Philip D. Nightingale

In situ evaluation of air-sea gas exchange parameterizations using novel conservative and volatile tracers

Measurements of air-sea gas exchange rates are reported from two deliberate tracer experiments in the southern North Sea during February 1992 and 1993. A conservative tracer, spores of the bacterium Bacillus globigii var. Niger, was used for the first time in an in situ air-sea gas exchange experiment. This nonvolatile tracer is used to correct for dispersive dilution of the volatile tracers and allows three estimations of the transfer velocity for the same time period. The first estimation of the power dependence of gas transfer on molecular diffusivity in the marine environment is reported. This allows the impact of bubbles on estimates of the transfer velocity derived from changes in the helium/sulphur hexafluoride ratio to be assessed. Data from earlier dual tracer experiments are reinterpreted, and findings suggest that results from all dual tracer experiments are mutually consistent. The complete data set is used to test published parameterizations of gas transfer with wind speed. A gas ex- change relationship that shows a dependence on wind speed intermediate between those ofLiss and Merlivat [1986] and Wanninkhof [1992] is found to be optimal. The dual tracer data are shown to be reasonably consistent with global estimates of gas exchange based on the uptake of natural and bomb-derived radiocarbon. The degree of scatter in the data when plotted against wind speed suggests that parameters not scaling with wind speed are also influencing gas exchange rates.

Measurements of air-sea gas transfer during an open ocean algal bloom

Measurements of air-sea gas transfer were made during the development of a large algal bloom that immediately followed in-situ iron enrichment in the equatorial Pacific Ocean. Gas transfer rates were determined from changes in the ratio of the deliberately released tracers sulphur hexafluoride and helium-3. This first application of the dual tracer technique in the open ocean produced estimates of the gas transfer rate that were similar in magnitude to those obtained previously from measurements in shelf seas. We found no evidence that gas transfer rates declined during the development of the algal bloom. Incorporation of the new open ocean measurements with other published dual tracer data gives a relationship between transfer velocity and wind speed that explains 82% of the total variance in the dataset.

Seasonal variation of dimethyl sulphide in the North Sea and an assessment of fluxes to the atmosphere

The distribution of DMS concentrations in surface waters of the southern North Sea is described for nine months (February–October) in 1989. Minimum concentrations in winter were 0.13 nM and the maximum, monthly mean concentration was 25 nM, in May, coincident with large blooms of Phaeocystis pouchetii, off the continental coast. Comparison with other North Sea data suggests that the interannual seasonal pattern of DMS concentrations is similar. Transfer velocities, for sea-to-air transfer of DMS are derived, comparing a number of methods, and some of the uncertainties in the flux calculation are discussed. Optimised flux data for the North Sea show a distinct annual cycle with monthly averages ranging from 0.2 to 16.4 μmol m−2 day−1 for February and June, respectively. Comparison with other data suggests that North Sea fluxes are very similar to other ocean areas in a similar latitude band and on an annual and seasonal basis. The potential impact of North Sea DMS fluxes on the European atmospheric sulphur budget is discussed.

Distribution and sea‐air fluxes of biogenic trace gases in the eastern Atlantic Ocean

A number of atmospherically important trace gases (dimethyl sulphide (DMS), methyl iodide (CH3I), and nonmethane hydrocarbons (NMHCs)) were measured simultaneously in the eastern Atlantic Ocean during May 1997. This investigation was part of the U.K. Atmospheric Chemistry Studies in the Oceanic Environment (ACSOE) Community Research Program and covered a 200 by 200 nautical mile (1 nautical mile is 1.852 km) area to the west of the Mace Head Atmospheric Research Station on the coast of Ireland. Different spatial and temporal patterns were observed for each of the gases, showing that distinct sources dominate their production in this region: specific species of phytoplankton (DMS), macroalgae (CH3I), total phytoplankton biomass (isoprene), and photochemistry (ethene). Sea-to-air fluxes of the gases are calculated for near and offshore domains, and their temporal variations are discussed. A simple photochemical box model has been used to assess the contributions of the gas fluxes to the levels of the gases observed at Mace Head, Results show that the area studied may constitute a substantial source of DMS, a weak source of CH 3 I, a small source of ethene at night, and an insignificant source of isoprene to atmospheric levels of these gases measured at Mace Head in western Ireland.

Ocean acidification and marine trace gas emissions

The oceanic uptake of man-made CO2 emissions is resulting in a measureable decrease in the pH of the surface oceans, a process which is predicted to have severe consequences for marine biological and biogeochemical processes [Caldeira K, Wickett ME (2003) Nature 425:365; The Royal Society (2005) Policy Document 12/05 (Royal Society, London)]. Here, we describe results showing how a doubling of current atmospheric CO2 affects the production of a suite of atmospherically important marine trace gases. Two CO2 treatments were used during a mesocosm CO2 perturbation experiment in a Norwegian fjord (present day: ∼380 ppmv and year 2100: ∼750 ppmv), and phytoplankton blooms were stimulated by the addition of nutrients. Seawater trace gas concentrations were monitored over the growth and decline of the blooms, revealing that concentrations of methyl iodide and dimethylsulfide were significantly reduced under high CO2. Additionally, large reductions in concentrations of other iodocarbons were observed. The response of bromocarbons to high CO2 was less clear cut. Further research is now required to understand how ocean acidification might impact on global marine trace gas fluxes and how these impacts might feed through to changes in the earths future climate and atmospheric chemistry.

