A comprehensive estimate for loss of atmospheric carbon tetrachloride (CCl4) to the ocean

© The Author(s), 2016. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Atmospheric Chemistry and Physics 16 (2016): 10899-10910, doi:10.5194/acp-16-10899-2016.Extensive undersaturations of carbon tetrachloride (CCl4) in Pacific, Atlantic, and Southern Ocean surface waters indicate that atmospheric CCl4 is consumed in large amounts by the ocean. Observations made on 16 research cruises between 1987 and 2010, ranging in latitude from 60° N to 77° S, show that negative saturations extend over most of the surface ocean. Corrected for physical effects associated with radiative heat flux, mixing, and air injection, these anomalies were commonly on the order of −5 to −10 %, with no clear relationship to temperature, productivity, or other gross surface water characteristics other than being more negative in association with upwelling. The atmospheric flux required to sustain these undersaturations is 12.4 (9.4–15.4) Gg yr−1, a loss rate implying a partial atmospheric lifetime with respect to the oceanic loss of 183 (147–241) yr and that  ∼  18 (14–22)  % of atmospheric CCl4 is lost to the ocean. Although CCl4 hydrolyzes in seawater, published hydrolysis rates for this gas are too slow to support such large undersaturations, given our current understanding of air–sea gas exchange rates. The even larger undersaturations in intermediate depth waters associated with reduced oxygen levels, observed in this study and by other investigators, strongly suggest that CCl4 is ubiquitously consumed at mid-depth, presumably by microbiota. Although this subsurface sink creates a gradient that drives a downward flux of CCl4, the gradient alone is not sufficient to explain the observed surface undersaturations. Since known chemical losses are likewise insufficient to sustain the observed undersaturations, this suggests a possible biological sink for CCl4 in surface or near-surface waters of the ocean. The total atmospheric lifetime for CCl4, based on these results and the most recent studies of soil uptake and loss in the stratosphere is now 32 (26–43) yr.This research could not have been done without the support of our various institutions and the programs through which they support science, including funds at various times from NASA’s Upper Atmosphere Research Program, the US Department of Energy, NOAA’s Climate Program Office, the Atmospheric and Geosciences sections of the National Science Foundation, and the National Research Council of the US National Academies of Science

A comprehensive estimate for loss of atmospheric carbon tetrachloride (CCl4) to the ocean

© The Author(s), 2016. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Atmospheric Chemistry and Physics 16 (2016): 10899-10910, doi:10.5194/acp-16-10899-2016.Extensive undersaturations of carbon tetrachloride (CCl4) in Pacific, Atlantic, and Southern Ocean surface waters indicate that atmospheric CCl4 is consumed in large amounts by the ocean. Observations made on 16 research cruises between 1987 and 2010, ranging in latitude from 60° N to 77° S, show that negative saturations extend over most of the surface ocean. Corrected for physical effects associated with radiative heat flux, mixing, and air injection, these anomalies were commonly on the order of −5 to −10 %, with no clear relationship to temperature, productivity, or other gross surface water characteristics other than being more negative in association with upwelling. The atmospheric flux required to sustain these undersaturations is 12.4 (9.4–15.4) Gg yr−1, a loss rate implying a partial atmospheric lifetime with respect to the oceanic loss of 183 (147–241) yr and that  ∼  18 (14–22)  % of atmospheric CCl4 is lost to the ocean. Although CCl4 hydrolyzes in seawater, published hydrolysis rates for this gas are too slow to support such large undersaturations, given our current understanding of air–sea gas exchange rates. The even larger undersaturations in intermediate depth waters associated with reduced oxygen levels, observed in this study and by other investigators, strongly suggest that CCl4 is ubiquitously consumed at mid-depth, presumably by microbiota. Although this subsurface sink creates a gradient that drives a downward flux of CCl4, the gradient alone is not sufficient to explain the observed surface undersaturations. Since known chemical losses are likewise insufficient to sustain the observed undersaturations, this suggests a possible biological sink for CCl4 in surface or near-surface waters of the ocean. The total atmospheric lifetime for CCl4, based on these results and the most recent studies of soil uptake and loss in the stratosphere is now 32 (26–43) yr.This research could not have been done without the support of our various institutions and the programs through which they support science, including funds at various times from NASA’s Upper Atmosphere Research Program, the US Department of Energy, NOAA’s Climate Program Office, the Atmospheric and Geosciences sections of the National Science Foundation, and the National Research Council of the US National Academies of Science

