| Summary: | An analytical method involving purge-and-trap and gas chromatographic techniques was developed for the determination of methyl chloride in aqueous samples. Vertical profiles of methyl chloride in the North-west Atlantic during spring showed higher concentrations above the seasonal thermocline and a general decrease with depth. Near-surface maxima were a common feature and several broad, less intense maxima were observed within the main thermocline. No simple relationship existed between concentrations of chlorophyll-a and methyl chloride. The average surface saturation was 268% using an average atmospheric mixing ratio of 625 pptv. The Henry's Law constant (H) for methyl chloride in seawater was measured at different temperatures by a dynamic stripping method and applied in the estimation of a sea-air flux of methyl chloride using the surface concentrations in the North-west Atlantic. The transfer velocity was corrected for temperature through the Schmidt Number. The calculated flux was $2.3 \times 10\sp $ g yr$\sp{-1}$, the lowest estimate to date. Methyl chloride was measured in Bedford Basin, Nova Scotia, during February to June. Low concentrations in waters from below the sill depth required a loss rate greater than that predicted from its rate of chemical hydrolysis in distilled water. Microbial destruction of methyl chloride in waters containing low levels of oxygen was suggested. A moderate positive correlation (r = 0.65) between methyl chloride and chlorophyll-a integrated over 0-20m depth could not be considered conclusive evidence of a phytoplankton source of methyl chloride due to the possibility of increasing emissions from macroalgae coincident with the bloom development. Net production of methyl chloride was observed in xenic unialgal cultures of cold and warm water phytoplankton. Further increases in methyl chloride observed after the death of all the phytoplankton cells were evidence for the existence of chemical and/or bacterial mechanisms converting organic substrates released by the phytoplankton to methyl chloride. When scaled using levels of chlorophyll-a, the laboratory rates can account for $<$0.1% of the estimated oceanic source of methyl chloride. Thesis (Ph.D.)--Dalhousie University (Canada), 1995.
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