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degradation rates in coastal seawater (Bedford Basin, Nova Scotia), using a stable isotope incubation ..... California, Irvine, Irvine, CA 92697-3100, USA.
GLOBAL BIOGEOCHEMICAL CYCLES, VOL. 17, NO. 2, 1057, doi:10.1029/2002GB001949, 2003

Biological degradation of methyl chloride in coastal seawater Ryszard Tokarczyk,1 Eric S. Saltzman,2 Robert M. Moore,1 and Shari A. Yvon-Lewis3 Received 18 June 2002; revised 16 December 2002; accepted 12 February 2003; published 30 May 2003.

[1] Methyl chloride (CH3Cl) is the most abundant halocarbon in the atmosphere, and

constitutes a significant fraction of the total atmospheric halogen burden. Chemical reactions of CH3Cl in seawater are slow, and it has been believed that the oceans are not an important sink for this compound. However, direct measurements of CH3Cl degradation rates in coastal seawater (Bedford Basin, Nova Scotia), using a stable isotope incubation technique, indicate rapid loss attributed to microbial activity. A series of weekly measurements from March 2000 to May 2001 yielded degradation rates ranging from 0–30% d1, with an annual mean of 7.4% d1. If biological uptake of CH3Cl occurs throughout the oceans at similar rates, the mean partial atmospheric lifetime of CH3Cl with respect to oceanic removal could be a few years, rather than several decades as previously thought. This rapid removal would make the oceans a major sink for CH3Cl and lower the overall atmospheric lifetime of CH3Cl from the current estimate of 1.3 to about 1.0 years. Measurements of the degradation rate of CH3Cl in open ocean waters are INDEX TERMS: 0322 Atmospheric needed in order to quantify the oceanic uptake rate. Composition and Structure: Constituent sources and sinks; 0315 Atmospheric Composition and Structure: Biosphere/atmosphere interactions; 1615 Global Change: Biogeochemical processes (4805); 4805 Oceanography: Biological and Chemical: Biogeochemical cycles (1615); KEYWORDS: methyl chloride, methyl halides, biogeochemistry of halocarbons Citation: Tokarczyk, R., E. S. Saltzman, R. M. Moore, and S. A. Yvon-Lewis, Biological degradation of methyl chloride in coastal seawater, Global Biogeochem. Cycles, 17(2), 1057, doi:10.1029/2002GB001949, 2003.

1. Introduction [2] Methyl chloride (CH3Cl) is a major source of atmospheric chlorine [Kurylo and Rodriguez, 1999], and a contributor to stratospheric ozone depletion. CH3Cl currently contributes about 12% of the total equivalent effective stratospheric chlorine. It is projected that this fraction will grow to as much as 30% later this century, as various solvents, CFCs, HCFCs, and other halogenated gases are purged from the atmosphere as a result of the Montreal Protocol [Madronich and Velders, 1999]. CH3Cl is largely natural in origin, with a complex biogeochemical cycle that is not well understood. The principal identified sources are terrestrial and include biomass burning and emission by wood-rotting fungi, and tropical plants [Lobert et al., 1999; Watling and Harper, 1998; Varner et al., 1999; Rhew et al., 2000; Yokouchi et al., 2000]. Oceanic emissions, which were previously thought to be the main source of atmospheric methyl chloride, have more recently been estimated to account for less than 12% of the global flux [Moore et al., 1 Department of Oceanography, Dalhousie University, Halifax, Nova Scotia, Canada. 2 Earth System Science, University of California, Irvine, Irvine, California, USA. 3 Atlantic Oceanographic and Meteorological Laboratory, NOAA, Miami, Florida, USA.