Atmospheric hydrogen sulfide over the equatorial Pacific (SAGA 3)

Atmospheric H2S concentrations were measured over the equatorial Pacific on leg 1 of the third Soviet-American Gases and Aerosols (SAGA 3) cruise during February and March 1990. Five N-S transects were made across the equator between Hawaii and American Samoa. The concentrations ranged from below the detection limit of 0.4 ± 0.5 (1 σ) to 14.4 ppt with an average value of 3.6 ± 2.3 ppt (1σ, n= 72). The highest concentrations were found on the easternmost two transects just south of the equator. The average concentration of 3.6 ppt observed on this cruise is the lowest reported value for background atmospheric H2S over the tropical oceans. A lack of correlation between 222Rn and H2S rules out a significant continental source. Model calculations indicate that the oceanic source of H2S in this region is in the range of 9 to 21 × 10−8 mol m−2 d−1. From this flux the concentration of free sulfide (H2S + S= ) in the surface mixed layer of the ocean is estimated to be in the range of 32 to 67 pmol L−1. In the atmosphere the oxidation of H2S produces SO2 at a rate of 2.1 to 4.4 × 10−11 mol m−3 d−1 which is only a small fraction of that estimated from the oxidation of dimethyl sulfide (DMS) in this region. A diurnal cycle was not observed in the H2S data recorded during this cruise

Atmospheric hydrogen sulfide over the equatorial Pacific (SAGA 3)

Atmospheric H2S concentrations were measured over the equatorial Pacific on leg 1 of the third Soviet-American Gases and Aerosols (SAGA 3) cruise during February and March 1990. Five N-S transects were made across the equator between Hawaii and American Samoa. The concentrations ranged from below the detection limit of 0.4 ± 0.5 (1 σ) to 14.4 ppt with an average value of 3.6 ± 2.3 ppt (1σ, n= 72). The highest concentrations were found on the easternmost two transects just south of the equator. The average concentration of 3.6 ppt observed on this cruise is the lowest reported value for background atmospheric H2S over the tropical oceans. A lack of correlation between 222Rn and H2S rules out a significant continental source. Model calculations indicate that the oceanic source of H2S in this region is in the range of 9 to 21 × 10−8 mol m−2 d−1. From this flux the concentration of free sulfide (H2S + S= ) in the surface mixed layer of the ocean is estimated to be in the range of 32 to 67 pmol L−1. In the atmosphere the oxidation of H2S produces SO2 at a rate of 2.1 to 4.4 × 10−11 mol m−3 d−1 which is only a small fraction of that estimated from the oxidation of dimethyl sulfide (DMS) in this region. A diurnal cycle was not observed in the H2S data recorded during this cruise

Atmospheric hydrogen sulfide over the equatorial Pacific (SAGA 3)

Atmospheric H2S concentrations were measured over the equatorial Pacific on leg 1 of the third Soviet-American Gases and Aerosols (SAGA 3) cruise during February and March 1990. Five N-S transects were made across the equator between Hawaii and American Samoa. The concentrations ranged from below the detection limit of 0.4 ± 0.5 (1 σ) to 14.4 ppt with an average value of 3.6 ± 2.3 ppt (1σ, n= 72). The highest concentrations were found on the easternmost two transects just south of the equator. The average concentration of 3.6 ppt observed on this cruise is the lowest reported value for background atmospheric H2S over the tropical oceans. A lack of correlation between 222Rn and H2S rules out a significant continental source. Model calculations indicate that the oceanic source of H2S in this region is in the range of 9 to 21 × 10−8 mol m−2 d−1. From this flux the concentration of free sulfide (H2S + S= ) in the surface mixed layer of the ocean is estimated to be in the range of 32 to 67 pmol L−1. In the atmosphere the oxidation of H2S produces SO2 at a rate of 2.1 to 4.4 × 10−11 mol m−3 d−1 which is only a small fraction of that estimated from the oxidation of dimethyl sulfide (DMS) in this region. A diurnal cycle was not observed in the H2S data recorded during this cruise