Sulfur metabolites in the pelagic ocean

Marine microorganisms play crucial roles in Earth's element cycles through the production and consumption of organic matter. One of the elements whose fate is governed by microbial activities is sulfur, an essential constituent of biomass and a crucial player in climate processes. With sulfur already being well studied in the ocean in its inorganic forms, organic sulfur compounds are emerging as important chemical links between marine phytoplankton and bacteria. The high concentration of inorganic sulfur in seawater, which can readily be reduced by phytoplankton, provides a freely available source of sulfur for biomolecule synthesis. Mechanisms such as exudation and cell lysis release these phytoplankton-derived sulfur metabolites into seawater, from which they are rapidly assimilated by marine bacteria and archaea. Energy-limited bacteria use scavenged sulfur metabolites as substrates or for the synthesis of vitamins, cofactors, signalling compounds and antibiotics. In this Review, we examine the current knowledge of sulfur metabolites released into and taken up from the marine dissolved organic matter pool by microorganisms, and the ecological links facilitated by their diversity in structures, oxidation states and chemistry.

Ion-induced sulfuric acid-ammonia nucleation drives particle formation in coastal Antarctica

Formation of new aerosol particles from trace gases is a major source of cloud condensation nuclei (CCN) in the global atmosphere, with potentially large effects on cloud optical properties and Earth's radiative balance. Controlled laboratory experiments have resolved, in detail, the different nucleation pathways likely responsible for atmospheric new particle formation, yet very little is known from field studies about the molecular steps and compounds involved in different regions of the atmosphere. The scarcity of primary particle sources makes secondary aerosol formation particularly important in the Antarctic atmosphere. Here, we report on the observation of ion-induced nucleation of sulfuric acid and ammonia-a process experimentally investigated by the CERN CLOUD experiment-as a major source of secondary aerosol particles over coastal Antarctica. We further show that measured high sulfuric acid concentrations, exceeding 10⁷ molecules cm^-3, are sufficient to explain the observed new particle growth rates. Our findings show that ion-induced nucleation is the dominant particle formation mechanism, implying that galactic cosmic radiation plays a key role in new particle formation in the pristine Antarctic atmosphere.

Atmospherically Relevant Radicals Derived from the Oxidation of Dimethyl Sulfide

The large number and amounts of volatile organosulfur compounds emitted to the atmosphere and the enormous variety of their reactions in various oxidation states make experimental measurements of even a small fraction of them a daunting task. Dimethyl sulfide (DMS) is a product of biological processes involving marine phytoplankton, and it is estimated to account for approximately 60% of the total natural sulfur gases released to the atmosphere. Ocean-emitted DMS has been suggested to play a role in atmospheric aerosol formation and thereby cloud formation. The reaction of ·OH with DMS is known to proceed by two independent channels: abstraction and addition. The oxidation of DMS is believed to be initiated by the reaction with ·OH and NO₃· radicals, which eventually leads to the formation of sulfuric acid (H₂SO₄) and methanesulfonic acid (CH₃SO₃H). The reaction of DMS with NO₃· appears to proceed exclusively by hydrogen abstraction. The oxidation of DMS consists of a complex sequence of reactions. Depending on the time of the day or altitude, it may take a variety of pathways. In general, however, the oxidation proceeds via chains of radical reactions. Dimethyl sulfoxide (DMSO) has been reported to be a major product of the addition channel. Dimethyl sulfone (DMSO₂), SO₂, CH₃SO₃H, and methanesulfinic acid (CH₃S(O)OH) have been observed as products of further oxidation of DMSO. Understanding the details of DMS oxidation requires in-depth knowledge of the elementary steps of this seemingly simple transformation, which in turn requires a combination of experimental and theoretical methods. The methylthiyl (CH₃S·), methylsulfinyl (CH₃SO·), methylsulfonyl (CH₃SO₂·), and methylsulfonyloxyl (CH₃SO₃·) radicals have been postulated as intermediates in the oxidation of DMS. Therefore, studying the chemistry of sulfur-containing free radicals in the laboratory also is the basis for understanding the mechanism of DMS oxidation in the atmosphere. The application of matrix-isolation techniques in combination with quantum-mechanical calculations on the generation and structural elucidation of CH₃SO_x (x = 0-3) radicals is reviewed in the present Account. Experimental matrix IR and UV/vis data for all known species of this substance class are summarized together with data obtained using other spectroscopic techniques, including time-resolved spectroscopy, electron paramagnetic resonance spectroscopy, and others. We also discuss the reactivity and experimental characterization of these species to illustrate their practical relevance and highlight spectroscopic techniques available for the elucidation of their geometric and electronic structures. The present Account summarizes recent results regarding the preparation, characterization, and reactivity of various radical species with the formula CH₃SO_x (x = 0-3).

