Natural climate variation, such as that caused by volcanoes, is the basis for identifying anthropogenic climate change. However, knowledge of the history of volcanic activity is inadequate, particularly concerning the explosivity of specific events. Some material is deposited in ice cores, but the concentration of glacial sulfate does not distinguish between tropospheric and stratospheric eruptions. Stable sulfur isotope abundances contain additional information, and recent studies show a correlation between volcanic plumes that reach the stratosphere and mass-independent anomalies in sulfur isotopes in glacial sulfate. We describe a mechanism, photoexcitation of SO 2 , that links the two, yielding a useful metric of the explosivity of historic volcanic events. A plume model of S(IV) to S(VI) conversion was constructed including photochemistry, entrainment of background air, and sulfate deposition. Isotopologue-specific photoexcitation rates were calculated based on the UV absorption cross-sections of 32 SO 2 , 33 SO 2 , 34 SO 2 , and 36 SO 2 from 250 to 320 nm. The model shows that UV photoexcitation is enhanced with altitude, whereas mass-dependent oxidation, such as SO 2 + OH, is suppressed by in situ plume chemistry, allowing the production and preservation of a mass-independent sulfur isotope anomaly in the sulfate product. The model accounts for the amplitude, phases, and time development of Δ 33 S/δ 34 S and Δ 36 S/Δ 33 S found in glacial samples. We are able to identify the process controlling mass-independent sulfur isotope anomalies in the modern atmosphere. This mechanism is the basis of identifying the magnitude of historic volcanic events.A ttribution of climate change relies on our understanding of natural climate variation. Volcanoes affect climate, but it is not easy to use proxy records to derive the climate impact of a given historical eruption, primarily because we lack knowledge about the volcanoes themselves. Some so-called Plinian eruptions penetrate the stratosphere, resulting in multiyear climate impacts (1). Volcanic sulfur dioxide (SO 2 ) in a plume is photooxidized in the atmosphere. In the troposphere, the sulfuric acid product is washed out as sulfate in acid rain in a matter of weeks. A Plinian eruption, in contrast, intensifies the stratospheric sulfate aerosol (SSA) layer (2), increasing the planet's albedo (3) and enhancing midlatitude O 3 depletion (4) for more than a year. Ice core records of sulfate provide an important record of volcanic activity (5), but the concentration of sulfate alone does not indicate the explosivity of the event and specifically, if the plume penetrated the stratosphere (6).A series of groundbreaking studies has shown that sulfur isotopes in sulfate from Plinian eruptions show mass-independent fractionation (MIF) (6-8). Carbonyl sulfide (OCS) is thought to be the main source of background SSA in volcanically quiescent periods (9), and the reactions breaking down OCS, mainly photolysis, show no evidence of sulfur MIF (10-13). MIF is not seen at the sour...

After the 1980s, atmospheric sulfate reduction is slower than the dramatic reductions in sulfur dioxide (SO2) emissions. However, a lack of observational evidence has hindered the identification of causal feedback mechanisms. Here, we report an increase in the oxygen isotopic composition of sulfate (Δ17OSO42−) in a Greenland ice core, implying an enhanced role of acidity-dependent in-cloud oxidation by ozone (up to 17 to 27%) in sulfate production since the 1960s. A global chemical transport model reproduces the magnitude of the increase in observed Δ17OSO42− with a 10 to 15% enhancement in the conversion efficiency from SO2 to sulfate in Eastern North America and Western Europe. With an expected continued decrease in atmospheric acidity, this feedback will continue in the future and partially hinder air quality improvements.

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