Copyright 2003 by the American Geophysical Union. 0886-6236/03/2002GB001949

1996]. Oceanic measurements of methyl chloride show that the low latitude ocean is supersaturated with respect to the overlying atmosphere, while the high latitude ocean is undersaturated [Moore et al., 1996]. Since chemical reactions of methyl chloride in seawater are very slow [Elliott and Rowland, 1995], these observations suggest the existence of an additional, most likely biological, loss mechanism at high latitudes. [3] Biological degradation of methyl bromide in seawater and in marine bacterial cultures has been demonstrated [King and Saltzman, 1997; Goodwin et al., 1998; Hoeft et al., 2000; Goodwin et al., 2001; Schaefer et al., 2002]. One strain of facultative methylotroph cultured from seawater has shown the ability to metabolize CH3Cl [Schaefer and Oremland, 1999], but no information is currently available about the rates of biological uptake in the oceans. [4] In this study, we utilize a stable isotope tracer technique to directly measure the loss rate of methyl chloride in Nova Scotia coastal waters between March 2000 and May 2001. These measurements reveal seasonally changing rates of methyl chloride degradation, with first order rate constants ranging from 0% per day in winter to 30% per day in summer. We attribute the observed CH3Cl degradation to microbial processes. These observations suggest that the loss rate for CH3Cl from the surface ocean due to biological uptake may be similar in magnitude to the loss by air-sea exchange, and can explain the methyl chloride undersaturation observed in high latitudes waters. These data indicate that oceans can be a significant global sink for atmospheric

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Figure 1. Location of the sampling site (solid circle) in Bedford Basin, Nova Scotia. CH3Cl, and that the lifetime of this gas in the atmosphere may be shorter than previously believed.

2. Methods 2.1. Sampling Location: Bedford Bay, Nova Scotia [5] We measured loss rates constants of 13CH3Cl over an annual cycle, from March 2000 until May 2001, at Compass Buoy station (444103000N, 633803000W) in the Bedford Basin (Nova Scotia). The Bedford Basin is the inner component of the Halifax harbor (Figure 1), with a surface area of 17 km2 and a volume of 5.6  108 m3. It is connected to the open Atlantic Ocean by a channel, with a minimum width of 400 m and sill depth of 20 m. The residence time of water in the basin is approximately 260 hours. Surface water temperature changes gradually from about 1C in winter to 20C in summer. The average salinity remains around 30, however freshwater runoff can occasionally lower it to 25 near the surface. Inorganic nutrients decrease sharply in the spring and increase in the fall, with nitrate approaching the limits of analytical detection in summer. Primary production in the basin, as on the adjacent shelf, is dominated by phytoplankton without significant contribution from seaweed [Li, 1998]. Overall exchange of shelf and inshore waters is controlled by alongshore winds driving Ekman transport, which exerts strong control of the nutrients and chlorophyll in the basin [Mitchell, 1991]. Weekly averages of temperature, salinity and nutrient concentrations in the basin are usually similar to those on the shelf [Petri et al., 1999], but local events related to runoff do occasionally occur. It has been estimated from the water exchange at the inlet mouth, that such events normally exist for a maximum of 3 – 4 days [Lewis and Platt, 1982]. At longer timescales, external physical forces control the basin dynamics. 2.2. Experimental Techniques: Sampling and Analysis [6] Samples were collected between 0830– 0900 (local time), just below the surface with a 20-L plastic bucket and

immediately transferred to 100-mL glass syringes. One aliquot of each sample was kept unfiltered, but was passed through a 63 mm pore size mesh to remove large particles and grazing organisms. These unfiltered samples were considered ‘‘live.’’ A second aliquot of each sample was filtered through a 0.2-mm Millipore filter to remove organisms and reduce biological activity. These filtered samples were used as control samples to verify that sampling handling did not introduce artifacts and to confirm the rate of chemical loss of CH3Cl in seawater. The syringes were kept closed, with no head space, submerged in a large volume of seawater, and transported to the laboratory. In the laboratory, they were placed in a temperature-controlled water bath at the temperature at which the samples were originally collected. [7] The methodology and instrumentation used in this study is essentially identical to that developed for methyl bromide loss rate studies [Tokarczyk and Saltzman, 2001; Tokarczyk et al., 2001]. Filtered and unfiltered seawater samples were spiked with 13CH3Cl to a concentration of approximately 1 nM and incubated for 12 – 14 hours. Aliquots (15 ml) were withdrawn from each syringe for analysis at 2-hour intervals. The concentration of 13CH3Cl was measured in the samples using purge and trap preconcentration, followed by gas chromatography with mass spectrometric detection. An isotope dilution method was used to improve the precision of the analysis, using a spike of CD3Cl introduced at analysis time. During analysis, ions m/z 53 and 55 were monitored, for 13CH3Cl and CD3Cl, respectively. [8] First-order loss rate constants were determined from the slope of a least squares regression of ln(13CH3Cl/13

