The concentration of carbonyl sulfide (OCS) has been determined in several preindustrial air samples extracted from a shallow ice core from Siple Dome, West Antarctica. The extraction of the air was carried out by shredding the core under vacuum, followed by analysis by gas chromatography with mass spectrometric detection. 11 ice core samples were analyzed, ranging in depth from 82.3 to 90.7 m. The ages assigned to OCS in the ice core samples range from 1616 to 1694 AD. The observed OCS mixing ratios range from 322.6 pptv to 442.3 pptv with a mean of 372.8 ± 37.2 pptv. These levels are significantly lower than the present day atmospheric mixing ratio of 500 pptv and suggest that the anthropogenic sources comprise approximately 25% of the present day OCS budget.

Alkyl nitrates have been observed in remote oceanic regions of the troposphere and in the surface ocean. The mechanism for their production in the oceans is not known. A likely source is the reaction of ROO + NO (where R is an alkyl group). Steady-state laboratory experiments show that alkyl nitrates are produced in the aqueous phase via this reaction, with branching ratios of 0.23 ± 0.04, 0.67 ± 0.03, and 0.71 ± 0.04 for methyl, ethyl, and propyl nitrate respectively. The branching ratios in aqueous solution are significantly higher than in the gas phase. Irradiation of surface seawaters yield rates of alkyl nitrate production on the order of 10⁻¹⁸ mol cm⁻³ s⁻¹, suggesting that the reaction of ROO and NO is an important source of alkyl nitrates in seawater.

Dimethylsulfide (DMS) atmospheric and oceanic concentrations and eddy covariance air/sea fluxes were measured over the N. Atlantic Ocean during July 2007 from Iceland to Woods Hole, MA, USA. Seawater DMS levels north of 55°N ranged from 3 to 17 nM, with variability related to the satellite-derived distributions of coccoliths and to a lesser extent, chlorophyll. For the most intense bloom region southwest of Iceland, DMS air/sea fluxes were as high as 300 μmol m⁻² d⁻¹, larger than current model estimates. The observations imply that gas exchange coefficients in this region are significantly greater than those estimated using most gas transfer parameterizations. South of 55°N, DMS levels were lower and the gas transfer coefficients were similar to those observed in other regions of the ocean. The data suggest that DMS emissions from the bloom region may be significantly larger than current estimates. The anomalous gas exchange coefficients likely reflect strong near-surface, water column DMS gradients influenced by physical and biological processes.

This paper presents the first ice core measurements of methyl bromide (CH₃Br). Samples from a shallow Antarctic ice core (Siple Dome, West Antarctica), ranging in mean gas dates from 1671 to 1942, had a mean CH₃Br mixing ratio of 5.8 ppt. These results extend the existing historical record derived from air and Antarctic firn air to about 350 years before present. Model simulations illustrate that the ice core results are consistent with estimates of the impact of anthropogenic activity (fumigation, combustion, and biomass burning) on the atmospheric CH₃Br burden, given the large current uncertainties in the modern atmospheric CH₃Br budget. A preindustrial scenario assuming no fumigation, no combustion, and a 75% reduction in biomass-burning sources yields a Southern Hemisphere mean mixing ratio of 5.8 ppt, in good agreement with the ice core results. There is a significant imbalance between the known CH₃Br sources and sinks in the modern atmospheric CH₃Br budget. The ice core data do not sufficiently constrain the model to determine how much of the “unknown source” was present in the preindustrial budget. The results do indicate that most of the southern hemispheric component of this “unknown source” is not anthropogenic.

Shipboard measurements of air/sea fluxes and sea surface concentrations of dimethylsulfide (DMS) were made over the tropical and midlatitude North Pacific Ocean. Atmospheric pressure chemical ionization mass spectrometry was used to measure DMS levels in ambient air and in air equilibrated with surface seawater drawn from a depth of 5 m. Air/sea fluxes were obtained using eddy covariance. Corrections and uncertainties involved in the calculation of fluxes from shipboard data are discussed. The surface ocean DMS concentrations measured during this study ranged from 1 to 10 nM, and atmospheric mixing ratios ranged from 20 to 1000 ppt. Air/sea fluxes ranged from 0.47 to 39 μmol m⁻² d⁻¹. Most of the variance in the fluxes can be accounted for by variations in sea surface concentration (37%) and wind speed (19%). Gas transfer coefficients derived from the measurements are generally consistent with those derived from deliberate inert gas tracer experiments and eddy covariance measurements of CO₂. The gas transfer coefficients exhibit wind speed dependence, but the variance in the data is sufficiently large that they do not constrain the functionality of the wind speed dependence of gas exchange.

Air-sea dimethylsulfide (DMS) fluxes and bulk air-sea gradients were measured over the Southern Ocean in February-March 2012 during the Surface Ocean Aerosol Production (SOAP) study. The cruise encountered three distinct phytoplankton bloom regions, consisting of two blooms with moderate DMS levels, and a high biomass, dinoflagellate-dominated bloom with high seawater DMS levels (> 15 nM). Gas transfer coefficients were considerably scattered at wind speeds above 5 m s-1. Bin averaging the data resulted in a linear relationship between wind speed and mean gas transfer velocity consistent with that previously observed. However, the wind-speed-binned gas transfer data distribution at all wind speeds is positively skewed. The flux and seawater DMS distributions were also positively skewed, which suggests that eddy covariance-derived gas transfer velocities are consistently influenced by additional, log-normal noise. A flux footprint analysis was conducted during a transect into the prevailing wind and through elevated DMS levels in the dinoflagellate bloom. Accounting for the temporal/spatial separation between flux and seawater concentration significantly reduces the scatter in computed transfer velocity. The SOAP gas transfer velocity data show no obvious modification of the gas transfer-wind speed relationship by biological activity or waves. This study highlights the challenges associated with eddy covariance gas transfer measurements in biologically active and heterogeneous bloom environments.

[1] In this study, direct measurements of the air/sea flux of acetone were made over the North Pacific Ocean. The results demonstrate that the net flux of acetone is into, rather than out of the oceans. The extrapolated global ocean uptake of 48 Tg yr⁻¹ requires a major revision of the atmospheric acetone budget. This result is consistent with a recent reevaluation of acetone photodissociation quantum yields.

Shipboard measurements of eddy covariance dimethylsulfide (DMS) air–sea fluxes and seawater concentration were carried out in the North Atlantic bloom region in June/July 2011. Gas transfer coefficients (k₆₆₀) show a linear dependence on mean horizontal wind speed at wind speeds up to 11 m s⁻¹. At higher wind speeds the relationship between k₆₆₀ and wind speed weakens. At high winds, measured DMS fluxes were lower than predicted based on the linear relationship between wind speed and interfacial stress extrapolated from low to intermediate wind speeds. In contrast, the transfer coefficient for sensible heat did not exhibit this effect. The apparent suppression of air–sea gas flux at higher wind speeds appears to be related to sea state, as determined from shipboard wave measurements. These observations are consistent with the idea that long waves suppress near-surface water-side turbulence, and decrease interfacial gas transfer. This effect may be more easily observed for DMS than for less soluble gases, such as CO₂, because the air–sea exchange of DMS is controlled by interfacial rather than bubble-mediated gas transfer under high wind speed conditions.