Huixiang Xie

Photochemical production of carbon disulphide in seawater

It is generally accepted that the ocean is an important source for atmospheric CS 2 , which makes a major contribution to the formation of COS in the atmosphere. The processes producing CS 2 in seawater, however, are essentially unknown. We report for the first time to our knowledge that marine photochemical reactions are identified as a significant source for oceanic CS 2 . Apparent quantum yield spectra of CS 2 production were obtained using water samples collected in the northeast Atlantic. Results indicate that it is mainly UV solar radiation (290-340 nm) which is responsible for CS 2 photoproduction. The photoproduction rate of CS 2 is positively correlated with absorbance at 350 nm, suggesting that the reactions are mediated by chromophoric dissolved organic matter (CDOM). Laboratory irradiations have confirmed that cysteine and cystine are efficient precursors of CS 2 and that OH radicals are likely to be important intermediates. Both the field survey and laboratory work point to similar mechanisms for photochemical production of CS 2 and COS in marine waters. A CS 2 production rate of 0.49 Tg yr -1 for the world oceans has been estimated using the quantum yield spectra from this work and the sea surface light field provided by Leifer [1988]. This estimate is of the same order of magnitude as the present estimate of the CS 2 flux from the ocean to the atmosphere based on surface saturation and wind speed.

Validated methods for sampling and headspace analysis of carbon monoxide in seawater

Abstract A headspace analysis system with well demonstrated precision and accuracy for measuring carbon monoxide (CO) in natural waters and for CO incubation experiments is described. High water/gas volume ratios are accurately set by injecting known volumes of CO-free air into known volumes of water in glass syringes. CO in equilibrated headspace gas is separated chromatographically and quantified by a mercuric oxide reduction detector. A water/gas ratio of ∼7 is sensitive and precise enough for determining low-level CO; sensitivity can be increased by raising the water/gas ratio. At a water/gas ratio of 7 (40 ml total), the analytical blank, precision, and accuracy are 0.02 nM (nanomolar), ±0.018 nM±2%, and better than ±10%, respectively. Recovery of CO from the water phase is ∼88%. The system is efficient, simple, convenient, rapid and robust; it responds linearly up to ∼12 nM, and can process ∼8–12 samples/h. Several applications are illustrated: studies elucidating subtle CO-contamination artifacts, microbial oxidation incubations, and an oceanic profile. Validated low-contamination sampling methods are presented, and contamination control measures are recommended. A detailed 0–200-m profile at BATS in summer shows less “deep” CO than previously reported, but there is CO well below the seasonal mixed layer (ML) and even at the 1% light level.

Spatiotemporal variations of dissolved organic carbon and carbon monoxide in first-year sea ice in the western Canadian Arctic

We monitored the spatiotemporal progression of dissolved organic carbon (DOC) and carbon monoxide (CO), along with general meteorological, hydrographic, and biological variables, in first-year sea ice in the western Canadian Arctic between mid-March and early July 2008. DOC and CO concentrations fluctuated irregularly in surface ice, but followed the concentration of ice algae in bottom ice, i.e., low at the start of ice algal accumulation, highly enriched during the peak-bloom and early post-bloom, and depleted again during sea ice melt. Vertical profiles of DOC and CO typically decreased downward in early spring and were variable in the melting season. In the presence of high bottom ice algal biomass in mid-spring, DOC and CO exhibited high concentrations in the bottom (DOC: 563 +/- 434 mu mol L(-1); CO: 82.9 +/- 84 nmol L(-1)) relative to the surface (DOC: 56 +/- 26 mu mol L(-1); CO: 16.8 +/- 7 nmol L(-1)). Landfast ice contained higher levels of DOC and CO than drifting ice. Cruise-mean DOC and CO inventories in sea ice were 87 +/- 51 mu mol m(-2) and 13.9 +/- 10 mu mol m(-2), respectively. Net productions of DOC and CO linked to the ice algal bloom were assessed to be 75 mu mol m(-2) and 13.2 mu mol m(-2). Sea ice in the study area was estimated to contribute 7.4 x 10(7) moles of CO a(-1) to the atmosphere. This study suggests that sea ice plays important roles in the cycling of organic carbon and trace gases.