Figure 2. 13CH3Cl degradation rates measured on May 31, 2000. Data are plotted as natural log(C/C0), where C is the measured concentration of 13CH3Cl during the incubation, and C0 is the initial concentration at the start of the incubation. The open and solid circles represent filtered and unfiltered aliquots of the same water sample, respectively. The slopes of the linear regression lines represent the degradation rate constants and the standard error of the slope for each line is given in brackets.

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Figure 3. Seasonal changes in the degradation rate constant of methyl chloride in Bedford Basin, Nova Scotia. Circles represent individual measurements of the 13CH3Cl degradation rate constant ks. Error bars represent the analytical uncertainty in each measurement (a = 0.05). The dotted line is surface water temperature. CH3Clinitial) versus time (Figure 2). The overall precision of the rate constant measurements varied from 0.01 – 0.04 d1 (95% confidence level). No loss of 13CH3Cl was observed in filtered samples indicating that the chemical breakdown of CH3Cl in seawater was undetectable over the applied incubation time and seawater temperature range, in agreement with laboratory rate constants [Elliott and Rowland, 1995]. Because rates were measured using 13CH3Cl, a minor correction due to isotopic fractionation should be applied in order to obtain rates for naturally occurring- CH3Cl, which consists primarily of 12CH3Cl. This fractionation factor has not been measured, and the rates in this study have not been corrected for this effect. Based on similar fractionation factors for chemical and microbial degradation of CH3Br [Tokarczyk and Saltzman, 2001; Miller et al., 2001], this effect is expected to increase the rate constant by about 7.5% of the measured value, which is less than the experimental uncertainty of most of the measurements.

2.3. Lifetime Calculations [9] A coupled surface ocean-atmosphere model was used to estimate the effect of oceanic loss rates on the atmospheric lifetime of methyl chloride [Yvon and Butler, 1996]. The model includes air/sea exchange, oceanic loss (via chemical or biological processes), and downward mixing across the thermocline. This model has recently been used to estimate the effect of oceanic uptake on the atmospheric lifetimes of a variety of gases, including CH3Cl, based on chemical losses in the ocean [Yvon-Lewis and Butler, 2002].

[10] These calculations were made assuming that the oceans and atmosphere are in steady state [Yvon and Butler, 1996], as follows: " !# 1 r Kw A kd ; ¼ tAO ntr H kd þ Kzw

ð1Þ

pffiffiffiffiffiffiffi where: kd ¼ ks þ Dzz kz . The model parameters are defined as follows: tAO is the partial atmospheric lifetime of CH3Cl with respect to oceanic losses; r is the fraction of atmospheric CH3Cl residing in troposphere; ntr is the number of moles of the troposphere; Kw is the air-sea gas transfer velocity; A is the surface area of the ocean; H is the solubility of CH3Cl; z is the mixed layer depth; kd is the oceanic degradation rate constant (corrected for vertical loss due to downward mixing from the mixed layer); ks is the degradation rate constant of CH3Cl in surface seawater; kz is the degradation rate constant at the mean thermocline temperature; Dz is the diffusivity through the thermocline. The two-box model calculation (described by equation (1)) was carried out for each cell of a 2  2 grid of monthly average sea surface temperature, 10 m wind speed, salinity [Woodruff et al., 1987], mixed layer depths, and thermocline diffusivities [Li et al., 1984].