Infrared studies of the reaction of methanesulfonic acid with trimethylamine on surfaces

Organosulfur compounds generated from a variety of biological as well as anthropogenic sources are oxidized in air to form sulfuric acid and methanesulfonic acid (MSA). Both of these acids formed initially in the gas phase react with ammonia and amines in air to form and grow new particles, which is important for visibility, human health and climate. A competing sink is deposition on surfaces in the boundary layer. However, relatively little is known about reactions after they deposit on surfaces. We report here diffuse reflectance infrared Fourier transform spectrometry (DRIFTS) studies of the reaction of MSA with trimethylamine (TMA) on a silicon powder at atmospheric pressure in synthetic air and at room temperature, either in the absence or in the presence of water vapor. In both cases, DRIFTS spectra of the product surface species are essentially the same as the transmission spectrum obtained for trimethylaminium methanesulfonate, indicating the formation of the salt on the surface with a lower limit to the reaction probability of γ > 10(-6). To the best of our knowledge, this is the first infrared study to demonstrate this chemistry from the heterogeneous reaction of MSA with an amine on a surface. This heterogeneous chemistry appears to be sufficiently fast that it could impact measurements of gas-phase amines through reactions with surface-adsorbed acids on sampling lines and inlets. It could also represent an additional sink for amines in the boundary layer, especially at night when the gas-phase reactions of amines with OH radical and ozone are minimized.

Photochemical oxidation of reduced sulfur compounds in an urban location based on short time monitoring data

This study examines the oxidation of reduced sulfur compounds (RSCs) in urban ambient air. The photochemical conversions of RSC (such as DMS, CS2, H2S, DMDS, and CH3SH) to a further oxidized form (e.g., SO2, MSA, and H2SO4) were assessed using a photochemical box model. For our model simulation of RSC oxidation, measurements were taken at an urban monitoring station in Seoul, Korea (37.6 degrees N, 127 degrees E) during three separate time periods (e.g., Sept. 17-18, Oct. 23, and Oct. 27-28, 2003). The results indicate that DMS and H2S were the dominant RSCs with concentrations of 370 +/- 140 and 110 +/- 60 pptv, respectively. The photochemical conversion of DMDS to SO2 was found to occur more efficiently than other RSCs. The overall results of our study suggest that photochemical conversion of RSCs accounted for less than 15% of the observed SO2 during the measurement period. The SO2 production from DMS oxidation (mainly by the reaction with OH) was found to be affected primarily by the abstraction channel due to high NOx levels during the experimental conditions.

Reaction Mechanisms and Kinetics for the Oxidations of Dimethyl Sulfide, Dimethyl Disulfide, and Methyl Mercaptan by the Nitrate Radical

This study examines the initial oxidation routes of the three major reduced sulfur compounds (CH(3)SH, CH(3)SCH(3), and CH(3)SSCH(3)) by the nitrate radical using density functional and ab initio methods. Stationary points along each reaction pathway are examined using different levels of theory and basis sets to ensure the convergence of the results. Kinetics calculations follow on the determined reaction pathways to obtain the rate constants. This study shows that sulfur compounds exhibit a general trend of hydrogen abstraction following the formation of an initial sulfur-nitrate complex. The results are in agreement with experimental work on CH(3)SCH(3) and CH(3)SH, while refuting a proposal of several previous studies that oxygen addition is the dominant oxidation pathway in the case of CH(3)SSCH(3). The rate constants obtained from kinetics calculations are consistent with experimental findings and exhibit negative temperature dependence. Overall, this study confirms the importance of nitrate in the oxidation of reduced sulfur compounds in the atmosphere.