Global Distribution of Chloroform in the Marine Boundary Layer

M.A.K. KHALIL

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

R.A. RASMUSSEN

Department of Environmental Science, Oregon Graduate Institute, Portland, Oregon 97291

Atmospheric chloroform comes from many natural and anthropogenic sources. We discuss its latitudinal distribution in the atmospheric boundary layer based on data taken at six sites, between January 1986 and January 1995, operated by the CMDL program. The interhemispherical ratio of concentrations is about 2.1 + 0.2 and the globally-averaged concentration is 17.3 + 1.3 pptv (+ values are 90% confidence limits).

In recent years there has been a growing interest in the global budgets of chlorine containing trace gases in the atmosphere because of their role in depleting the stratospheric ozone layer and in affecting tropospheric ozone. The most important of these gases are the chlorofluorocarbons, but other gases such as methylchloride, chloroform, trichloroethylene, perchloroethylene, dichloromethane and similar compounds also contribute to the reservoir of relatively long-lived chlorine containing gases with lifetimes between a month and several years. Little is known about the global budgets and biogeochemical cycles of these gases.

In this paper we describe the salient features of the global distribution of chloroform (CHCl₃). Chloroform is emitted in substantial quantities from the oceans and soils. In addition, there are many anthropogenic sources, including emissions from chlorination of water, chemical manufacturing, and combustion processes [Graedel, 1978; Khalil et al., 1983, 1990].

Between 1986 and 1994 we obtained flask samples from eight sites distributed around the world. This sampling is part of our cooperative project with the CMDL program supplemented with our own sites. The sites are Barrow, Alaska (BRW) (71.16°N, 156.5°W), Cape Meares, Oregon (CM 45.5°N, 124°W), Cape Kumukahi and Mauna Loa, Hawaii (CK and MLO 19.3°N, 154.5°W), Samoa (SA 14.1S, 170.6°W), Cape Grim, Tasmania (CG 42S, 145°E), Palmer Station (PS 64.46°S, 64.05°W) and the South Pole (SP 90°S). Data from these sites are reported in Figure 1. It is important to note that these concentrations represent a long-term climatology of chloroform concentrations and that these measurements represent the marine boundary layer. There is evidence that over land, concentrations can be substantially higher at some latitudes.
 

Fig. 1: The long-term concentration of chloroform in the marine boundary layer. Data shown here were taken between January 1986 and January 1995. From north to south (sine latitude = +1, the sites are Barrow, Alaska; Cape Meares, Oregon; Cape Kumukahi, Hawaii; Samoa; Cape Grim, Tasmania; Palmer Station, Antarctica; and the South Pole.

Table 1 shows the concentration of chloroform at the five main sites where there are enough data to estimate mean annual concentrations for each of the years between January 1986 and January 1995. The results show that there are no secular trends, although the concentrations do tend to increase and decrease over short periods of time. The trends, over the entire period, are generally not statistically significant at the 5% level. Concentrations seemed to have increased between 1986 and 1990 and then have been falling to the present.

TABLE 1. Annual and Long-Term Averages of Chloroform Concentrations (pptv)

Acknowledgments. This work was supported in part by grants from NSF (ATM-8109047) and DOE (DE-FG06-85ER6031). Additional support was provided by the Biospherical Research Corporation and the Andarz Co. We thank the staffs of the CMDL program, the Baseline Cape Grim Station, and CSIRO for collecting samples. We thank Don Stearns, Jim Mohan, Bob Dalluge, Rohith Gunawardena (OGI), Martha Shearer and Francis Moraes ( PSU), for their contributions to the project.

REFERENCES

Graedel, T.E., Chemical Compounds in the Atmosphere, Academic Press, NY, 440 pp., 1978.Khalil, M.A.K., R.A. Rasmussen, and S.D. Hoyt, Atmospheric chloroform: Ocean-air, exchange and global mass balance, Tellus, 35B, 266-274, 1983.

Khalil, M.A.K., R.A. Rasmussen, J.R. French, and J. Holt, The influence of termites on atmospheric trace gases, J. Geophys. Res., 95, 3619-3634, 1990.

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