28. R. J. Schoelkopf et al., Phys. Rev. Lett. 78, 3370 (1997). 29. G. L. Ingold, Yu. V. Nazarov, in Single-Charge Tunnel- ing, H. Grabert, M. H. Devoret, Eds. (Plenum, New York, 1992), pp. 21–106. 30. Y.Nakamura,C.D.Chen,J.S.Tsai, Phys.Rev.Lett. 79, 2328 (1997). 31. G. Johansson, e-Print available at http://xxx.lanl.gov/ abs/cond-mat/0210539. 32. A. A. Clerk, S. M. Girvin, A. K. Nguyen, A. D. Stone, Phys. Rev. Lett. 89, 176804 (2002). 33. We thank Y. Nakamura, K. Harmans, P. Hadley, Y. Nazarov, H. Mooij, D. Bagrets, and Y. Blanter for discussions.Weacknowledgethetechnicalassistance of R. Schouten, B. van der Enden, and M. van Oos- sanen.SupportedbytheDutchOrganisationforFun- damental Research (FOM), and U.S. Army Research Office (grant DAAD19-02-1-0700). Supporting Online Material www.sciencemag.org/cgi/content/full/301/5630/203/ DC1 Materials and Methods SOM Text Figs. S1 and S2 5 March 2003; accepted 16 June 2003 Chloride Methylation by Plant Pectin: An Efficient Environmentally Significant Process John T. G. Hamilton, 1,2 * W. Colin McRoberts, 1 Frank Keppler, 2 Robert M. Kalin, 3 David B. Harper 2 Atmospheric chloromethane (CH 3 Cl) plays an important role in stratospheric ozone destruction, but many uncertainties exist regarding the strengths of its sources and sinks and particularly regarding the processes generating this naturally occurring gas. Evidence is presented here that CH 3 Cl is produced in many terrestrial environments by a common mechanism. Abiotic conversion of chloride to CH 3 Cl occurs readily in plant material, with the widespread plant component pectin acting as a methyl donor. Significant CH 3 Cl emissions from senescent and dead leaves were observed at ambient temperatures; those emissions rose dramatically when temperatures increased. This ubiquitous process acting in terrestrial ecosystems and during biomass burning could contribute the bulk of atmospheric CH 3 Cl. Chloromethane (CH 3 Cl), the most abundant atmospheric halocarbon, contributes 16% of the organic chlorine in the troposphere and is the most important naturally occurring gas involved in stratospheric ozone depletion (1, 2). Much research in recent years has focused on identifying and quantifying the sources and sinks of CH 3 Cl and attempting to under- stand the mechanisms of its production and degradation in nature. Despite these efforts, there is still a large uncertainty in the budget, with the best estimate of known sources be- ing 1 to 2 Tg less than the estimated global sink of 4 Tg year -1 (2). Until 1996, most of the CH 3 Cl input to the atmosphere was be- lieved to originate from the oceans, but more recent investigations have indicated that the marine source is relatively small (3) and that terrestrial sources, predominantly tropical, dominate the atmospheric budget (4–8). The mechanisms of CH 3 Cl formation in terrestrial ecosystems are far from clear. Emissions of CH 3 Cl by plants and fungi have been tenta- tively ascribed to enzymic processes, but there is little convincing evidence that the kinetics of the enzymes isolated would permit significant release of CH 3 Cl in vivo (1, 9–11). An abiotic route for halomethane for- mation from humic substances in soil has been proposed (12), but its environmental importance is unknown. Although it has long (13) been recognized that the volatilization of Cl – as CH 3 Cl occurs during biomass burning, the mechanism remains uncertain. Emissions of CH 3 Cl by biomass burning, estimated at 1 Tg year -1 (5, 14 ), have been ascribed to either free-radical processes during the com- bustion of cellulose (15, 16 ) or to the char- coal-catalyzed reaction of gaseous methanol and HCl produced by pyrolysis (17 ). Here we describe the abiotic release of CH 3 Cl by se- nescent or dead plant material, both foliar and woody, at ambient and elevated temperatures and suggest that its formation from pectin may be the common process underlying CH 3 Cl emissions in a variety of ecosystems and during biomass burning. We first investigated the fate of chloride in leaf and woody tissues of several plant species as temperature was progressively in- creased from 150° to 350°C over a 1.5-hour period in air (Fig. 1, A and B). CH 3 Cl emis- sions of 40 to 2600 ng g -1 of dry weight (dw) min -1 were observed at the initial tempera- ture of 150°C, but rose dramatically as the temperature was increased to 350°C. For most samples, CH 3 Cl release had terminated by the time 300°C was reached. When bio- mass was heated under N 2 , CH 3 Cl release exhibited a temperature profile similar to that obtained under aerobic conditions (Fig. 1A). The efficiency of conversion of Cl – to CH 3 Cl during heating of both leaf and woody tissues of a variety of species was high, with quan- titative yields being obtained from leaves of Norway maple (6.1 mg of CH 3 Cl g -1 dw) and from wood of horse chestnut (0.55 mg of CH 3 Cl g -1 dw) and Eucalyptus spp (1.3 to 1.7 mg of CH 3 Cl g -1 dw) (table S1) (18). For biomass with higher Cl – levels, the percent- age conversion was lower, although the actu- al amounts of CH 3 Cl produced could be very substantial (for example, 4.5 mg g -1 dw for ryegrass and 20 mg g -1 dw for the saltmarsh halophyte Batis maritima). The comparatively low temperature associ- ated with CH 3 Cl emission and the lack of an O 2 requirement clearly demonstrate that heating alone, rather than combustion, can readily ac- count for the release of CH 3 Cl during biomass burning [during which Cl – volatilization as CH 3 Cl has been estimated at around 3% (5, 14 )]. Our results are consistent with observa- tions that the highest yields of CH 3 Cl occur during the ignition phase of a fire, during smouldering combustion, and on the burning of foliage rather than wood (19, 20). Given that the leaf and woody tissues of a broad range of species yield large quantities of CH 3 Cl when heated, the chemical origin of the CH 3 moiety of the halomethane must be located in an abundant and ubiquitous plant structural component. One possible candidate is the biopolymer lignin, which can possess up to 15% methoxyl content. Accordingly, we measured CH 3 Cl production upon incre- mental heating of several types of extracted lignin in the presence of Cl – . Although sub- stantial emissions of CH 3 Cl were observed at elevated temperatures, little volatilization of Cl – occurred below 250°C. Furthermore, leaf tissue contains little, if any, lignified cell wall material, so CH 3 Cl released during the heat- ing of leaf material is unlikely to be lignin- derived. Another important structural compo- nent located in the primary cell wall is pectin, a polysaccharide composed primarily of -(1-4)–linked galacturonic acid units. The pectin content of the cell wall material of leaves normally ranges between 7 and 23% (21). The chemical determination of pectin content in wood is difficult, but immunolog- ical techniques indicate significant quantities (22). The pectin content of mature aspen wood has been estimated at 3% (23). A 1 Department of Agriculture and Rural Development forNorthernIreland,NewforgeLane,BelfastBT95PX, UK. 2 School of Agriculture and Food Science, Queen’s University Belfast, Newforge Lane, Belfast BT9 5PX, UK. 3 Questor and Environmental Engineering Re- search Centres, Queen’s University Belfast, Belfast BT9 5AG, UK. *To whom correspondence should be addressed. E- mail: jack.hamilton@dardni.gov.uk R EPORTS 11 JULY 2003 VOL 301 SCIENCE www.sciencemag.org 206