Influence of energetic wind and waves on gas transfer in a large wind–wave tunnel facility

Air–water gas exchange experiments were carried out in a large wind wave tunnel in Marseille, France, to investigate gas transfer processes under energetic wind and wave fields, where macroscale breaking waves create bubble plumes (white caps) and turbulence on the water surface. We measured the gas transfer velocities of N2O, DMS, He, SF6, CH3Br, and total air. Their diffusivity and solubility span a large range, allowing us to investigate gas transfer mechanisms under a variety of physical conditions. We observed that the gas transfer velocities varied with friction velocity in a linear manner. Gas transfer in the presence of pure wind waves is generally consistent with the surface renewal model, as the gas transfer velocity has a strong dependence on diffusivity with an exponent of 0.53(±0.02). Contrary to expectations, the bubble plumes generated by breaking waves contributed relatively little in our pure wind wave experiments. Superposition of mechanically generated waves onto the wind waves in the high wind regime attenuated DMS gas transfer (as a function of friction velocity) across the air–water interface by ~20% compared with gas transfer under pure wind waves, implying suppression of gas transfer directly across the sheared water surface. Greater transfer of less soluble gases may result from bubble-mediated gas transfer.

Biological production of methyl bromide in the coastal waters of the North Sea and open ocean of the northeast Atlantic

Two separate studies in different oceanic regions provide evidence for the production of methyl bromide (CH3Br) by the prymnesiophyte Phaeocystis. A sampling program to study the seasonal cycle of CH3Br in a coastal area demonstrated that the seawater was supersaturated with respect to CH3Br for over 3 months of the year. The greatest saturation was observed during a bloom of Phaeocystis. Also, in situ field measurements demonstrated that CH3Br was supersaturated over a large region of the northeast Atlantic. A positive correlation was observed between CH3Br and dimethylsulphoniopropionate (DMSP), indicating that there was a source common to both compounds. An accessory pigment, hexanoyloxyfucoxanthin, which indicates the presence of prymnesiophytes, also correlated positively with CH3Br.

Microbial methanol uptake in northeast Atlantic waters

Methanol is the predominant oxygenated volatile organic compound in the troposphere, where it can significantly influence the oxidising capacity of the atmosphere. However, we do not understand which processes control oceanic concentrations, and hence, whether the oceans are a source or a sink to the atmosphere. We report the first methanol loss rates in seawater by demonstrating that 14C-labelled methanol can be used to determine microbial uptake into particulate biomass, and oxidation to 14CO2. We have found that methanol is used predominantly as a microbial energy source, but also demonstrated its use as a carbon source. We report biological methanol oxidation rates between 2.1 and 8.4 nmol l−1 day−1 in surface seawater of the northeast Atlantic. Kinetic experiments predict a Vmax of up to 29 nmol l−1 day−1, with a high affinity Km constant of 9.3 nM in more productive coastal waters. We report surface concentrations of methanol in the western English channel of 97±8 nM (n=4) between May and June 2010, and for the wider temperate North Atlantic waters of 70±13 nM (n=6). The biological turnover time of methanol has been estimated between 7 and 33 days, although kinetic experiments suggest a 7-day turnover in more productive shelf waters. Methanol uptake rates into microbial particles significantly correlated with bacterial and phytoplankton parameters, suggesting that it could be used as a carbon source by some bacteria and possibly some mixotrophic eukaryotes. Our results provide the first methanol loss rates from seawater, which will improve the understanding of the global methanol budget.

Low molecular weight halocarbons in the Humber and Rhine estuaries determined using a new purge-and-trap gas chromatographic method

A number of chlorinated and brominated low molecular weight hydrocarbons (halocarbons) have been measured in and adjacent to the North Sea estuaries of the Humber and the Rhine. The measurements have been carried out using a newly constructed purge-and-trap sample work-up system coupled to megabore gas chromatography with electron capture detection. The results show that whereas the Humber is a pronounced source of the anthropogenic halocarbons carbon tetrachloride and perchloroethylene, the input from the Rhine into the North Sea of these compounds is more modest. Some halocarbons normally considered as mainly or even exclusively of natural origin are released from the two investigated estuaries into the North Sea. A distinct patch of high concentrations of the naturally produced compound bromoform was observed in the southwestern North Sea. The results have also been used to examine some of the halocarbons for common sources.

Emissions of CH3Br, organochlorines, and organoiodines from temperate macroalgae

The production rates of a range of low molecular weight halogenated organics have been determined in cultures of five temperate species of macroalgae collected from the north coast of Norfolk, England. Compounds studied included CH3Br, the chlorinated organics CH3Cl, CH2Cl2 and CHCl3, and the iodinated organics CH3I, C2H5I, and CH2ClI. Measurements of a wider range of halocarbon concentrations in an isolated rockpool and in air over the seaweed bed were also conducted to evaluate the local impact of the seaweeds on halocarbon concentrations in the natural environment. Estimates for the global emissions of some of the key halogenated compounds from macroalgae have been derived. In general macrophytes appear not to be globally significant producers of the particular halocarbons studied. In coastal regions, however, the impact on local atmospheric composition and chemistry could be greater.