Air bubbles released from an underwater nozzle emit an acoustical pulse which is of interest both for the study of bubble detachment and for elucidating the mechanism of sound generation by a newly formed bubble. In this paper we calculate theoretically the sequence of bubble shapes from a given nozzle and show that there is for each nozzle a bubble of maximum volume vmax Assuming that the bubble becomes detached at its ‘neck’, and that the volume of the detached bubble equals the volume V* of the undetached bubble above its ’neck’, we determine for each nozzle diameter D an acoustic frequency f* corresponding to 'slow’ bubble release.Experiments show that the acoustic frequency, hence the bubble size, depends on the rate of air.flow to the bubble, but for slow rates of flow the frequency f is very close to the theoretical frequency f*.High-speed photographs suggest that when the bubble pinches off. the limiting form of the surface is almost a cone. This is accounted for by assuming a line sink along the axis of symmetry. Immediately following pinch-off there is evidence of the formation of an axial jet going upwards into the bubble. This may play a part in stimulating the emission of sound.

Air bubbles released from an underwater nozzle emit an acoustical pulse that is of interest both for the study of bubble detachment and for elucidating the mechanism of sound generation by a newly formed bubble. In this paper, the sequence of bubble shapes is calculated theoretically from a given nozzle, and it is shown that there is for each nozzle a bubble of maximum volume Vmax. Assuming that the bubble becomes detached at its “neck,” and that the volume of the detached bubble equals the volume V* of the undetached bubble above its “neck,” it is determined for each nozzle diameter D an acoustic frequency f* corresponding to “slow” bubble release. Experiments show that the acoustic frequency, hence the bubble size, depends on the rate of air flow to the bubble, but for slow rates of flow the frequency f is very close to the theoretical frequency f*. High-speed photographs suggest that when the bubble pinches off, the limiting form of the surface is almost a cone. This is accounted for by assuming a line sink along the axis of symmetry. Immediately following pinch off, there is evidence of the formation of an axial jet going upward into the bubble. This may play a part in stimulating the emission of sound.

The technique of chiral phase capillary gas chromatography was applied to investigate the degradation and transport of the persistent chiral pesticide R-hexachlorocyclohexane (R-HCH) in the Lake Ontario environment. Chiral analysis gave the enantiomeric ratios (ERs) of R-HCH in samples taken May-October 1993 from the lake and its atmosphere. ERs of (+)R-HCH/(-)R-HCH for Lake Ontario surface and deep water samples were similar and averaged 0.85 ( 0.02 as compared with a value of 1.00 for the R-HCH standard. Higher ERs were observed in water samples from the Niagara River (0.91 ( 0.02) and from precipitation (1.00 ( 0.01). Air samples of R-HCH measured at 10 m above the lake show a seasonal variability with values near 1.00 in spring and fall and minimum values in individual samples near 0.90 in summer. A simple air-water gas transfer model demonstrates that enantiomeric ratios <1.0 in air are derived from equilibration of the air with the water during transport of the air mass over the lake. Based on the measured ERs, it is calculated that as much as 60% of the R-HCH in air above Lake Ontario was derived from the lake itself. These results strongly support the need for overwater air measurements to provide better estimates of airwater gas transfer fluxes of persistent organic pollutants.

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We investigated the seasonality of the air-water gas transfer of hexachlorocyclohexanes (HCH) in Lake Ontario. These organochlorine pesticides were measured in air and water samples collected on weekly cruises on Lake Ontario from May to October 1993. Air concentrations ranged from 51 to 200 pg m -3 for R-HCH and from 8 to 133 pg m -3 for γ-HCH. Surface water concentrations of R-HCH showed a small seasonal variation: mean values decreased from 922 ( 73 pg L -1 in May to 679 ( 47 pg L -1 in August and gradually increased to 897 ( 76 pg L -1 by October. Levels of γ-HCH did not show seasonal differences and averaged 357 ( 25 in surface waters. Air-water fugacity gradients of R-and γ-HCH were used to predict the direction of air-water gas exchange of these isomers. Reversals in exchange direction were observed for both compounds on time scales of days, weeks, and months. On average, air-water gas transfer of HCH was depositional in May and early June, reversed to volatilizational by August, and returned to depositional for both HCH isomers in October. Net fluxes of R-HCH calculated using a two-film gas exchange model ranged from -50 (deposition) to 25 ng m -2 day -1 (volatilization). Net fluxes of γ-HCH ranged from -63 to 7 ng m -2 day -1 . We estimate that 15 kg of R-HCH was removed and 37 kg of γ-HCH was added to Lake Ontario by air-water gas transfer from May to October 1993.