Preindustrial volcanic sulfate aerosol is underestimated in the Arctic: implications for radiative forcing

Thesis (Master's)--University of Washington, 2022The Arctic has warmed at a rate of twice the global average since 1980. An estimated sixty percent of greenhouse-gas-induced warming in the Arctic has been offset by aerosols in the past century. However, aerosols are the largest uncertainty in estimates of radiative forcing (RF) in large part due to unknown preindustrial aerosol abundance. Here we quantify the relative importance of natural sources of Arctic sulfate during the preindustrial by measuring the concentrations of non-sea salt sulfate (nssSO42-) and sulfur isotopic composition of sulfate (d34S(SO42-)) in ice-core samples from Summit, Greenland between 1200 and 1850 CE. The mean ice-core volcanic sulfur fraction (fvolc) of nssSO42- is 63 ± 10%, approximately twice as abundant as biogenic sulfate from the oxidation of dimethyl sulfide (DMS). We compare these results to a global model, which uses the same volcanic emissions inventory as most climate models, and find that the model underestimates the volcanic preindustrial passive degassing emissions in regions affecting the Arctic by a factor of three. Enhanced volcanic emissions from passive degassing in the model based on ground-based observations of SO2 and hydrogen sulfide (H2S) reconciles the discrepancy between the model and ice-core observations. Higher preindustrial volcanic sulfur emissions results in a higher aerosol RF by +0.11 to +0.31 W m-2, depending on the assumed present-day volcanic emissions. Our findings suggest that preindustrial and possibly also present-day volcanic non-eruptive emissions are underestimated with significant implications for RF