An on-line method for S-isotope analysis is described. Samples are combusted in an elemental analyzer. SO2 is separated from other combustion gases by gas chromatography, and the gases enter the ion source of the mass spectrometer through a split interface. Integrated peak areas for 32S02+ and 34SC>2+ are compared to the response for a standard gas sample to determine the S^S value. 534S values of samples analyzed using the on-line method correspond linearly with those achieved from the same sample prepared off-line, where Kiba reduction followed by oxidation of the sulfur to SO2 is carried out prior to S-isotope analysis against a known standard. With the described on-line method, the amount of sulfur necessary for S-isotope analysis is reduced to about 10 #tg of S per analysis. The time needed for on-line preparation and measurement is less than one-third of the off-line procedure.Stable sulfur-isotope analysis is often associated with many problems concerning sample preparation and mass spectrometric determination. The 34S/32S ratios are most commonly determined after SO2 has been generated out of natural materials. Sample preparation always requires several chemical transformation steps to finally produce SO2 out of the S-containing compounds. The standard preparation techniques used for most sulfur-bearing samples start with conversion of all sulfur to BaSO*, which is then reduced to H2S by one of the following three procedures: graphite reduction at 1000 °C,' reduction with tin(II)-strong phosphoric acid (Kiba's reagent) at 300 °C in a stream of nitrogen,2 or application of a HI-H3PO4-HCI reduction solution.3 The generated H2S is converted to Ag2S, which is finally oxidized to SO2. One major disadvantage of these methods is that they all need relatively large amounts of the original material to obtain sufficient sulfur as the numerous chemical processes require rather high amounts of sulfur (3-7 mg of S). Although

[1] The recent decline in sea ice cover in the Arctic Ocean could affect the regional radiative forcing via changes in sea ice-atmosphere exchange of dimethyl sulfide (DMS) and biogenic aerosols formed from its atmospheric oxidation, such as methanesulfonic acid (MSA). This study examines relationships between changes in total sea ice extent north of 70 N and atmospheric MSA measurement at Alert, Nunavut, during 1980Nunavut, during -2009 at Barrow, Alaska, during 1997 and at Ny-Ålesund, Svalbard, for 1991. During the 1980-1989 and 1990 periods, summer (July-August) and June MSA concentrations at Alert decreased. In general, MSA concentrations increased at all locations since 2000 with respect to 1990 values, specifically during June and summer at Alert and in summer at Barrow and Ny-Ålesund. Our results show variability in MSA at all sites is related to changes in the source strengths of DMS, possibly linked to changes in sea ice extent as well as to changes in atmospheric transport patterns. Since 2000, a late spring increase in atmospheric MSA at the three sites coincides with the northward migration of the marginal ice edge zone where high DMS emissions from ocean to atmosphere have previously been reported. Significant negative correlations are found between sea ice extent and MSA concentrations at the three sites during the spring and June. These results suggest that a decrease in seasonal ice cover influencing other mechanisms of DMS production could lead to higher atmospheric MSA concentrations.

One year of aerosol particle observations from Alert, Nunavut shows that new particle formation (NPF) is common during clean periods of the summertime Arctic associated with attendant low condensation sinks and with the presence of methane sulfonic acid (MSA), a product of the atmospheric oxidation of dimethyl sulfide (DMS). The clean aerosol time periods, defined using the distribution of refractory black carbon number concentrations, increase in frequency from June through August as the anthropogenic influence dwindles. During the clean periods, the number concentrations of particles that can act as cloud condensation nuclei (CCN) increase from June through August suggesting that DMS, and possibly other oceanic organic precursors, exert significant control on the Arctic summertime submicron aerosol, a proposition supported by simulations from the GEOS-Chem-TOMAS global chemical transport model with particle microphysics. The CCN increase for the clean periods across the summer is estimated to be able to increase cloud droplet number concentrations (CDNC) by 23-44 cm -3 , comparable to the mean CDNC increase needed to yield the current global cloud albedo forcing from industrial aerosols. These results suggest that DMS may contribute significantly to modification of the Arctic summer shortwave cloud albedo, and they offer a reference for future changes in the Arctic summer aerosol.

However, in that study the analytical sensitivity was not available to determine the isotopic composition of individual weekly aerosol samples during summer when aerosol concen-11,619

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