cl (arithmetic mean: −83.2 ± 15.2‰) ranged from −56‰ to −114‰; that from dipterocarp trees (−87.4 ± 12.3‰) was on average more depleted in 13C than that from tree ferns (−61.9 ± 9.7‰). The isotopic ratio was lower than that of CH3Cl produced by other known sources (e.g., biomass burning and salt marshes), with the exception of that by dead leaves. Using the distinctive isotope ratio of CH3Cl emitted from tropical plants together with previously reported isotopic data of CH3Cl sources and sinks to an isotopic mass balance calculation, global CH3Cl emission by tropical plants was estimated to be approximately 1500–3000 Gg yr−1 with uncertainties of 30–60%, which could account for 30–50% of the global emission.

1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Experiments
  5. 3. Results and Discussion
  6. Acknowledgments
  7. References
  8. Supporting Information

[2] Methyl chloride (CH3Cl) is the most abundant halocarbon in the remote atmosphere and it plays an important role in the chlorine-catalyzed destruction of stratospheric ozone. The atmospheric mixing ratio of CH3Cl (∼550 pptv) is maintained by emissions from predominantly natural sources and losses mainly caused by reaction with OH radicals in the atmosphere. Because of the wide variety of natural sources, including oceans [Moore et al., 1996], biomass burning [Lobert et al., 1999], tropical plants [Yokouchi et al., 2002, 2007], wood-rotting fungi [Harper, 1985], coastal salt marshes [Rhew et al., 2000], and senescent or dead leaves [Hamilton et al., 2003], its atmospheric budget includes major uncertainties [World Meteorological Organization (WMO), 2007].

[3] A mass balance approach using stable isotope data of CH3Cl may allow the uncertainties associated with various sources to be reduced. The development of a technique for measuring the stable carbon isotopic composition of CH3Cl [Rudolph et al., 1997] allowed Thompson et al. [2002] to calculate for the first time the isotopic mass balance of atmospheric CH3Cl, although their isotopic database was very limited. Recent findings on important isotopic information for the removal of CH3Cl by reaction with OH radicals [Gola et al., 2005] and CH3Cl emission from senescent or dead leaves [Keppler et al., 2004] led Keppler et al. [2005] to reanalyze the CH3Cl mass balance, and they have proposed that senescent plants and leaf litter in tropical and subtropical regions are likely the largest global sources of CH3Cl (1800 to 2500 Gg yr−1) to the atmosphere. However, isotopic data to reliably constrain the atmospheric budget are still lacking for some important sources. Although tropical plants are the potential largest source of CH3Cl [WMO, 2007], their isotopic signatures have only been reported for a few species of tropical ferns [Harper et al., 2003] and no isotopic measurements have been reported for dipterocarp trees, the largest group of CH3Cl-emitting tropical plants, with estimated emissions of 910 Gg yr-1 from those growing in Southeast Asia alone [Yokouchi et al., 2002]. In this study, we report the stable carbon isotopic compositions of CH3Cl emitted from tropical dipterocarp trees and tree ferns, and estimate the global source strength of tropical plants by using the isotope mass balance approach.

2. Experiments

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Experiments
  5. 3. Results and Discussion
  6. Acknowledgments
  7. References
  8. Supporting Information

[4] Leaves were collected from 14 species of tropical plants grown in glasshouses at the Arboricultural Research Institute (University of Tokyo), Tsukuba Botanical Garden, or the Biotron Section of the National Institute for Environmental Studies (see Table 1). The dipterocarp trees at the Arboricultural Research Institute had been transplanted from a tropical rain forest on Borneo Island. Foliar emission gas was collected by using the vial method [Yokouchi et al., 2007]. Mature, healthy, whole leaves or pinnae were detached from the plants and placed in a 40-ml screw-cap vial sealed with a Mini-inert sampling cap. One to two days after sampling, headspace gas (1 ml) in the vial was collected with a gas-tight syringe.