3. Results [11] During the period between March 2000 and May 2001, 39 rate constant measurements were made, yielding values ranging from 0.00 to 0.30 d1(Figure 3). Experimental data from an individual rate constant measurement are shown in

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Figure 2, illustrating that the observed decays were first-order and that the degradation rate constant in unfiltered samples often far exceeded that in filtered samples. The annual mean degradation rate, as calculated from averaging the periods March 2000 – February 2001 or June 2000 – May 2001, is 0.07 ± .0.08 d1 (1s, n = 34 and 30, respectively). [12] The rate constants exhibit complex seasonality, with maxima at the beginning and end of the warm season. The largest rate constant occurred in June reaching 30% per day. A slightly smaller maximum occurred in October, equivalent to a loss of 17% per day, followed by a rapid decrease to below 10% per day during November and December. During January – February the CH3Cl degradation rate constant approached the detection limit of the analytical method (0.00 ± 0.02 d1). During both years, the spring increase in the rate constant occurred just at the onset of spring warming in March. [13] The factors controlling the variability in CH3Cl loss rate constants are not known. No consistent relationships were found between the observed degradation rate constants and ancillary parameters collected at this time series station, including nutrient concentrations, seawater salinity, chlorophyll contents or total bacterial abundance. The June maximum in the rate constant coincided with the maximum abundance of bacteria in coastal waters [Li and Dickie, 2001], while the October maximum coincided with the maximum abundance of microzooplankton. The latter suggests a possible link between abundance of these grazers and CH3Clutilizing species (likely bacteria). However, it is also possible that there is an optimal temperature range, outside which the rate of biological degradation slows down significantly. It should also be noted that all of the samples in this study were collected during the morning. No measurements were made at other times of day, and if significant diel variability in degradation exists, it could systematically bias the results.

4. Discussion [14] The existence of an oceanic biological sink has significant implications for the CH3Cl budget and lifetime, both in the ocean and atmosphere. For oceanic waters between 10 and 20C, the rate constant for chemical loss of CH3Cl varies between 0.07 and 0.33 yr1 [Elliott and Rowland, 1995], as compared with an annual average degradation rate constant of 27 ± 28 yr1 from this study. Using the coupled surface ocean-atmosphere model described above, a global mean area-weighted rate constant for chemical loss of CH3Cl is calculated to be 0.54 yr1 [Yvon-Lewis and Butler, 2002]. Including vertical mixing in the water column brings the total aquatic loss rate constant to 0.80 yr1. This yields an oceanic lifetime of about 1.25 years and a partial atmospheric lifetime with respect to oceanic losses of about 70 years. These loss processes have a negligible effect on the overall tropospheric lifetime of CH3Cl, which is calculated to be 1.3 years, based on the reaction with OH radicals [Kurylo and Rodriguez, 1999; DeMore et al., 1997]. However, using the mean annual degradation rate constant observed in this study, 27.0 yr1, the oceanic lifetime is reduced to 0.03 years and the partial atmospheric lifetime of CH3Cl with respect to the oceanic

loss is reduced to 4.1 years. Considering both the OH loss and oceanic loss gives an overall atmospheric lifetime of CH3Cl of 1.0 years. [15] The partial atmospheric lifetime of a gas with respect to oceanic loss is highly sensitive to oceanic degradation at low loss rate constants [Butler, 1994]. At very high loss rate constants, air/sea transfer rather than in situ loss limits the gas flux into the ocean, and the atmospheric lifetime approaches a constant limiting value. For CH3Cl, assuming instantaneous loss in the ocean yields a minimum partial atmospheric lifetime with respect to oceanic loss of approximately 2.0 years, corresponding to a total atmospheric lifetime of 0.8 years. This calculation places a strong upper limit on the rate at which the ocean can remove atmospheric CH3Cl. [16] The observed degradation rates can explain the reported undersaturation of cold North Atlantic waters with respect to atmospheric concentrations of CH3Cl. The minimum degradation rate constant (ks) needed to support an observed saturation anomaly is estimated from the expression   Dg Kw  ; ðks Þmin ¼ z 100 þ Dg

