Geochemical Journal, Vol. 35, pp. 137 to 144, 2001
LETTER Distribution of methyl chloride, methyl bromide, and methyl iodide in the marine boundary air over the western Pacific and southeastern Indian Ocean
HONG-JUN LI ,1,2 YOKO YOKOUCHI, 1* HAJIME AKIMOTO2 and YASUSHI NARITA 3 1
Division of Environmental Chemistry, National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba, Ibaraki 305-0042, Japan 2 Research Center for Advanced Science and Technology, University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-0041, Japan 3 Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan (Received July 26, 2000; Accepted January 27, 2001) Methyl chloride (CH 3Cl), methyl bromide (CH3Br), and methyl iodide (CH 3I) in marine boundary air were measured over the western Pacific and the southeastern Indian Ocean during the period of December 1996–February 1997. The mean concentrations of CH 3Cl, CH3Br, and CH 3I were 623, 10.3, and 1.1 pptv, respectively, and their highest concentrations were observed in the tropics. The enhancement of CH 3Cl concentration in the tropics, particularly near islands, was consistent with the recently reported finding on its high emissions from tropical coastal lands. The enhancement of CH3Br in the tropics was not related to the closeness of the sampling sites to islands, suggesting that land sources were not so important for CH3Br as for CH 3Cl. The atmospheric CH3I concentrations showed a little higher concentration in the tropics and in the southern hemisphere (SH) than in the northern hemisphere (NH). This finding suggests that its higher emission from the tropics and SH in austral summer compensates the higher rate of photolytic decay of this compound in the regions.
et al., 2000). Methyl bromide (CH3 Br) is the most important carrier of bromine into the stratosphere, and may contribute to the stratospheric ozone depletion. This compound has anthropogenic sources besides natural sources, and its use for fumigation is now being phased out in accordance with the Montreal Protocol and its amendments (UNEP, 1997). The ocean has been thought to be a major natural source for CH3Br (Singh and Kanakidou, 1993), but some cruise studies have shown that the ocean is more likely to be a net sink (source; 56 Gg/y, sink; 77 Gg/y) (Lobert et al., 1995). Current assessments of the global budget of CH 3Br show an imbalance between the known sources and sinks, with its source(s) of 70 Gg/y still need-
I NTRODUCTION Methyl halides are known to be natural sources of reactive halogens that destroy ozone in both the stratosphere and the troposphere. Among the halocarbons, methyl chloride (CH3 Cl) is the most abundant species and a major natural source of ClO x in the stratosphere (Graedel and Keene, 1995). The ocean (Singh et al., 1983), biomass burning (Lobert et al., 1991) and fungal activity in rotten woods (Harper, 1985) are known sources for this compound. Recent studies have provided evidence that terrestrial systems, especially coastal areas can contribute significantly to CH3Cl in the global atmosphere (Yokouchi et al., 2000a; Rhew *Corresponding author (e-mail:
[email protected])
137
138
H-J. Li et al.
ing to be accounted for (Yvon and Butler, 1997). Methyl iodide (CH3I), being a major natural source of iodine in the atmosphere, is much more reactive than CH3Br or CH 3Cl. The compound releases iodine atoms mostly in the troposphere through photolytic decay (Lovelock et al., 1973; Zafiriou, 1974), leading to ozone destruction in the boundary layer through rapid production of IO radicals and regeneration of I atoms on photolysis (Chameides and Davis, 1980; McFiggans et al., 2000). Observations up to several pptv IO in the marine boundary layer have been reported (Carpenter et al., 1999). Biological activity in surface waters of the ocean is considered to be a major source of CH3I (Singh et al., 1983; Gschwend et al., 1985; Manley and Dastoor, 1988; Reifenhauser and Heumann, 1992), although photochemical production in the open ocean has also been suggested as a dominant pathway (Moore and Zafiriou, 1994; Happell and Wallace, 1996). For further understanding of the distribution of the sources of these methyl halides, more comprehensive measurements are required. Here we report concurrent measurements of the latitudinal distribution of CH3 Cl, CH 3 Br, and CH3I, as well as C2Cl 4, in the western Pacific, the southeastern Indian Ocean, the South China Sea and East China Sea during a cruise conducted from December 1996 to February 1997. We discuss factors controlling the distribution of these compounds.