Table 1. CH3Cl Emission and Their δ13C Values Obtained From Glasshouse-Grown Tropical Plants
SpeciesGlasshouseaTypeHeight (m)nFlux (μg g−1 h−1)δ13C (‰)
  • a

    Glasshouse: ARI, Arboricultural Research Institute, University of Tokyo; TBG, Tsukuba Botanikal Garden; NIES, National Institute for Environmental Studies.

Dipterocarp Tree (Dipterocarpaceae)
Anisoptera sp.ARIpot0.51n.d.-
Dipterocarpus grancilisARIground740.11 ± 0.14−113.5 ± 13.5
D. obtusifoliusARIpot11n.d.-
Dryobalanops lanceolataARIpot140.02 ± 0.01−94.4 ± 7.0
Hopea odorataARIground731.04 ± 0.63−72.6 ± 7.7
H. odorataARIpot220.05 ± 0.01−92.2 ± 5.0
H. odorataARIpot220.08 ± 0.06−89.2 ± 0.8
H. odorataARIpot220.10 ± 0.07−70.4 ± 2.8
Shorea balangeranARIpot11n.d.-
S. guisoTBGground550.22 ± 0.07−74.5 ± 3.4
S. leprosulaARIpot150.09 ± 0.12−90.5 ± 6.0
S. multifloraARIground740.61 ± 0.18−72.1 ± 3.9
S. roxburghiiARIground730.27 ± 0.23−86.5 ± 4.9
S. roxburghiiARIpot220.08 ± 0.06−103.4 ± 16.6
S. roxburghiiARIpot220.04 ± 0.03−88.4 ± 9.5
S. roxburghiiARIpot220.10 ± 0.07−77.3 ± 6.5
S. roxburghiiTBGground540.20 ± 0.03−89.6 ± 7.2
S. smithianaARIpot130.40 ± 0.16−96.7 ± 6.8
Mean (dipterocarp tree)     −87.4 ± 12.3
 
Tree Fern (Marattiaceae)
Angiopteris lygodiifoliaNIESpot0.510.20−56.4
 
Tree Fern (Cyatheaceae)
Cyathea lepiferaNIESpot210.07−73.1
C. podophyllaNIESpot0.530.20 ± 0.15−56.1 ± 4.6
Mean (tree fern)−61.9 ± 9.7

[5] The headspace gas samples were preconcentrated in a trap consisting of a 1/16-inch diameter stainless steel tube packed with solid adsorbent material (2 mg Carbopack B and 10 mg Carboxene 1000, Supelco), and cooled to −30°C with a free piston stirling cooler (MA-SCUC04, Shinyei). The preconcentration trap in which CH3Cl was collected was then flash-heated to 180°C with a Nichrome wire heater, and the thermally desorbed CH3Cl was transferred in a flow of 1 ml/min of helium to a PoraBOND separation column (20 m long, 0.32 mm ID; Chrompack) in a gas chromatography oven (HP-6890, Agilent). The stable carbon isotopic ratio of CH3Cl was measured with an isotope ratio mass spectrometer (Finnigan MAT 252) in continuous flow mode following the combustion of the CH3Cl to CO2 by passing it through a CuO/Pt combustion tube at 960°C. The isotopic composition of the CO2 produced from CH3Cl was measured against a CO2 reference gas (δ13CVPDB = −31.73‰). The precision of the isotopic measurements was deduced from replicate δ13C analyses of a gravimetrically prepared standard gas (Taiyo Toyo Sanso) containing 5 ppmv CH3Cl in nitrogen, to be <0.3‰ for samples containing more than 300 pmol C. All the isotopic measurements were conducted with the precision less than 0.3‰. The mixing ratio of CH3Cl was determined from the 44CO2 peak area of CH3Cl relative to that of the standard gas. The precision of the mixing ratio measurements was better than 10%. The CH3Cl emission rate was calculated from the increase in its concentration in the vial after sealing, the weight of the oven-dried leaves, the volume of the vial, and the storage time.

