Ice-core observations of methane sulfonic acid (MSA) are used as a proxy for past oceanic biogenic productivity because MSA originates solely from the oxidation of dimethyl sulfide (DMS) emitted by ocean phytoplankton. The use of MSA as a proxy for biogenic productivity relies on the assumption that the branching ratio of production of MSA versus sulfur dioxide (SO2) from DMS oxidation remains constant over time. However, recent Greenland ice-core observations of MSA and biogenic sulfate over the last 800 years show that the ratio of MSA-to-biogenic sulfate (MSA/bioSO4) has not remained constant. We hypothesize that recent trends in MSA are driven by changes in oxidant abundances (e.g., NOx) that lead to a reduced yield of MSA and increased yield of SO2 during oxidation of DMS. We propose to drill shallow ice cores at Summit, Greenland covering the last 30 years of snow accumulation and measure ion and MSA concentrations and sulfur isotopes of sulfate. The last 30 years will cover the time when anthropogenic NOx emissions from North America and Europe began to decline (after the mid-1990s). This will yield an additional 16 years of data compared to our current record extending from 1200 C.E. through 2006. We hypothesize that the MSA/bioSO4 ratio continues to increase from the mid-1990s to the present day due to decreases in NOx emissions in North America and Europe. To assist data interpretation, we will utilize a global chemical transport model GEOS-Chem in order to quantify the role of different oxidants on DMS oxidation as these oxidants have changed due to anthropogenic emissions. We will measure sulfate isotopes at sub-seasonal resolution over the last 30 years of snow accumulation from the proposed shallow ice cores in addition to select, discrete samples from archived ice in the preindustrial. We hypothesize that DMS emissions peak earlier in in the year today than in the preindustrial due to the earlier sea-ice melt resulting from Arctic warming. Measuring biogenic sulfate at seasonal resolution since the preindustrial will allow us to investigate changes in the seasonality of biogenic sulfur aerosol in the Arctic resulting from changes in Arctic climate. This is a collaborative project between PIs in the US and the UK, and the UK part of the project will be supported by NERC.