The Global Distribution of Atmospheric Methyl Chloride

M.A. Khalil

Department of Physics, Portland State University, Portland, Oregon 97207-0751

R.A. Rasmussen

Department of Environmental Sciences and Engineering, Oregon Graduate Institute, Portland 97291

Introduction

Methyl chloride is the most abundant chlorine containing gas in the Earth’s atmosphere. It is also the largest natural source of chlorine to the stratosphere. As such there is considerable interest in its global cycle which includes its sources, sinks, and trends.

We have taken measurements of methyl chloride and other gases at the polar, middle, and tropical latitudes of both hemispheres. The sites are Pt. Barrow, Alaska (71.16ºN, 156.5ºW); Cape Meares, Oregon (45.5ºN, 124ºW); Cape Kumukahi and Mauna Loa, Hawaii (19.3ºN, 154.5ºW); Cape Matatula, Samoa (14.1ºS, 170.6ºW); Cape Grim, Tasmania (42ºS, 145ºE); and Antarctica (South Pole at 90ºS and Palmer Station at 65.46ºS, 64.05ºW). Most of these sites are also part of the NOAA-CMDL global monitoring network. Our present data extend back to 1981, but not all sites were sampled until 1984 or so. In this paper we will tabulate annual average concentrations of methyl chloride (1981-1996) and discuss the salient features of the data.

Methods

Samples were collected in 0.8-L internally electropolished stainless steel canisters. These containers preserve the concentrations of most trace gases for periods much longer than the time between sample collection and analysis [Edgerton, 1985]. This time, between sampling and analysis, varies from site to site; it is as short as a few days for Cape Meares and can be as long as 6-9 months for samples from Antarctica. Triplicate samples are collected once per week. The samples are analyzed using a gas chromatograph equipped with an electron capture detector as described in earlier publications [Rasmussen and Khalil, 1980; Rasmussen et al., 1980].

Results

The data obtained are given in Table 1 as annual averages. These data have some noteworthy features that we will discuss next. The first is that concentrations in the tropical latitudes are higher than at middle and polar latitudes. This latitudinal bump is on average about 37 pptv or about 6% (tropics to pole difference). It was also observed in earlier work but has not been seen in certain experiments on board ship cruises or aircraft flights. One reason is that there are seasonal variations in the concentrations at various latitudes, and the latitudinal bump can virtually disappear during some seasons. Samples taken over a short period are affected by the seasonal cycle and are not directly comparable to the long-term annualized latitudinal distribution as represented in Table 1.

The general features of the latitudinal distribution suggest that the major sources of methyl chloride must be in the tropics. This conclusion is further strengthened by the fact that the abundance of hydroxyl radicals (OH) is much greater in the tropics (by a factor of 3 or so) compared to the middle latitudes. Since reaction with OH is the main process by which methyl chloride is removed from the atmosphere, it takes a large tropical source to sustain the high con-centrations observed. Estimates of the total regional sources, based on these observations, suggest that some 85% of the emissions may be from the tropical regions (±30 degrees latitude, Khalil and Rasmussen [1997]). The total emissions needed to balance the observed concentrations are about 3.6 ± 0.4 Tg yr^-1 and the amount of methyl chloride in the atmosphere is calculated to be 5 Tg yr^-1, giving an estimated average atmospheric lifetime of 1.3 years.

Another important question is whether the concentration of methyl chloride is changing over long time scales. Our data show trends, but these trends are not large enough to be unequivocal. For the 13-year period between January 1984 and December 1996, the trends calculated by linear regression, based on the annual average data, are shown in Figure 1 (there are data at all sites during this period). During this period there is an indication of a decrease at Barrow (-0.3 ± 0.2% yr^-1) and Cape Meares (-0.7 ± 0.2% yr^-1) representing the middle northern hemisphere latitudes. No significant trends are seen in the tropics, and a weak and barely statistically significant increase is seen at Tasmania (+0.3 ± 0.2% yr^-1). Trends can also be calculated by taking all years of data into account, although in this case the results are over different time periods depending on the site. The overall pattern remains nearly the same. Trends are negative at all sites except Tasmania, but are statistically significant only at Barrow (-0.3 ± 0.2 % yr^-1), Cape Meares (-0.7 ± 0.2% yr^-1) and Samoa (-0.3 ± 0.2% yr^-1). In all cases the trends are small and difficult to explain. We speculate that these trends are likely to be related to changes in regional biomass burning. One alternative explanation, among others, would be a slow increase in OH concentrations in the northern hemisphere. The long time series of data shown here are discussed in more detail by Khalil and Rasmussen [1997]. These data are currently being used to construct a budget of methyl chloride that can be further tested by field experiments.

TABLE 1. Annually Averaged Concentrations of Methyl Chloride at Sites in the Polar, Middle and Tropical Latitudes of Both Hemispheres*

*Samples were taken at sea level. Concentrations are reported as mixing ratios in parts per trillion.

Fig. 1. The trends of methyl chloride (pptv yr^-1) (January 1984-December 1996).

Acknowledgments. We thank the staffs of the CMDL program, the Baseline Cape Grim Station, and CSIRO for collecting samples for us. We thank Don Stearns, Jim Mohan, Bob Dalluge (OGI), and Martha Shearer (PSU) for their contributions to this project. The analysis of the data were supported in part by the Chemical Manufacturers Association (Chlorine Chemistry Council) and the European Chemical Industry Council (Euro Chlor) through a grant to the University of Virginia as part of the Reactive Chlorine Emission Inventory activity of GEIA. Additional support was provided by the resources of the Biospherics Research Corporation and the Andarz Co.

References

Edgerton, S.A., Gaseous tracers in receptor modeling: Methylchloride emissions from wood combustion, Ph.D. dissertation, Oregon Graduate Center, Beaverton, 1985.

Khalil, M.A.K., and R.A.Rasmussen, Atmospheric methyl chloride, Rpt. No. 01-97, Dept. of Physics, Portland State Univ., Portland, OR, 1997.

Rasmussen, R.A., and M.A.K. Khalil, Atmospheric halocarbons: Measurements and analyses of selected trace gases, Proc., NATO Adv. Study Ins. on Atmos. Ozone, edited by A.C. Aikin, Dept. Transp., Washington, D.C., 1980.

Rasmussen, R.A., L.E. Rasmussen, M.A.K. Khalil, and R.W. Dalluge, Concentration distribution of methylchloride in the atmosphere, J. Geophys. Res., 85, 7350-7356, 1980.

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