tion: 6-L canisters lined with fused silica (Silicocan, Restek Co., Ltd.) with a ball valve, and 3-L canisters with surfaces finished by electrochemical buffing (ECB, Ultrafinish Technology Co., Ltd.) with a diaphragm valve. Seventy-six samples collected during the cruise were subjected to gas chromatography/mass spectrometry (GC/MS) analysis after being transported to the laboratory. Details of the canister sampling and analytical methods have been published elsewhere (Li et al., 1999). The detection limits (s/n = 3) for CH3Cl, CH3Br, CH3 I, and C2Cl 4 were 2, 0.6, 0.3, and 0.7 pg, respectively, corresponding to about 0.1 to 1.7 pptv for a 500-ml air sample. Relative standard deviations for actual air samples calculated from duplicate analyses of 30 ambient samples were 1.1% for CH3Cl (av. 741 pptv), 3.4% for CH3Br (av. 12.2 pptv), and 7.4% for CH 3I (av. 1.1 pptv) (Li et al., 1999). A 6-month storage experiment confirmed that no significant change in the concentration of these gases occurred during storage (Yokouchi et al., 1999).
EXPERIMENTAL Samples were collected during the cruise KH96-5 of R/V Hakuho Maru of the Ocean Research Institute, University of Tokyo, from 19 December 1996 to 18 February 1997. The cruise track (see Fig. 1) covers a wide latitudinal range from 35°N to 45°S during the northern winter season or the southern summer. Air sampling was done twice a day, at 11:00 and 23:00 hours (local time), on the upper deck four of Hakuho Maru, on the upwind side of the vessel. Two types of evacuated stainless steel canisters with inert surfaces were used for the collec-
Fig. 1. KH-96-5 Cruise of the Hakuho Maru over the western Pacific-southeastern Indian Ocean from December 1996 to February 1997.
CH3Cl, CH 3Br and CH3I in marine boundary air
RESULTS AND D ISCUSSION Latitudinal distributions of CH3Cl, CH3Br, and CH3I observed during the cruise are shown in Figs. 2–4, together with that of C2Cl 4 shown in Fig. 5. Samples collected from different tracks (A to B, B to C, or C to A in Fig. 1) are represented by different symbols in the figures. The variations of the three methyl halides differed from one another and from that of C2Cl 4, which decreased gradually from north to south, reflecting a significant industrial emission. For track A–B, a reference
139
dataset from the 39th Japanese Antarctic Research Expedition cruise (R/V Shirase, which sailed from 31°N to 69°S between 14 November and 16 December 1997) (Yokouchi et al., 2000a), covering in part the voyage between lat 31°N and lat 50°S, whose results are added as a gray line in Figs. 2 and 3. The data on the methyl halides were divided into three groups; NH (15°N–34°N), tropics (15°N–15°S) and SH (15°S–44°S), and their mean values are given in Table 1 with the standard deviations.
Fig. 2. Latitudinal variation in the concentration of CH 3Cl from cruise KH-96-5 of the Hakuho Maru from 19 December 1996 to 18 February 1997. Gray line: CH3Cl data from the Shirase cruise in 1997.
Fig. 3. Latitudinal variation in the concentration of CH 3Br from cruise KH-96-5 of the Hakuho Maru from 19 December 1996 to 18 February 1997. Gray line: CH 3Br data from the Shirase cruise in 1997. (The data in this figure have been published in our previous paper (Yokouchi et al., 2000b).)
140
H-J. Li et al.
Fig. 4. Latitudinal variation in the concentration of CH3I from cruise KH-96-5 of the Hakuho Maru from 19 December 1996 to 18 February 1997.
Fig. 5. Latitudinal variation in the concentration of C2Cl 4 from cruise KH-96-5 of the Hakuho Maru from 19 December 1996 to 18 February 1997.
Methyl chloride The concentration of CH3Cl ranged from 505 pptv to 946 pptv with a mean value of 623 pptv (Fig. 2, Table 1). Levels of CH3 Cl were higher in the tropics (mean: 648 pptv) than in NH (mean: 590 pptv) or SH (mean: 589 pptv) (Table 1), as was the case in the Shirase cruise. The highest concentration was observed near the Andaman Islands (11°42′ N). Calculation of backtrajectory of airmasses was done for all the sampling points using NIES-CGER program, and the results showed that enhancement of CH3Cl was mostly
related to the influence of land air-masses: As high as 946 pptv and 848 pptv of CH3Cl were observed near the Andaman Island and near Malaysian Peninsula, respectively, where the airmass came from the island or from the peninsula (Fig. 6). Therefore, our finding on the enhancement of atmospheric CH3Cl in the present study strongly supports the idea that a significant amount of CH3 Cl is emitted from tropical coastal lands (Yokouchi et al., 2000a). Although biomass burning could also be a cause of higher CH3Cl in the tropics (Crutzen et al., 1979; Talbot et al., 1996), our sam-
CH3Cl, CH 3Br and CH3I in marine boundary air
141
Table 1. Mean and standard deviation of atmospheric concentrations of CH3Cl, CH3Br and CH3I in each region of NH, tropics and SH CH3 Cl mean
CH3 Br
CH3 I
s.d.
mean
s.d.
mean
s.d.