ð2Þ

where Kw ranges from 2000 to 3000 m yr1 (10 to 0C) and Dg is the gas saturation anomaly. Dg is defined as 100(pgw  pga)/pga, where pgw and pga are the partial pressures of the gas in seawater and air. A positive saturation anomaly indicates that the seawater is oversaturated with respect to the overlying atmosphere. [17] Saturation anomalies as low as 24% have been observed in cold Labrador Sea surface waters [Moore et al., 1996], which cannot be explained by chemical losses and vertical mixing alone. A loss rate constant of approximately 7 yr1 is needed to support this level of undersaturation. This far exceeds the rate constant for the known chemical loss of CH3Cl in the seawater, but falls well within the range of the degradation rate constants observed in this study. The actual degradation rate required to explain the Labrador Sea undersaturation could be larger, if there were in situ production of CH3Cl in those waters. The annual average degradation rate constant observed in this study can support a saturation anomaly of 40%, in the absence of in situ production. [18] The only known in situ oceanic source of CH3Cl is the chloride substitution of CH3Br and CH3I. It is estimated that this source can account for 40– 75% of the net flux of CH3Cl emitted from the oceans to the atmosphere [Moore et al., 1996; Moore, 2000]. The annual average rate constant for biological degradation (27.0 yr1) obtained from this study is similar to the global mean time constant for air-sea exchange (Kw/z = 23.5 yr1). Since the oceanic production of CH3Cl must balance both the air/sea flux and in situ degradation, there must be a significant additional in situ oceanic source of CH3Cl. This source is estimated to be approximately 0.3 to 0.7 Tg yr1, and is presumed to be biological. Laboratory culture studies have demonstrated production of CH3Cl by some species of marine macro and micro-algae [Manley and Dastoor, 1987; Tait and Moore, 1995]. However, extrapolation of the production rates observed in the laboratory is not sufficient to explain the required oceanic production rate. There are many possible

TOKARCZYK ET AL.: BIOLOGICAL DEGRADATION OF METHYL CHLORIDE

reasons for this, including the fact that laboratory cultures are very different from natural phytoplankton in terms of species and growing conditions. Furthermore, these experiments yield net production rates of methyl chloride that mask any possible biological consumption processes.

5. Conclusions [19] This study presents the first observations of degradation of CH3Cl in ocean waters. These results provide a preliminary estimate of the potential strength of the biological sink for CH3Cl in the ocean and its effect on the overall atmospheric lifetime of this compound. These estimates are based on the extrapolation of Bedford Basin results to the entire ocean, and should therefore be viewed with caution. Open ocean and coastal measurements are needed in a variety of environments on a seasonal basis in order to provide a firm basis for regional or global extrapolations. However, it has recently been demonstrated that biological loss of CH3Br occurs at measurable rates across wide regions of the North Atlantic and Pacific Oceans [King and Saltzman, 1997; Tokarczyk and Saltzman, 2001; Tokarczyk et al., 2001]. Therefore it is reasonable to suspect that biological degradation of CH3Cl in oceanic waters is a widespread phenomenon that has a significant impact on the global CH3Cl budget. Measurements of the degradation rate of CH3Cl in the open ocean are clearly needed in order to better constrain the atmospheric lifetime of CH3Cl and the role of the oceans in the biogeochemical cycle of this compound.

[20] Acknowledgments. We thank the Bedford Institute of Oceanography for allowing us to use their research boat and Loyd Baker, for his assistance during the field work. We are thankful to W. Li, W. Reeburgh, and M. Prather for comments during the preparation of this manuscript. This research was supported by NSERC, the NSF, NASA, and the NOAA Atmospheric Chemistry Program.