Carbon disulfide in the North Atlantic and Pacific oceans

Carbon disulfide (CS2) was determined in surface waters of the North Atlantic and Pacific Oceans. The mean concentrations (and ranges) of CS2 in open ocean waters were 13.4 (7.8–26.1) pM S (picomol sulfur per liter) for the North Atlantic and 14.6 (7.2–27.5) pM S for the Pacific. The concentrations in the coastal waters of the North Atlantic averaged 26.4 pM S and ranged from 17.9 to 40.4 pM S. Warm waters generally contained higher levels of CS2 than did cold waters. All the study areas were found to be supersaturated in CS2 relative to the atmosphere based on calculations from published CS2 mixing ratios in the marine boundary layer and their Henrys law constants. Sea-to-air fluxes of CS2 were estimated using exchange velocities for spot and climatological wind speeds. The global oceanic flux extrapolated from this study is 0.18 Tg CS2 yr−1 and in the range 0.13–0.24 Tg CS2 yr−1. It is suggested that microbial processes, photochemical reactions, and phytoplankton activity are potential sources for oceanic CS2.

Dark production of carbon monoxide (CO) from dissolved organic matter in the St. Lawrence estuarine system Implication for the global coastal and blue water CO budgets

We investigated the thermal (dark) production of carbon monoxide (CO) from dissolved organic matter (DOM) in the water column of the St. Lawrence estuarine system in spring 2007. The production rate, Q(co), decreased seaward horizontally and downward vertically. Q(co) exhibited a positive, linear correlation with the abundance of chromophoric dissolved organic matter (CDOM). Terrestrial DOM was more efficient at producing CO than marine DOM. The temperature dependence of Q(co) can be characterized by the Arrhenius equation with the activation energies of freshwater samples being higher than those of salty samples. Q(co) remained relatively constant between pH 4-6, increased slowly between pH 6-8 and then rapidly with further rising pH. Ionic strength and iron chemistry had little influence on Q(co). An empirical equation, describing Q(co) as a function of CDOM abundance, temperature, pH, and salinity, was established to evaluate CO dark production in the global coastal waters (depth 200 m). Both the coastal and global dark source strengths are significant compared to the corresponding photochemical CO source strengths (coastal: similar to 2.9 Tg CO-C a(-1); global: similar to 50 Tg CO-C a(-1)). Steady state deepwater CO concentrations inferred from Q(co) and microbial CO uptake rates are < 0.1 nmol L-1.

Carbon disulphide production in laboratory cultures of marine phytoplankton

Abstract Carbon disulphide (CS 2 ) data were collected from axenic monocultures of six species of marine phytoplankton. The tested species included Chaetoceros calcitrans , Phaeodactylum tricornutum , Phaeocystis sp., Porphyridium purpureum , Synechococcus sp. and Isochrysis sp. For a period of between two weeks and forty days, substantial accumulation of CS 2 was found in the cultures of C . calcitrans , P . tricornutum and Phaeocystis sp ., whereas the change of CS 2 concentration in the remaining cultures was insignificant. C . calcitrans had a potential for CS 2 production about 10 times higher than P . tricornutum or Phaeocystis sp. The formation of the compound was strongly dependent on the physiological state of the cultured species. More investigation is needed to elucidate the mechanisms responsible for the formation of this sulphur compound in these cultures.

Carbon Monoxide Photoproduction: Implications for Photoreactivity of Arctic Permafrost-Derived Soil Dissolved Organic Matter

Apparent quantum yields of carbon monoxide (CO) photoproduction (AQY(CO)) for permafrost-derived soil dissolved organic matter (SDOM) from the Yukon River Basin and Alaska coast were determined to examine the dependences of AQY(CO) on temperature, ionic strength, pH, and SDOM concentration. SDOM from different locations and soil depths all exhibited similar AQY(CO) spectra irrespective of soil age. AQY(CO) increased by 68% for a 20 °C warming, decreased by 25% from ionic strength 0 to 0.7 mol L(-1), and dropped by 25-38% from pH 4 to 8. These effects combined together could reduce AQY(CO) by up to 72% when SDOM transits from terrestrial environemnts to open-ocean conditions during summer in the Arctic. A Michaelis-Menten kinetics characterized the influence of SDOM dilution on AQY(CO) with a very low substrate half-saturation concentration. Generalized global-scale relationships between AQY(CO) and salinity and absorbance demostrate that the CO-based photoreactivity of ancient permaforst SDOM is comparable to that of modern riverine DOM and that the effects of the physicochemical variables revealed here alone could account for the seaward decline of AQY(CO) observed in diverse estuarine and coastal water bodies.