0.7 1.2 1.1 1.1
0.2 0.7 0.4 0.6
R/V Hakuho (KH-96-5) NH (15°N–34°N) (n = 11) Tropics (15°N–15°S) (n = 44) SH (15°S–44°S) (n = 21) All data (n = 76)
590 648 589 623
34 84 59 77
12.0 10.8 8.6 10.3
1.1 1.5 0.6 1.7
R/V Shirase NH (15°N–31°N) (n = 3) Tropics (15°N–15°S) (n = 7) SH (15°S–44°S) (n = 8) All data (n = 18)
581 675 565 611
53 121 27 92
11.1 10.4 8.2 9.5
1.1 1.0 1.1 1.6
Fig. 6. Two-day back trajetory of air masses where high concentration of CH3Cl was observed. (a) 11 °42 ′ N, 92 °49′ E, January 30, 03:00 (GST), (b) 7°53 ′ N, 97°8 ′ E, February 2, 03:00 (GST).
pling period (December, 1996) was not the burning season in those regions. The mean concentration of CH 3Cl excluding tropical data was 590 pptv, which was close to the average concentration observed over the open Pacific (552 pptv: Moore et al., 1996) and Atlantic (541 pptv: Koppmann et al., 1993) Oceans. For the atmospheric CH3 Cl over the land, very few data are available, making it difficult to estimate its global burden. Further studies including observation
over the land and identification of land sources are required to understand atmospheric CH 3 Cl budget. Methyl bromide The latitudinal variation in CH 3 Br in the present study (Fig. 3) was similar to that found in previous studies (Lobert et al., 1995). The concentration differed significantly between the NH and SH (NH: 12.0 pptv, SH: 8.6 pptv), and the
142
H-J. Li et al.
mean concentration was 10.3 pptv. The higher concentration in NH is consistent with the greater anthropogenic emissions there, although the NH/ SH ratio of CH3 Br was not so remarkable as that of C2Cl4 due to the longer lifetime of CH 3Br (0.7– 1.0 y, Yvon-Lewis and Butler, 1997; Yokouchi et al., 2000b). However, the highest level of CH3Br (14.8 pptv at 1.9°N, 14.0 pptv at 2.9°S, 13.5 pptv at 7.9°N) was observed in the tropics, where anthropogenic emissions are considered to be much lower than in the northern mid-latitudes, suggesting some natural emission. These data were used elsewhere (Yokouchi et al., 2000b) to estimate the natural flux by subtracting out the simulated distribution of industrial CH3Br, thus showing much higher natural emissions in the tropics than in the higher latitudes. However, natural sources responsible for the high level of CH3 Br in the tropics have not been discussed. The known natural sources for CH3 Br were the ocean (56 Gg/y) and biomass burning (20 Gg/y, partly including anthropogenic one) (WMO, 1998). Recent studies have shown new land sources for CH3Br, some types of higher plants (Gan et al., 1998) and coastal salt marshes (Rhew et al., 2000). From an observational study at Cape Hedo of Okinawa Island (subtropical), significant contribution of oceanic source to the atmospheric CH3Br had been suggested, based on the positively correlated variations of CH3Br and CH 3I (mostly ocean-origin) under summertime calm condition (RCH3Br-CH3I = 0.77, cf., R CH3Br-CH3Cl = –0.07) (Li et al., 1999). In this study, however, the distribution of atmospheric CH3 Br was not so closely related to CH3I (R = 0.18 in NH, 0.60 in the tropics, 0.61 in SH), giving no stronger evidence for the oceanic sources, although their correlation in the atmospheric concentration might have been decreased during long-transport due to large difference in lifetime (several days for CH3 I, 0.7–1 y for CH3Br). On the other hand, no enhancement of CH3Br was found near the Andaman Islands, where the highest CH3Cl level was detected in the air masses transported from the island (Fig. 6(a)). This finding suggests that land sources are not so responsible for the increased concentration of
CH3Br in the tropics as they are for the measured CH3Cl levels. Methyl iodide CH3I concentrations ranged from 0.46 pptv to 4.9 pptv, with a mean value of 1.1 pptv (Fig. 4, Table 1), which was consistent with the previous measurements, 0.3–1.8 pptv at Mace Head (Carpenter et al., 1999);
