Daniel B. King

Experimental determination of the diffusion coefficient of dimethylsulfide in water

Estimates of the sea-to-air flux of dimethylsulfide (DMS) are based on sea surface concentration measurements and gas exchange calculations. Such calculations are dependent on the diffusivity of DMS (DDMS), which has never been experimentally determined. In this study the diffusivity of DMS in pure water was measured over a temperature range of 5°–30°C. The measurements were made using a dynamic diffusion cell in which the diffusing gas flows over one side of an agar gel membrane and the inert gas flows over the other side. The diffusion coefficient can be estimated from either time dependent or steady state analysis of the data, with an estimated uncertainty of less than 8% (1σ) in each measurement. A best fit to all the experimental results yields the equation DDMS (in cm2 sec−1) = 0.020 exp (−18.1/RT), where R = 8.314 × 10−3 kJ mole−1 K−1 and T is temperature in kelvin. The values of DDMS obtained in this study were 7–28% larger than estimates from the empirical formula of Hayduk and Laudie (1974) which has previously been used for DMS in gas exchange calculations. Applying these values to seawater results in an increase of less than 5% in the global oceanic flux of DMS.

Methyl iodide: Atmospheric budget and use as a tracer of marine convection in global models

biomass burning are also included in the model. The model captures 40% of the variance in the observed seawater CH3I(aq) concentrations. Simulated concentrations at midlatitudes in summer are too high, perhaps because of a missing biological sink of CH3I(aq). We define a marine convection index (MCI) as the ratio of upper tropospheric (8–12 km) to lower tropospheric (0–2.5 km) CH3I concentrations averaged over coherent oceanic regions. The MCI in the observations ranges from 0.11 over strongly subsiding regions (southeastern subtropical Pacific) to 0.40 over strongly upwelling regions (western equatorial Pacific). The model reproduces the observed MCI with no significant global bias (offset of only +11%) but accounts for only 15% of its spatial and seasonal variance. The MCI can be used to test marine convection in global models, complementing the use of radon-222 as a test of continental convection. INDEX TERMS: 0312 Atmospheric Composition and Structure: Air/sea constituent fluxes (3339, 4504); 0322 Atmospheric Composition and Structure: Constituent sources and sinks; 0368 Atmospheric Composition and Structure: Troposphere—constituent transport and chemistry; KEYWORDS: methyl iodide, marine convection, atmospheric tracer, global budget of methyl iodide

Oceanic distributions and emissions of short‐lived halocarbons

[1] Using data from seven cruises over a 10-year span, we report marine boundary layer mixing ratios (i.e., dry mole fractions as pmol mol 1 or ppt), degrees of surface seawater saturation, and air-sea fluxes of three short-lived halocarbons that are significant in tropospheric and potentially stratospheric chemistry. CHBr3 ,C H2Br2, and CH3I were all highly supersaturated almost everywhere, all the time. Highest saturations of the two polybrominated gases were observed in coastal waters and areas of upwelling, such as those near the equator and along ocean fronts. CH3I distributions reflected the different chemistry and cycling of this gas in both the water and the atmosphere. Seasonal variations in fluxes were apparent where cruises overlapped and were consistent among oceans. Undersaturations of these gases were noted at some locations in the Southern Ocean, owing to mixing of surface and subsurface waters, not necessarily biological or chemical sinks. The Pacific Ocean appears to be a much stronger source of CHBr3 to the marine boundary layer than the Atlantic. The high supersaturations, fluxes, and marine boundary layer mixing ratios in the tropics are consistent with the suggestion that tropical convection could deliver some portion of these gases and their breakdown products to the upper troposphere and lower stratosphere.

Removal of methyl bromide in coastal seawater: Chemical and biological rates

A stable isotope tracer technique was used to investigate the loss rate of methyl bromide in surface ocean waters. Unfiltered and 0.2 μm-filtered or autoclaved aliquants of Biscayne Bay seawater samples were spiked with 13CH3Br at roughly 10–100 times ambient concentrations (50–800 pM) and incubated for 10–30 hours. The concentration of 13CH3Br was monitored using gas chromatography with isotope dilution mass spectrometry, with CD3Br as the isotope spike. Removal rates in unfiltered aliquants were significantly faster than in the 0.2 μm-filtered or autoclaved aliquants, indicating that some of the loss of methyl bromide was associated with particulate matter. Filtration experiments indicate that the particulate material responsible for methyl bromide loss is between 0.2 and 1.2 μm in diameter, suggesting that bacteria are likely to be responsible. The particulate-related removal of methyl bromide was inhibited by autoclaving, supporting a biological mechanism.

