Record of pre-industrial atmospheric sulfate in continental interiors

Sulfate aerosols affect climate by scattering radiation and by changing the microphysical properties of water clouds. In much of the continental interiors that are overwhelmed by anthropogenic sulfate today, the nature of pre-industrial atmospheric sulfate remains pure speculation, which hampers our ability to quantify anthropogenic perturbation on climate and uncertainties in global climate models. Here we show that sequential leaching and multiple-isotope measurement enabled us to effectively distinguish sulfate of different origins, including pre-industrial atmospheric sulfate, retained in certain outcropping carbonates. Data from two interior sites in northern China show that one of the sulfate endmembers consistently has an unusually positive 17O anomaly (Δʹ17O at ~+1.8‰) and characteristic δ18O (~1–5‰) and δ34S (~5–10‰) values. We interpret this sulfate endmember to be integrated pre-industrial atmospheric sulfate from at least the last a few thousands of years. A triple oxygen isotope enabled GEOS-Chem chemical transport model revealed a higher Δʹ17O value in northern China and the south-western United States in the past, consistent with our data. Overall, pre-industrial atmospheric sulfate aerosol chemistry in the interior of northern China and south-western United States had a higher pH in cloud water, which may have led to a less cloud cover due to cleaner air and coarser aerosol sizes than today. Atmospheric sulfate aerosols—which could cool the atmosphere—were formed in less acidic cloud water in continental interiors in pre-industrial time than today, according to a triple oxygen isotope analysis of sulfate in weathering carbonates.