Dimethyl sulfide, tropospheric

Another interesting applieation of MDGC is in the rapid determination of isoprene (the most reaetive hydroearbon speeies) and dimethyl sulfide (DMS) (the major souree of sulfur in the marine troposphere and a preeursor to eloud formation) in the atmosphere (16). The deteetion limits were 5 and 25 ng 1 respeetively. 

A high-speed sensor for the assay of dimethyl sulfide in the marine troposphere based on its CL reaction with F2 was recently reported [18]. Sample air and F2 in He were introduced at opposite ends of a reaction cell with a window at one end. The production of vibrationally excited HF and electronically excited fluorohydrocarbon (FHC) produced CL emission in the wavelength range 450-650 nm, which was monitored via photon counting. Dimethyl sulfide could be determined in the 0-1200 pptv (parts per trillion by volume) concentration range, with a 4-pptv detection limit. 

Table 8.17 summarizes the rate constants and estimated tropospheric lifetimes of some of these sulfur compounds with respect to reaction with OH or NO-,. The assumed concentrations of these oxidants chosen for the calculations are those characteristic of more remote regions, which are major sources of reduced sulfur compounds such as dimethyl sulfide (DMS). It is seen that OH is expected to be the most important sink for these compounds and that NO, may also be important, for example, for DMS oxidation (see also Chapter 6.J). 

Barnes, I., K. H. Becker, and I. Patroescu, The Tropospheric Oxidation of Dimethyl Sulfide A New Source of Carbonyl Sulfide, Geophys. Res. Lett., 21, 2389-2392 (1994b). 

In the last 150 years the anthropogenic emission of sulfur has increased dramatically, primarily due to combustion processes [1]. In the 1950s anthropogenic emission surpassed natural emission and the atmospheric sulfur cycle is one of the most perturbed biogeochemical cycles [1,2]. The oceans are the largest natural source of atmospheric sulfur emissions, where sulfur is emitted in a reduced form, predominantly as dimethyl sulfide (DMS) and to a much lesser extent carbonyl sulfide (OCS) and carbon disulfide (CS2) [3]. Ocean emitted DMS and CS2 are initially oxidised to OCS, which diffuses through the troposphere into the stratosphere where further oxidation to sulfur dioxide (SO2), sulfur trioxide (SO3) and finally sulfuric acid (H2SO4) occurs [1-4]. 

An example in which formation of a carbon radical is not the initial reaction is provided by the atmospheric reactions of organic sulfides and disulfides. They also provide an example in which rates of reaction with nitrate radicals exceed those with hydroxyl radicals. 2-dimethylthiopropionic acid is produced by algae and by the marsh grass Spartina alternifolia, and may then be metabolized in sediment slurries under anoxic conditions to dimethyl sulfide (Kiene and Taylor 1988), and by aerobic bacteria to methyl sulfide (Taylor and Gilchrist 1991). It should be added that methyl sulfide can be produced by biological methylation of sulfide itself (HS ) (Section 6.11.4). Dimethyl sulfide — and possibly also methyl sulfide — is oxidized in the troposphere to sulfur dioxide and methanesulfonic acids. 

Dimethyl sulfide — and possibly also methyl sulfide — is oxidized in the troposphere to sulfuric and methanesulfonic acids, and it has been suggested that these compounds may play a critical role in promoting cloud formation... 

Thomton DC, Bandy AR. 1993. Sulfur dioxide and dimethyl sulfide in the central Pacific troposphere. Journal of Atmospheric Chemistry 17 1-13. 

Fig. 10-9. Flux diagram for sulfur in the unperturbed marine atmosphere. Fluxes are given in units of p.gS/m2day. Numbers in boxes indicate column densities in units of p.gS/m2. DMS, Dimethyl sulfide MSA, methane sulfonic acid (associated with the aerosol). The mixing ratio of S02 is 60 ng S/m3, independent of altitude. The mixing ratio of SOis 280 ng S/m3 in the boundary layer and 80 ng S/m3 in the free troposphere. Contrary to the model of Kritz (1982), the fluxes are confined to the boundary layer. There exists no significant net flux into or out of the free troposphere. The dry deposition velocity for S02 is 5mm/s.

Dimethyl sulfide (DMS), through its oxidation to sulfate in the troposphere, acts as a source of cloud condensation nuclei, thus potentially influencing the radiative balance of the atmosphere. DMS is formed in sea water through the microbial decomposition of dimethyl sulfonioproprionate (DMSP), a compound believed to act as an osmolyte in certain species of marine phytoplankton. The flux of DMS to the atmosphere is controlled by its concentration in surface sea waters, which is controlled in turn by the rate of its decomposition. Estimates indicate that 7-40% of the total turnover of DMS in the surface waters of the Pacific Ocean is due to the photosensitized destruction of this compound, illustrating the potential importance of this pathway in controlling the flux of DMS to the atmosphere. 

These phenomena are of great significance in environmental chemistry, either owing to the solubilities of air pollutants in water or owing to their ability to migrate from water to the atmosphere. A good example is dimethyl sulfide, which is emitted by oceanic phytoplankton and constitutes the major natural source of sulfur in the troposphere. 

As for the primary trace chemical species in the remote natural atmosphere where there is no influence of anthropogenic activities, methane (CH4) emitted from lakes and marshes, biogenic volatile organic compounds (BVOCs), nitric oxide (NO) from natural soils and lightning, dimethyl sulfide (DMS) from marine organisms, O3 descended from the stratosphere can be conceived. Among them, the most important chemical species that is subjected to photolysis by the actinic flux (see Sect. 3.5) in the troposphere is O3 and NO2 (see Sects. 4.2.1 and 4.2.2). 

Akimoto, H., Kimitaka, K., Nakazawa, T., Washida, N. (eds.) Chemistry of the Tropospheric Atmosphere and Global Environment, Gakkai Shuppan Center, Toyko (2002). (in Japanese) Albu, M., Barnes, I., Becker, K.H., Patroescu-Klotz, I., Mocano, R., Benter, T. Rate coefficients for the gas-phase reaction of OH radicals with dimethyl sulfide Temperature and O2 partial pressure dependence. Phys. Chem. Chem. Phys. 8, 728-736 (2006)... 

Aerosols are also important in stratospheric photochemistry. They are thought to be formed as a result of the oxidation of dimethyl sulphide in the troposphere. This is formed from decaying organic matter (e.g. oceanic algae) and is emitted into the air where it breaks down to form carbonyl sulfide COS. This is chemically stable... 

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