[6] To evaluate whether detaching the leaves from the plants affected the carbon isotope ratio, we compared the detached leaf results with those obtained by the branch enclosure method [e.g., Saito and Yokouchi, 2006] in two Hopea odorata plants and two Shorea roxburghii plants. In both species, the carbon isotope ratios measured by the vial method were similar with those measured by the branch enclosure method: the differences were <4 ‰ for H. odorata and <2‰ for S. roxburghii.

3. Results and Discussion

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Experiments
  5. 3. Results and Discussion
  6. Acknowledgments
  7. References
  8. Supporting Information

3.1. Carbon Isotope Signature of Tropical Plant-Derived CH3Cl

[7] Table 1 shows the observed emission rates of CH3Cl from glasshouse-grown tropical plants and its stable carbon isotopic composition. Of the 14 species examined, 11 were found to emit CH3Cl (> 0.01 μg g−1 h−1; fluxes from Anisoptera sp., Dipterocarpus obtusifolius, and Shorea balangeran were below the determination limit). Among these 11 species, 5 species of the Dipterocarpaceae family were shown in this study for the first time to be CH3Cl-emitting plants (Dipterocarpus gracilis, Dryobalanops lanceolata, H. odorata, Shorea leprosula, and Shorea smithiana). CH3Cl emission rates from the other 6 species were similar to those in our previous studies [Yokouchi et al., 2002, 2007; Saito and Yokouchi, 2006].

[8] The stable carbon isotope ratios of CH3Cl show wide variability. The CH3Cl most depleted in 13C was released from D. gracilis (−113.5 ± 13.5‰), and the most enriched from Angiopteris lygodiifolia (−56.4‰). Besides the isotopic difference between different species, there can be great variation of up to 17‰ in individual plants. On average, δ13C values of CH3Cl emitted from Dipterocarpaceae trees (mean: −87.4 ± 12.3‰) were lighter than those from tree ferns (mean: −61.9 ± 9.7‰). We compared the results from H. odorata and S. roxburghii between potted young trees and mature trees in the ground. Although CH3Cl fluxes from mature plants were larger than those from young plants, as previously reported [Yokouchi et al., 2002], the δ13C values were similar between young and mature plants. We also compared our data with the only data previously reported for tropical plants (−72.7 ± 1.4‰ for Cyathea smithii and −69.3 ± 0.9‰ for Angiopteris evecta) [Harper et al., 2003], and found that, overall, the values that we obtained were more depleted by about 12‰, although those for some tree ferns (Cyathea podophylla and A. lygodiifolia) were more enriched. The isotopic signatures of CH3Cl emitted from tropical plants were more depleted relative to that from most of other sources, such as biomass burning (−45.1 ± 0.6‰ [Rudolph et al., 1997] and −51.7 ± 12.7‰ [Czapiewski et al., 2002]), wood-rotting fungi (−43.3 ± 0.2‰ [Harper et al., 2001)), and salt marshes (−62 ± 3‰ [Bill et al., 2002] and −65.7 ± 3.4‰ [Harper et al., 2001]), with the exception of that from senescent or dead leaves (−73‰ to −147‰ [Keppler et al., 2004]).

[9] The mean δ13C value for all of the CH3Cl-emitting plants tested in this study was −83.3 ‰. Thus, the δ13C value of the emitted CH3Cl was depleted by more than 50‰ relative to that of the plant biomass, because the δ13C value of C3-metabolized plant biomass is generally around −27‰ [e.g., Oleary, 1988]. Because leaves would contain no CH3Cl pool, isotopic fractionation associated with emission is probably negligible. Thus, this large fractionation should be attributed to the production mechanism in plant leaves. Two mechanisms have been proposed: a biogenic and an abiotic mechanism of CH3Cl production.

[10] In the biogenic mechanism, CH3Cl is synthesized in plant cells via the enzymatic transfer of a methyl group from a donor, S-adenosyl-l-methionine (SAM), to the acceptor chloride. The methyl group of SAM has been reported to be isotopically depleted relative to plant biomass by about 13‰ [Weilacher et al., 1996]. Further, enzymatic transmethylation can cause substantial fractionation (e.g., in the reaction involving catechol-O-methyltransferase) [Hegazi et al., 1979]. Thus, a similar large kinetic isotope effect (KIE) in the enzymatic methylation of chloride utilizing 13C-depleted SAM as the methyl donor could account for the observed large fractionation.

