3.1. Carbon Isotope Signature of Tropical Plant-Derived CH3Cl
 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].
 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]).
 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.
 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.
 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.  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
 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]:
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. , who based on the best estimates of source and sink strengths of WMO  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:
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.
 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  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 Aa||Scenario Bb|
|Sources (total)||4399||5743|| |
|Tropical and subtropical plants|| || ||−83 ± 15c|
| ||2900||4244|| |
|Tropical senescent or dead leaves|| || ||−135 ± 12d|
|Biomass burning||611||611||−47 ± 12e|
|Oceans||508||508||−38 ± 4f|
|Salt marshes||170||170||−64 ± 3g|
|Incineration/industrial||162||162||−52 ± 9i|
|Sinks (total)||4399||5743|| |
|OH reaction||3994||3994||59 ± 8j|
|Soil||256||1600||47 ± 3k|
 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:
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‰).
 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.