[PAOCQ] Becky Alexander (U of Washington)

Abstract: Sulfur-containing aerosol cools the climate. The magnitude of the cooling effect of anthropogenic sulfate aerosol is uncertain due in large part to unknown natural abundances. We use Arctic ice core measurements of sulfur aerosols and their isotopes since the preindustrial era to quantify the relative abundance and trends of natural sulfur aerosols from volcanoes and marine phytoplankton. We find that sulfur emissions from volcanic passive degassing are significantly underestimated in volcanic emissions inventories, mainly due to the lack of hydrogen sulfide emissions. The underestimate in volcanic emissions results in an overestimate of the cooling from anthropogenic sulfate aerosol. We find that the amount of sulfur aerosol from marine phytoplankton has remained relatively constant over the last 800 years. The sulfur aerosol methanesulfonic acid (MSA) has decreased since the preindustrial era but is compensated for by an increase in biogenic sulfate aerosol. The observed decrease in MSA has previously been interpreted as being driven by declines in emissions from marine phytoplankton and marine primary productivity. Our results suggest that the decline in MSA is instead driven by anthropogenic-induced changes in atmospheric chemistry. We use a global chemical transport model to investigate the cause of the observed trends in the relative abundance of MSA and biogenic sulfate aerosol and the ability of several new sulfur oxidation chemical mechanisms to reproduce the observed trends.