[11] The abiotic mechanism, in contrast, involves thermal denaturation of pectin in plant leaves [Hamilton et al., 2003]. An important feature of the mechanism is that the production rate depends strongly on the water content of the leaves: the CH3Cl emission increase with decreasing water content. Hamilton et al. [2003] originally proposed this mechanism for the production of CH3Cl in senescent or dead leaves, and they suggested that this abiotic route may also be responsible for CH3Cl production in the leaves of Dipterocarpaceae trees since the leaves have relatively low water content. However, it is uncertain as to whether or not the water content (∼55% [Hamilton et al., 2003] and ∼65% (this study)) is low enough to cause abiotic production of CH3Cl in living tropical plants.

3.2. Estimation of CH3Cl Emissions From Tropical Plants by the Isotopic Mass Balance Approach

[12] The isotopic ratio of global average CH3Cl in the atmosphere reflects the average δ13C from all sources weighted by the source strengths and the average KIE associated with all sinks weighted by the sink strengths [e.g., Miller et al., 2002]:

  • equation image

where δ13Catm and δ13Cisource are the δ13C values of CH3Cl in the atmosphere and in each source i, respectively. Φ is the proportion of each source i and sink j to the total source and sink strengths, respectively. ɛjsink is the isotopic fractionation constant of each sink j (in per mil), defined as 1000(α − 1), where α is KIE. Equation (1) has been used by Keppler et al. [2005], who based on the best estimates of source and sink strengths of WMO [2003] in which the tropical plant source strength is 910 Gg yr−1, to deduce the δ13C value of the missing CH3Cl source. However, as mentioned above, the source strength of tropical plants was estimated based on tropical plants growing in Southeast Asia alone [Yokouchi et al., 2002], and thus the global source strength is likely to be larger than the estimate. On the other hand, global three-dimensional simulations of atmospheric CH3Cl [Lee-Taylor et al., 2001; Yoshida et al., 2004] have shown that the missing source of CH3Cl (2400–2900 Gg yr−1) was attributed to the tropical terrestrial vegetation sources, which include tropical plants and tropical senescent or dead leaves. Thus, first we determined the average δ13C value of the tropical terrestrial vegetation sources (δ13Ctropical) under the assumption that the isotopic imbalance could be attributed to the tropical sources by using the following equation:

  • equation image

where δ13Ctotal is the weighted average δ13C value of total sources and was determined by equation (1) using atmospheric average [Thompson et al., 2002] and the weighted average for all sinks. δ13Cother is the weighted average δ13C value of sources other than the tropical terrestrial vegetation sources. Φtropical is the proportion of tropical and subtropical source strength to the total source strength.

[13] The fluxes and isotopic data used for the mass balance calculation are shown in Table 2. We present two scenarios for global source and sink strengths. In scenario A, we chose the values estimated by the model simulation of atmospheric CH3Cl [Yoshida et al., 2004]. In scenario B, we set the soil sink strength to be 1600 Gg yr−1, which has been taken from Harper and Hamilton [2003] who estimated the value based on the assumption that a partial lifetime for CH3Cl with respect to the soil sink is equivalent to that derived previously for CH3Br. The corresponding increment of source strength (1344 Gg yr−1) was attributed to the tropical terrestrial vegetation sources. We assumed that the isotopic values of the wetlands source and the ocean sink, for which no isotopic values were available, to be similar to those of salt marshes and soil, respectively.