References Butler, J. H., The potential role of the ocean in regulating atmospheric CH3Br, Geophys. Res. Lett., 21, 185 – 188, 1994. DeMore, W. B., S. P. Sander, D. M. Golden, R. F. Hampson, M. J. Kurylo, C. J. Howard, A. R. Ravishankara, C. E. Kolb, and M. J. Molina, Chemical kinetics and photochemical data for use in stratospheric modeling, JPL Publ., 97 – 4, 1997. Elliott, S., and F. S. Rowland, Methyl halide hydrolysis rates in natural waters, J. Atmos. Chem., 20, 229 – 236, 1995. Goodwin, K. D., J. K. Schaefer, and R. S. Oremland, Bacterial oxidation of dibromomethane and methyl bromide in natural waters and enrichment cultures, Appl. Environ. Microbiol., 64, 4629 – 4636, 1998. Goodwin, K. D., R. K. Varner, P. M. Crill, and R. S. Oremland, Consumption of tropospheric levels of methyl bromide by C1 compound-utilizing bacteria and comparison to saturation kinetics, Appl. Environ. Microbiol., 67, 5437 – 5443, 2001. Hoeft, S. E., D. R. Rogers, and P. T. Visscher, Metabolism of methyl bromide and dimethyl sulfide by marine bacteria isolated from coastal and open waters, Aquat. Microbiol. Ecol., 21, 221 – 230, 2000. King, D. B., and E. S. Saltzman, Removal of methyl bromide in coastal seawater: Chemical and biological rates, J. Geophys. Res., 102, 18,715 – 18,721, 1997. Kurylo, M. J., and J. M. Rodriguez, Short-lived ozone-related compounds, in Scientific Assessment of Ozone Depletion, 1998, edited by D. L. Albritton et al., Rep. 44, pp. 2.1 – 2.56, World Meteorol. Org., Geneva, 1999. Lewis, M. R., and T. Platt, Scales of variability in estuarine ecosystems, in Estuarine Comparisons, edited by V. S. Kennedy, pp. 3 – 20, Academic, San Diego, Calif., 1982. Li, W. K. W., Annual average abundance of heterotrophic bacteria and Synechococcus in surface ocean waters, Limnol. Oceanogr., 43, 1746 – 1753, 1998.

