CH3Cl and CH3Br are among the most important halogenated compounds that cause ozone depletion in the lower stratosphere. Unlike the anthropogenic Chlorofluorocarbons (CFCs), methyl halides have both man made and natural sources, which make the global budgets of these two compounds much more complicated. Large imbalances still exist between the estimated magnitudes of known sources and sinks after a decade of investigation. The importance of the ocean in the global budgets of CH3Cl and CH3Br is the focus of this dissertation, since, through air-sea gas exchange, the ocean plays a major role as a source or sink of these compounds to the atmosphere.

Improved techniques were developed for the measurement of CH3Cl and CH3Br in air and water samples, using a cryogenic purge-and-trap pre-concentration system with capillary Gas Chromatography / Electron Capture Detector (GC/ECD). Field measurements were made on several ocean cruises covering a wide latitudinal range, and in a fresh water system. Based on the collected data, a mathematical model was made to improve estimates of the importance of the global ocean as a natural source/sink of these compounds to the atmosphere.

Measurements of the atmospheric, surface water and water column distributions of CH3Cl and CH3Br were obtained in different regions, selected to be representative of the open sea (mid- to low-latitude North Pacific Ocean, high-latitude sub-Antarctic South Pacific Ocean), coastal water (Saanich Inlet, BC, Canada; Puget Sound, WA, USA), and fresh water system (Lake Washington and its tributary rivers, WA, USA).

The open ocean was found to be a net source of CH3Cl to the atmosphere in mid-latitude and tropical regions and a net sink in colder regions. A relatively wide range in near-surface concentrations was observed, corresponding to relative saturation factors (r.s.f.) between 0.46 and 2.10 (r.s.f. = 1.00 at equilibrium). In contrast, the open ocean is a net sink for CH3Br in most areas measured, except upwelling zones, with observed surface concentrations corresponding to r.s.f. of 0.25 - 1.37; Coastal waters were found to be a net source of CH3Cl and CH3Br in summer, and a net sink for both in winter. Concentration corresponds to r.s.f. 0.61 - 2.05 for CH3Cl, and 0.28 - 1.88 for CH3Br; Lake Washington was found to be a net source of CH3Br and a slight sink for CH3Cl in summer (t > 15°C), while in winter (t = ~ 8°C) the lake acted as a net sink for both compounds. Considering the small surface area of the global fresh water system (0.28 % of the ocean), the results indicate that emission from fresh waters contributes a negligible amount to the global budgets.

Air-sea gas exchange model based on wind speed and observed dis-equilibrium of these gases in near-surface waters were used to estimate the in situ production/consumption rates and the global air-sea fluxes. Calculated in situ production rates range from –4.3 to +4.8 pM/day (negative numbers represent in situ consumption) for CH3Cl, and -0.11 to +0.25 pM/day for CH3Br. Extrapolating the observations made in this study to the global ocean indicate that the open ocean acts as a net source of CH3Cl to the atmosphere, with an annual flux of 580 - 1030 Gg from water to air; and as a net sink of CH3Br, with an annual flux of 48 - 70 Gg from air to water. These results decrease the gap between previously estimated source and sink terms in the global budget for CH3Cl, yet worsen the discrepancy between the source and sink terms for CH3Br, suggesting that there are one or more significant sources (most likely terrestrial) being underestimated in current global budgets for CH3Br.