Table 2. Global Source and Sink Strengths of CH3Cl and Their Relevant Isotopic Data Used for the Isotope Mass Balance Calculations
 Source or Sink Strength (Gg yr−1)δ13C of Source or KIE of Sinks (‰)
Scenario AaScenario Bb
Sources (total)43995743 
Tropical and subtropical plants  −83 ± 15c
 29004244 
Tropical senescent or dead leaves  −135 ± 12d
Biomass burning611611−47 ± 12e
Oceans508508−38 ± 4f
Salt marshes170170−64 ± 3g
Wetlands4848-h
Incineration/industrial162162−52 ± 9i
 
Sinks (total)43995743 
OH reaction3994399459 ± 8j
Ocean149149-h
Soil256160047 ± 3k

[14] Average δ13C values of the tropical terrestrial vegetation sources were calculated to be −111 ± 11‰ in scenario A and −101 ± 8‰ in scenario B. The uncertainties were calculated by using the propagation of random error technique. If our mean value of the δ13C measurements, −83 ± 15‰ (mean ± 1σ), is representative of tropical live plants globally, then a more 13C-depleted source is required to account for the isotopic imbalance between tropical live plants and the tropical terrestrial vegetation sources. One source, senescent or dead leaves, produces CH3Cl more depleted in 13C than our measurements of tropical plants [Keppler et al., 2004]. Here, we assume that the source producing the lightest CH3Cl in tropical/subtropical regions is senescent or dead leaves, although field observational evidence of a significant emission of CH3Cl from this source is currently lacking. Using the isotopic signatures of tropical live plants (δ13Cplant) and dead leaves (δ13Cdead), we determined the contribution of tropical live plants (Φplant) relative to the tropical terrestrial vegetation sources as follows:

  • equation image

Then, we obtained the global tropical live plant emission of CH3Cl by multiplying the calculated contribution by the strength of the tropical terrestrial vegetation sources. In this way, we estimated the relative contribution of tropical live plants to be 0.5 for scenario A and 0.7 for scenario B, amounting 1450 ± 870 Gg yr−1 and 2900 ± 1020 Gg yr−1, respectively. However, it should be noted that the above estimates may be lower limits, because the δ13C value of CH3Cl emitted from dead leaves at ambient temperature, which has not been reported, is likely to be more depleted than that used in the calculation, because the δ13C value of CH3Cl emitted from dead leaves is temperature dependent [Keppler et al., 2004]. We estimated the source strength of dead leaves to be lower than about 1400 Gg yr−1 in both scenarios; this value is lower than the values (1800–2500 Gg yr−1) estimated by using the isotope mass balance approach [Keppler et al., 2005]. This difference derives mainly from the difference in the scenarios and secondarily from the difference in the δ13C signatures of tropical live plants (Δ ≈ 12‰).

[15] The present budget calculation based on isotopic information provides additional evidence supporting previous findings of strong emissions of CH3Cl from tropical and subtropical plants to the atmosphere. However, we emphasize that the above calculation is associated with considerable potential uncertainty because 1) it is based on a small number of measurements of tropical plants and the averaged isotopic ratio was not weighted by the fluxes, 2) glasshouse-grown tropical live plants may have different isotope ratios than those in the field, and 3) the vial method might not take into account possible diurnal variability in the isotope ratios. To more reliably constrain the CH3Cl budget, comprehensive field studies that include isotopic measurements should be conducted in tropical and subtropical forests.

Acknowledgments

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Experiments
  5. 3. Results and Discussion
  6. Acknowledgments
  7. References
  8. Supporting Information

[16] We thank Yoshihiro Watanabe of the Arboricultural Research Institute, Tomohisa Yukawa of the Tsukuba Botanical Garden, and the staff members of the Biotron Section of the National Institute for Environmental Studies for their assistance in sample collection.

References

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Experiments
  5. 3. Results and Discussion
  6. Acknowledgments
  7. References
  8. Supporting Information
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Supporting Information

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Experiments
  5. 3. Results and Discussion
  6. Acknowledgments
  7. References
  8. Supporting Information
FilenameFormatSizeDescription
grl24217-sup-0001-t01.txtplain text document2KTab-delimited Table 1.
grl24217-sup-0002-t02.txtplain text document1KTab-delimited Table 2.

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