Measurement of the diffusion coefficient of sulfur hexafluoride in water

Sulfur hexafluoride has been widely used in field studies and laboratory experiments to develop a relationship between gas transfer and wind speed. The interpretation of the data from such studies requires the diffusion coefficient of SF{sub 6} (D{sub SF6}), which has not previously been measured. In this study, D{sub SF6} has been determined in pure water and in 35%NaCl over a temperature range of 5-25{degrees}C. The measurements were made using a continuous-flow diffusion cell where SF{sub 6} flows beneath an agar gel membrane while helium flows above the gel. The experimental data for pure water yielded the following equation: D{sub SF6}=0.029 exp ({minus}19.3/RT, where R is the gas constant and T is temperature in kelvins). Measurements of D{sub SF6} in 35% NaCl were not significantly different from the pure water values. On the basis of this data, the authors estimate the Schmidt numbers for seawater over the temperature range 5-25{degrees}C to be Sc=3016.1{minus}172.00t+4.4996t{sup 2}{minus}0.047965t{sup 3}, where t is temperature in degrees Celsius. 31 refs., 3 figs., 2 tabs.

Implications of methyl bromide supersaturations in the temperate North Atlantic Ocean

Methyl bromide saturation anomalies measured in the springtime North Atlantic and summertime North Pacific Oceans during 1998 revealed persistent supersaturations in the temperate waters of the northeastern Atlantic but undersaturations in tropical waters of both oceans. A comparison of data from this study with those from a previous cruise to the northeastern Atlantic suggests that methyl bromide is cycled seasonally in these waters and perhaps in all temperate open-ocean waters. This means that the calculated net flux of methyl bromide into the oceans is slightly less negative than previously reported. With these new insights we estimate that the global air-sea flux of methyl bromide ranges from −11 to −20 Gg yr−1. Data combined from this and three previous cruises support a flux dependence upon sea surface temperature, as reported recently by Groszko and Moore [1998]. Whereas sea surface temperature can account for 40–70% of the observed variability in methyl bromide globally, it is able to reproduce only a small fraction of the observed seasonal cycle in the temperate northeastern Atlantic. The development of reliable predictions of air-sea fluxes of methyl bromide will require information on additional variables as well.

The oceans: A source or a sink of methyl bromide?

The global ocean/atmosphere flux of methyl bromide has been estimated from shipboard measurements of the saturation anomaly. When such data are extrapolated globally on the basis of constant saturation anomaly, the ocean is a net sink for methyl bromide [Lobert et al, 1995]. The same data can also be extrapolated on the basis of steady-state production rate of methyl bromide in the water column, allowing regional and seasonal variations in temperature to affect the saturation anomaly. We have carried out this type of extrapolation, and we found that the oceans are a strong net source of methyl bromide to the atmosphere. The difference arises mainly due to slow degradation rates in water of higher latitudes. A reduction of the applied production rate by more than 35% is needed in order to switch the ocean from a source to a sink of methyl bromide. These results demonstrate the sensitivity of current estimates of oceanic flux to assumptions about methyl bromide production and destruction in the water column.

Performance of the HPLC/fluorescence SO2detector during the GASIE instrument intercomparison experiment

Sulfur dioxide (SO2) in synthetic air and diluted ambient air was measured as part of the Gas-Phase Sulfur Intercomparison Experiment (GASIE) using the high performance liquid chromatography (HPLC)/fluorescence technique. SO2 was analyzed by equilibrating the gaseous sample with aqueous SO2, sulfite, and bisulfite, then converting the aqueous S(IV) to an isoindole derivative. The derivative was separated by reversed phase HPLC and detected via fluorescence. The system was calibrated with mixtures of SO2 in zero air prepared from an SO2 permeation device through a two-stage dilution system. The instrument has a 4-min. sample integration time and a measurement period of 9-min. During the GASIE intercomparison the lower limit of detection averaged 3.6 parts per trillion by volume (pptv). The precision of replicate measurements over the entire intercomparison period was better than 5% at the 20 pptv level. Instrument performance was unaffected by the interferent gases included in the GASIE protocol (H2O, O3, NOx, DMS, CO, CO2, and CH4). During diluted ambient air tests, the HPLC/fluorescence technique exhibited an approximately 10% reduction in response relative to some other techniques. The cause of this apparent calibration change is not understood.