Distributions and fluxes of methyl halides in natural waters

Improved techniques were developed for the measurement of methyl chloride (CH₃Cl) and methyl bromide (CH₃Br) in air and water samples, using a purge-and-trap preconcentration system with electron capture gas chromatography. Field measurements were made in the ocean and in fresh water systems to improve estimates of the importance of these regions as natural sources/sinks of these compounds to the atmosphere.; Measurements of the atmospheric, surface water and water column distributions of CH₃Cl and CH₃Br were obtained in three regions, selected to be representative of the open ocean, coastal water and fresh water respectively: (a) on long meridional sections in the Pacific Ocean (from 10°N–40°N along 152°W/158°W; and from 45°–67°S along 170°E/170°W); (b) in Saanich Inlet (BC, Canada) and Puget Sound (Seattle, WA); (c) in Lake Washington and its tributary rivers (Seattle, WA).; The open ocean was found to be a net source of CH₃Cl to the atmosphere in warm regions (t > 15°C, latitude < 40°C), and a net sink in colder regions. The open ocean was found to generally be a net sink for CH₃Br. Coastal waters were found to be a net source of both compounds in summer (t > 15°C), and a net sink for both in winter (t = $\sim$8°C). Lake Washington was found to be a net source of CH₃ Br and a slight sink for CH₃Cl in summer (t > 15°C), while in winter (t = $\sim$8°C) the lake acted as a net sink for both compounds.; Models based on wind speed and observed disequilibrium of these gases in near-surface waters were used to estimate the in situ production/consumption rates, and the global air-sea fluxes of these gases. 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. Fresh water systems contribute a negligible amount of methyl halides to the atmosphere.