Late autumn to spring changes in the inorganic and organic carbon dissolved in the water column at Scholaert Channel, West Antarctica

Abstract The temporal changes in dissolved inorganic (DIC) and organic carbon concentrations (DOC) were monitored from late autumn to spring 2006 in the Scholaert Channel, West Antarctic Peninsula. Surface DIC spanned a small range (2163.3 to 2194.5 μmol kg-1), increasing from late autumn to winter and then decreasing in spring. An excess of DOC (7.0–63.6 μmol l-1), against a deepwater background concentration of 44 μmol l-1, existed in the surface mixed layer throughout the sampling period. Mass-balance budgeting indicates that the DIC dynamics were primarily governed by remineralization in winter and by primary production in spring despite very low biomass of both autotrophic and heterotrophic organisms. The net community production (7.3 mmol C m-2 d-1) in spring was mainly partitioned to DOC accumulation (3.6 mmol m-2 d-1) and downward export of particulate organic carbon (POC) (2.9 mmol m-2 d-1) rather than POC accretion (0.8 mmol m-2 d-1) in the surface mixed layer. The study area acted as a source of CO2 to the atmosphere in winter (∼0.8 mmol m-2 d-1) and a sink in spring (2.3–5.3 mmol m-2 d-1), and hence was not a one-way CO2 sink as had been previously hypothesized for marginal sea ice zones.

Distribution and spectral characteristics of chromophoric dissolved organic matter in a coastal bay in northern China

The absorption spectra of chromophoric dissolved organic matter (CDOM), along with general physical, chemical and biological variables, were determined in the Bohai Bay, China, in the springs of 2011 and 2012. The absorption coefficient of CDOM at 350 nm (a350) in surface water ranged from 1.00 to 1.83 m⁻¹ (mean: 1.35 m⁻¹) in May 2011 and from 0.78 to 1.92 m⁻¹ (mean: 1.19 m⁻¹) in April 2012. Little surface-bottom difference was observed due to strong vertical mixing. The a350 was weakly anti-correlated to salinity but positively correlated to chlorophyll a (Chl-a) concentration. A shoulder over 260-290 nm, suggestive of biogenic molecules, superimposed the overall pattern of exponentially decreasing CDOM absorption with wavelength. The wavelength distribution of the absorption spectral slope manifested a pronounced peak at ca. 300 nm characteristic of algal-derived CDOM. All a250/a365 ratios exceeded 6, corresponding to CDOM molecular weights (Mw) of less than 1 kDa. Spectroscopically, CDOM in the Bohai Bay differed substantively from that in the Haihe River, the bays dominant source of land runoff; photobleaching of the riverine CDOM enlarged the difference. Results point to marine biological production being the principal source of CDOM in the Bohai Bay during the sampling seasons. Relatively low runoff, fast dilution, and selective photodegradation are postulated to be among the overarching elements responsible for the lack of terrigenous CDOM signature in the bay water.

Investigations on the sorption of a toxaphene model congener, the B7-1450, on marine sediments

Sorption is a natural process that takes place in sediments or soils and changes the mobility and availability of hydrophobic organic compounds, such as toxaphene pesticide in the environment. The sorption of the 2-exo,3-endo,5-exo,8,9,10,10-heptachlorobornane (B7-1450), used as a model compound of the toxaphene heptachlorobornane congeners found in sediments, was investigated for the first time through a series of batch sorption experiments. The losses of B7-1450 due to adsorption onto glass walls and to evaporation occurring during analytical treatment steps were corrected. The study showed that these specific losses ranged from 2% to 3.5% for the glass walls adsorption and can be as high as 15% for the evaporation treatment. The sorption coefficients, K(d) and K(oc), of B7-1450 could be overestimated by >30%, particularly for low-concentration samples, if the losses were not corrected. Loss correction equations were established, validated and applied to determine sorption coefficients for the B7-1450 congener. The K(oc) values for B7-1450 determined over a gradient of concentrations ranged from 3.5x10(4) to 6.5x10(4)mlg(-1), revealing a strong affinity of B7-1450 for marine sediments.