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Li, W. K. W., and P. M. Dickie, Monitoring phytoplankton, bacterioplankton and viroplankton in a coastal inlet (Bedford Basin) by flow cytometry, Cytometry, 44, 236 – 246, 2001. Li, Y. H., T. H. Peng, and W. S. Broecker, The average vertical mixing coefficient for the oceanic thermocline, Tellus, Ser. B, 36, 212 – 217, 1984. Lobert, J. M., W. C. Keene, J. A. Logan, and R. Yevich, Global chlorine emissions from biomass burning: Reactive chlorine emissions inventory, J. Geophys. Res., 104, 8373 – 8389, 1999. Madronich, S., and G. J. M. Velders, Halocarbon scenarios for the future ozone layer and related consequences, in Scientific Assessment of Ozone Depletion, 1998, edited by D. L. Albritton et al., Rep. 44, pp. 11.1 – 11.38, World Meteorol. Org., Geneva, 1999. Manley, S. L., and M. N. Dastoor, Methyl halide (CH3X) production from the giant kelp, Macrocystis, and estimates of the global CH3X production by kelp, Limnol. Oceanogr., 32, 709 – 715, 1987. Miller, L. G., R. M. Kalin, S. E. McCauley, J. T. G. Hamilton, D. B. Harper, D. B. Millet, R. S. Oremland, and A. H. Goldstein, Large carbon isotope fractionation associated with oxidation of methyl halides by methylotrophic bacteria, Proc. Natl. Acad. Sci. U.S.A., 98, 5833 – 5837, 2001. Mitchell, M. R., The influence of local wind forcing on the low-frequency variations of chlorophyll-a in a small marine basin, Cont. Shelf Res., 11, 53 – 66, 1991. Moore, R. M., The solubility of a suite of low molecular weight organochlorine compounds in seawater and implications for estimating the marine source of methyl chloride to the atmosphere, Chemosphere: Global Change Sci., 2, 95 – 99, 2000. Moore, R. M., W. Groszko, and S. J. Niven, Ocean-atmosphere exchange of methyl chloride: Results from northwest Atlantic and Pacific Ocean studies, J. Geophys. Res., 101, 28,529 – 28,538, 1996. Petri, B., P. Yeats, and P. Strain, Nitrate, silicate and phosphate atlas for the Scotian Shelf and the Gulf of Maine, Can. Tech. Rep. Hydrogr. Ocean Sci., 203, 1 – 96, 1999. Rhew, R. C., B. R. Miller, and R. F. Weiss, Natural methyl bromide emissions from coastal salt marshes, Nature, 403, 292 – 295, 2000. Schaefer, J. K., and R. S. Oremland, Oxidation of methyl halides by the facultative methylotroph strain IMB-1, Appl. Environ. Microbiol., 65, 5035 – 5041, 1999. Schaefer, J. K., K. D. Goodwin, I. R. McDonald, J. C. Murrell, and R. S. Oremland, Leisingera methylohalidivorans gen. nov., sp. nov., a marine methylotroph that grows on methyl bromide, Int. J. Syst. Evol. Microbiol., 52, 851 – 859, 2002. Tait, V. K., and R. M. Moore, Methyl chloride production in phytoplankton cultures, Limnol. Oceanogr., 40, 189 – 195, 1995. Tokarczyk, R., and E. S. Saltzman, Methyl bromide loss rates in surface waters of the North Atlantic Ocean, Caribbean Sea and eastern Pacific (8N – 45N), J. Geophys. Res., 106, 9843 – 9851, 2001. Tokarczyk, R., K. D. Goodwin, and E. S. Saltzman, Methyl bromide loss rate constants in the North Pacific Ocean, Geophys. Res. Lett., 28, 4429 – 4432, 2001. Varner, R. K., P. M. Crill, and R. W. Talbot, Wetlands: A potentially significant source of atmospheric methyl bromide and methyl chloride, Geophys. Res. Lett., 26, 2433 – 2436, 1999. Watling, R., and D. B. Harper, Chloromethane production by wood-rotting fungi and an estimate of the global flux to the atmosphere, Mycol. Res., 102, 769 – 787, 1998. Woodruff, S. D., R. J. Slutz, R. L. Jenne, and P. M. Steurer, A comprehensive ocean-atmosphere data set, Bull. Am. Meteorol. Soc., 68, 521 – 527, 1987. Yokouchi, Y., Y. Nojiri, L. A. Barrie, D. Toom-Sauntry, T. Machida, Y. Inuzuka, H. Akimoto, H.-J. Li, Y. Fujinuma, and S. Aoki, A strong source of methyl chloride to the atmosphere from tropical coastal land, Nature, 403, 295 – 298, 2000. Yvon, S. A., and J. H. Butler, An improved estimate of the lifetime of atmospheric CH3Br, Geophys. Res. Lett., 23, 53 – 56, 1996. Yvon-Lewis, S. A., and J. H. Butler, The effect of oceanic uptake on the atmospheric lifetime of selected trace gases, J. Geophys. Res., 107, 4414, doi:10.1029/2001JD001267, 2002.



R. Moore and R. Tokarczyk, Dept. of Oceanography, Dalhousie University, Nova Scotia, Canada B3H4J1. E. Saltzman, Earth Systems Science, 220 Rowland Hall, University of California, Irvine, Irvine, CA 92697-3100, USA. S. A. Yvon-Lewis, Atlantic Oceanographic and Meteorological Laboratory, NOAA, Miami, FL 33149, USA.